Chapter 27 Extending MySQL

27.1 MySQL Internals

This chapter describes a lot of things that you need to know when working on the MySQL code. To track or contribute to MySQL development, follow the instructions in Section 2.9.3, “Installing MySQL Using a Development Source Tree”. If you are interested in MySQL internals, you should also subscribe to our internals mailing list. This list has relatively low traffic. For details on how to subscribe, please see Section 1.6.1, “MySQL Mailing Lists”. Many MySQL developers at Oracle Corporation are on the internals list and we help other people who are working on the MySQL code. Feel free to use this list both to ask questions about the code and to send patches that you would like to contribute to the MySQL project!

27.1.1 MySQL Threads

The MySQL server creates the following threads:

  • Connection manager threads handle client connection requests on the network interfaces that the server listens to. On all platforms, one manager thread handles TCP/IP connection requests. On Unix, this manager thread also handles Unix socket file connection requests. On Windows, a manager thread handles shared-memory connection requests, and another handles named-pipe connection requests. The server does not create threads to handle interfaces that it does not listen to. For example, a Windows server that does not have support for named-pipe connections enabled does not create a thread to handle them.

  • Connection manager threads associate each client connection with a thread dedicated to it that handles authentication and request processing for that connection. Manager threads create a new thread when necessary but try to avoid doing so by consulting the thread cache first to see whether it contains a thread that can be used for the connection. When a connection ends, its thread is returned to the thread cache if the cache is not full.

    For information about tuning the parameters that control thread resources, see Section 9.12.5.1, “How MySQL Uses Threads for Client Connections”.

  • On a master replication server, connections from slave servers are handled like client connections: There is one thread per connected slave.

  • On a slave replication server, an I/O thread is started to connect to the master server and read updates from it. An SQL thread is started to apply updates read from the master. These two threads run independently and can be started and stopped independently.

  • A signal thread handles all signals. This thread also normally handles alarms and calls process_alarm() to force timeouts on connections that have been idle too long.

  • If InnoDB is used, there will be additional read and write threads by default. The number of these are controlled by the innodb_read_io_threads and innodb_write_io_threads parameters. See Section 15.14, “InnoDB Startup Options and System Variables”.

  • If the server is started with the --flush_time=val option, a dedicated thread is created to flush all tables every val seconds.

  • If the event scheduler is active, there is one thread for the scheduler, and a thread for each event currently running. See Section 22.4.1, “Event Scheduler Overview”.

mysqladmin processlist only shows the connection, replication, and event threads.

27.1.2 The MySQL Test Suite

The test system that is included in Unix source and binary distributions makes it possible for users and developers to perform regression tests on the MySQL code. These tests can be run on Unix.

You can also write your own test cases. For information about the MySQL Test Framework, including system requirements, see the manual available at http://dev.mysql.com/doc/mysqltest/2.0/en/.

The current set of test cases does not test everything in MySQL, but it should catch most obvious bugs in the SQL processing code, operating system or library issues, and is quite thorough in testing replication. Our goal is to have the tests cover 100% of the code. We welcome contributions to our test suite. You may especially want to contribute tests that examine the functionality critical to your system because this ensures that all future MySQL releases work well with your applications.

The test system consists of a test language interpreter (mysqltest), a Perl script to run all tests (mysql-test-run.pl), the actual test cases written in a special test language, and their expected results. To run the test suite on your system after a build, type make test from the source root directory, or change location to the mysql-test directory and type ./mysql-test-run.pl. If you have installed a binary distribution, change location to the mysql-test directory under the installation root directory (for example, /usr/local/mysql/mysql-test), and run ./mysql-test-run.pl. All tests should succeed. If any do not, feel free to try to find out why and report the problem if it indicates a bug in MySQL. See Section 1.7, “How to Report Bugs or Problems”.

If one test fails, you should run mysql-test-run.pl with the --force option to check whether any other tests fail.

If you have a copy of mysqld running on the machine where you want to run the test suite, you do not have to stop it, as long as it is not using ports 9306 or 9307. If either of those ports is taken, you should set the MTR_BUILD_THREAD environment variable to an appropriate value, and the test suite will use a different set of ports for master, slave, and NDB). For example:

shell> export MTR_BUILD_THREAD=31
shell> ./mysql-test-run.pl [options] [test_name]

In the mysql-test directory, you can run an individual test case with ./mysql-test-run.pl test_name.

If you have a question about the test suite, or have a test case to contribute, send an email message to the MySQL internals mailing list. See Section 1.6.1, “MySQL Mailing Lists”.

27.2 The MySQL Plugin API

MySQL supports a plugin API that enables creation of server components. Plugins can be loaded at server startup, or loaded and unloaded at runtime without restarting the server. The API is generic and does not specify what plugins can do. The components supported by this interface include, but are not limited to, storage engines, full-text parser plugins, and server extensions.

For example, full-text parser plugins can be used to replace or augment the built-in full-text parser. A plugin can parse text into words using rules that differ from those used by the built-in parser. This can be useful if you need to parse text with characteristics different from those expected by the built-in parser.

The plugin interface is more general than the older user-defined function (UDF) interface.

The plugin interface uses the plugin table in the mysql database to record information about plugins that have been installed permanently with the INSTALL PLUGIN statement. This table is created as part of the MySQL installation process. Plugins can also be installed for a single server invocation with the --plugin-load option. Plugins installed this way are not recorded in the plugin table. See Section 6.5.2, “Installing and Uninstalling Plugins”.

MySQL supports an API for client plugins in addition to that for server plugins. This is used, for example, by authentication plugins where a server-side plugin and a client-side plugin cooperate to enable clients to connect to the server through a variety of authentication methods.

Additional Resources

The book MySQL 5.1 Plugin Development by Sergei Golubchik and Andrew Hutchings provides a wealth of detail about the plugin API. Despite the fact that the book's title refers to MySQL Server 5.1, most of the information in it applies to later versions as well.

27.2.1 Types of Plugins

The plugin API enables creation of plugins that implement several capabilities:

The following sections provide an overview of these plugin types.

Storage Engine Plugins

The pluggable storage engine architecture used by MySQL Server enables storage engines to be written as plugins and loaded into and unloaded from a running server. For a description of this architecture, see Section 16.11, “Overview of MySQL Storage Engine Architecture”.

For information on how to use the plugin API to write storage engines, see MySQL Internals: Writing a Custom Storage Engine.

Full-Text Parser Plugins

MySQL has a built-in parser that it uses by default for full-text operations (parsing text to be indexed, or parsing a query string to determine the terms to be used for a search). The built-in full-text parser is supported with InnoDB and MyISAM tables.

MySQL also has a character-based ngram full-text parser that supports Chinese, Japanese, and Korean (CJK), and a word-based MeCab parser plugin that supports Japanese, for use with InnoDB and MyISAM tables.

For full-text processing, parsing means extracting words (or tokens, in the case of an n-gram character-based parser) from text or a query string based on rules that define which character sequences make up a word and where word boundaries lie.

When parsing for indexing purposes, the parser passes each word to the server, which adds it to a full-text index. When parsing a query string, the parser passes each word to the server, which accumulates the words for use in a search.

The parsing properties of the built-in full-text parser are described in Section 13.9, “Full-Text Search Functions”. These properties include rules for determining how to extract words from text. The parser is influenced by certain system variables that cause words shorter or longer to be excluded, and by the stopword list that identifies common words to be ignored. For more information, see Section 13.9.4, “Full-Text Stopwords”, and Section 13.9.6, “Fine-Tuning MySQL Full-Text Search”.

The plugin API enables you to use a full-text parser other than the default built-in full-text parser. For example, if you are working with Japanese, you may choose to use the MeCab full-text parser. The plugin API also enables you to provide a full-text parser of your own so that you have control over the basic duties of a parser. A parser plugin can operate in either of two roles:

  • The plugin can replace the built-in parser. In this role, the plugin reads the input to be parsed, splits it up into words, and passes the words to the server (either for indexing or for token accumulation). The ngram and MeCab parsers operate as replacements for the built-in full-text parser.

    You may choose to provide your own full-text parser if you need to use different rules from those of the built-in parser for determining how to split up input into words. For example, the built-in parser considers the text case-sensitive to consist of two words case and sensitive, whereas an application might need to treat the text as a single word.

  • The plugin can act in conjunction with the built-in parser by serving as a front end for it. In this role, the plugin extracts text from the input and passes the text to the parser, which splits up the text into words using its normal parsing rules. This parsing is affected by the innodb_ft_xxx or ft_xxx system variables and the stopword list.

    One reason to use a parser this way is that you need to index content such as PDF documents, XML documents, or .doc files. The built-in parser is not intended for those types of input but a plugin can pull out the text from these input sources and pass it to the built-in parser.

It is also possible for a parser plugin to operate in both roles. That is, it could extract text from noncleartext input (the front end role), and also parse the text into words (thus replacing the built-in parser).

A full-text plugin is associated with full-text indexes on a per-index basis. That is, when you install a parser plugin initially, that does not cause it to be used for any full-text operations. It simply becomes available. For example, a full-text parser plugin becomes available to be named in a WITH PARSER clause when creating individual FULLTEXT indexes. To create such an index at table-creation time, do this:

CREATE TABLE t
(
  doc CHAR(255),
  FULLTEXT INDEX (doc) WITH PARSER parser_name
) ENGINE=InnoDB;

Or you can add the index after the table has been created:

ALTER TABLE t ADD FULLTEXT INDEX (doc) WITH PARSER parser_name;

The only SQL change for associating the parser with the index is the WITH PARSER clause. Searches are specified as before, with no changes needed for queries.

When you associate a parser plugin with a FULLTEXT index, the plugin is required for using the index. If the parser plugin is dropped, any index associated with it becomes unusable. Any attempt to use a table for which a plugin is not available results in an error, although DROP TABLE is still possible.

For more information about full-text plugins, see Section 27.2.4.4, “Writing Full-Text Parser Plugins”. MySQL 5.7 supports full-text plugins with MyISAM and InnoDB.

Daemon Plugins

A daemon plugin is a simple type of plugin used for code that should be run by the server but that does not communicate with it. MySQL distributions include an example daemon plugin that writes periodic heartbeat messages to a file.

For more information about daemon plugins, see Section 27.2.4.5, “Writing Daemon Plugins”.

INFORMATION_SCHEMA Plugins

INFORMATION_SCHEMA plugins enable the creation of tables containing server metadata that are exposed to users through the INFORMATION_SCHEMA database. For example, InnoDB uses INFORMATION_SCHEMA plugins to provide tables that contain information about current transactions and locks.

For more information about INFORMATION_SCHEMA plugins, see Section 27.2.4.6, “Writing INFORMATION_SCHEMA Plugins”.

Semisynchronous Replication Plugins

MySQL replication is asynchronous by default. With semisynchronous replication, a commit performed on the master side blocks before returning to the session that performed the transaction until at least one slave acknowledges that it has received and logged the events for the transaction. Semisynchronous replication is implemented through complementary master and client plugins. See Section 18.3.9, “Semisynchronous Replication”.

For more information about semisynchronous replication plugins, see Section 27.2.4.7, “Writing Semisynchronous Replication Plugins”.

Audit Plugins

The MySQL server provides a pluggable audit interface that enables information about server operations to be reported to interested parties. Audit notification occurs for these operations (although the interface is general and the server could be modified to report others):

  • Write a message to the general query log (if the log is enabled)

  • Write a message to the error log

  • Send a query result to a client

Audit plugins may register with the audit interface to receive notification about server operations. When an auditable event occurs within the server, the server determines whether notification is needed. For each registered audit plugin, the server checks the event against those event classes in which the plugin is interested and passes the event to the plugin if there is a match.

This interface enables audit plugins to receive notifications only about operations in event classes they consider significant and to ignore others. The interface provides for categorization of operations into event classes and further division into event subclasses within each class.

When an audit plugin is notified of an auditable event, it receives a pointer to the current THD structure and a pointer to a structure that contains information about the event. The plugin can examine the event and perform whatever auditing actions are appropriate. For example, the plugin can see what statement produced a result set or was logged, the number of rows in a result, who the current user was for an operation, or the error code for failed operations.

For more information about audit plugins, see Section 27.2.4.8, “Writing Audit Plugins”.

Authentication Plugins

MySQL supports pluggable authentication. Authentication plugins exist on both the server and client sides. Plugins on the server side implement authentication methods for use by clients when they connect to the server. A plugin on the client side communicates with a server-side plugin to provide the authentication information that it requires. A client-side plugin may interact with the user, performing tasks such as soliciting a password or other authentication credentials to be sent to the server. See Section 7.3.8, “Pluggable Authentication”.

Pluggable authentication also enables proxy user capability, in which one user takes the identity of another user. A server-side authentication plugin can return to the server the name of the user whose identity the connecting user should have. See Section 7.3.9, “Proxy Users”.

For more information about authentication plugins, see Section 27.2.4.9, “Writing Authentication Plugins”.

Password-Validation Plugins

The MySQL server provides an interface for writing plugins that test passwords. Such a plugin implements two capabilities:

For information about writing this type of plugin, see Section 27.2.4.10, “Writing Password-Validation Plugins”.

Protocol Trace Plugins

MySQL supports the use of protocol trace plugins: client-side plugins that implement tracing of communication between a client and the server that takes place using the client/server protocol. This capability can be used in MySQL 5.7.2 and up.

For more information about protocol trace plugins, see Section 27.2.4.11, “Writing Protocol Trace Plugins”.

Query Rewrite Plugins

As of MySQL 5.7.6, MySQL Server supports query rewrite plugins that can examine and possibly modify statements received by the server before the server executes them. A query rewrite plugin takes statements either before or after the server has parsed them.

A preparse query rewrite plugin has these characteristics:

  • The plugin enables rewriting of SQL statements arriving at the server before the server processes them.

  • The plugin receives a statement string and may return a different string.

A postparse query rewrite plugin has these characteristics:

  • The plugin enables statement rewriting based on parse trees.

  • The server parses each statement and passes its parse tree to the plugin, which may traverse the tree. The plugin can return the original tree to the server for further processing, or construct a different tree and return that instead.

  • The plugin can use the mysql_parser plugin service for these purposes:

    • To activate statement digest calculation and obtain the normalized version of statements independent of whether the Performance Schema produces digests.

    • To traverse parse trees.

    • To parse statements. This is useful if the plugin constructs a new statement string from the parse tree. The plugin can have the server parse the string to produce a new tree, then return that tree as the representation of the rewritten statement.

For more information about plugin services, see Section 27.3, “MySQL Services for Plugins”.

Preparse and postparse query rewrite plugins share these characteristics:

  • If a query rewrite plugin is installed, the --log-raw option affects statement logging as follows:

    • Without --log-raw, the server logs the statement returned by the query rewrite plugin. This may differ from the statement as received.

    • With --log-raw, the server logs the original statement as received.

  • If a plugin rewrites a statement, the server decides whether to write it to the binary log (and thus to any replication slaves) based on the rewritten statement, not the original statement. If a plugin rewrites only SELECT statements to SELECT statements, there is no impact on binary logging because the server does not write SELECT statements to the binary log.

  • If a plugin rewrites a statement, the server produces a Note message that the client can view using SHOW WARNINGS. Messages have this format, where stmt_in is the original statement and stmt_out is the rewritten statement:

    Query 'stmt_in' rewritten to 'stmt_out' by a query rewrite plugin
    

MySQL distributions include a postparse query rewrite plugin named Rewriter. This plugin is rule based. You can add rows to its rules table to cause SELECT statement rewriting. For more information, see Section 6.5.5, “The Rewriter Query Rewrite Plugin”.

Query rewrite plugins use the same API as audit plugins. For more information about audit plugins, see Section 27.2.4.8, “Writing Audit Plugins”.

Keyring Plugins

As of MySQL 5.7.11, MySQL Server supports keyring plugins that enable internal server components and plugins to securely store sensitive information for later retrieval.

All MySQL distributions include a keyring plugin named keyring_file. MySQL Enterprise Edition distributions include a keyring plugin named keyring_okv. See Section 7.5.4, “The MySQL Keyring”.

For more information about keyring plugins, see Section 27.2.4.12, “Writing Keyring Plugins”.

27.2.2 Plugin API Characteristics

The server plugin API has these characteristics:

  • All plugins have several things in common.

    Each plugin has a name that it can be referred to in SQL statements, as well as other metadata such as an author and a description that provide other information. This information can be examined in the INFORMATION_SCHEMA.PLUGINS table or using the SHOW PLUGINS statement.

  • The plugin framework is extendable to accommodate different kinds of plugins.

    Although some aspects of the plugin API are common to all types of plugins, the API also permits type-specific interface elements so that different types of plugins can be created. A plugin with one purpose can have an interface most appropriate to its own requirements and not the requirements of some other plugin type.

    Interfaces for several types of plugins exist, such as storage engines, full-text parser, and INFORMATION_SCHEMA tables. Others can be added.

  • Plugins can expose information to users.

    A plugin can implement system and status variables that are available through the SHOW VARIABLES and SHOW STATUS statements.

  • The plugin API includes versioning information.

    The version information included in the plugin API enables a plugin library and each plugin that it contains to be self-identifying with respect to the API version that was used to build the library. If the API changes over time, the version numbers will change, but a server can examine a given plugin library's version information to determine whether it supports the plugins in the library.

    There are two types of version numbers. The first is the version for the general plugin framework itself. Each plugin library includes this kind of version number. The second type of version applies to individual plugins. Each specific type of plugin has a version for its interface, so each plugin in a library has a type-specific version number. For example, a library containing a full-text parser plugin has a general plugin API version number, and the plugin has a version number specific to the full-text plugin interface.

  • The plugin API implements security restrictions.

    A plugin library must be installed in a specific dedicated directory for which the location is controlled by the server and cannot be changed at runtime. Also, the library must contain specific symbols that identify it as a plugin library. The server will not load something as a plugin if it was not built as a plugin.

  • Plugins have access to server services.

    The services interface exposes server functionality that plugins can access using ordinary function calls. For details, see Section 27.3, “MySQL Services for Plugins”.

In some respects, the server plugin API is similar to the older user-defined function (UDF) API that it supersedes, but the plugin API has several advantages over the older interface. For example, UDFs had no versioning information. Also, the newer plugin interface eliminates the security issues of the older UDF interface. The older interface for writing nonplugin UDFs permitted libraries to be loaded from any directory searched by the system's dynamic linker, and the symbols that identified the UDF library were relatively nonspecific.

The client plugin API has similar architectural characteristics, but client plugins have no direct access to the server the way server plugins do.

27.2.3 Plugin API Components

The server plugin implementation comprises several components.

SQL statements:

  • INSTALL PLUGIN registers a plugin in the mysql.plugin table and loads the plugin code.

  • UNINSTALL PLUGIN unregisters a plugin from the mysql.plugin table and unloads the plugin code.

  • The WITH PARSER clause for full-text index creation associates a full-text parser plugin with a given FULLTEXT index.

  • SHOW PLUGINS displays information about server plugins.

Command-line options and system variables:

  • The --plugin-load option enables plugins to be loaded at server startup time.

  • The plugin_dir system variable indicates the location of the directory where all plugins must be installed. The value of this variable can be specified at server startup with a --plugin_dir=dir_name option. mysql_config --plugindir displays the default plugin directory path name.

For additional information about plugin loading, see Section 6.5.2, “Installing and Uninstalling Plugins”.

Plugin-related tables:

  • The INFORMATION_SCHEMA.PLUGINS table contains plugin information.

  • The mysql.plugin table lists each plugin that was installed with INSTALL PLUGIN and is required for plugin use. For new MySQL installations, this table is created during the installation process.

The client plugin implementation is simpler:

  • For the mysql_options() C API function, the MYSQL_DEFAULT_AUTH and MYSQL_PLUGIN_DIR options enable client programs to load authentication plugins.

  • There are C API functions that enable management of client plugins.

To examine how MySQL implements plugins, consult the following source files in a MySQL source distribution:

  • In the include/mysql directory, plugin.h exposes the public plugin API. This file should be examined by anyone who wants to write a plugin library. plugin_xxx.h files provide additional information that pertains to specific types of plugins. client_plugin.h contains information specific to client plugins.

  • In the sql directory, sql_plugin.h and sql_plugin.cc comprise the internal plugin implementation. sql_acl.cc is where the server uses authentication plugins. These files need not be consulted by plugin developers. They may be of interest for those who want to know more about how the server handles plugins.

  • In the sql-common directory, client_plugin.h implements the C API client plugin functions, and client.c implements client authentication support. These files need not be consulted by plugin developers. They may be of interest for those who want to know more about how the server handles plugins.

27.2.4 Writing Plugins

To create a plugin library, you must provide the required descriptor information that indicates what plugins the library file contains, and write the interface functions for each plugin.

Every server plugin must have a general descriptor that provides information to the plugin API, and a type-specific descriptor that provides information about the plugin interface for a given type of plugin. The structure of the general descriptor is the same for all plugin types. The structure of the type-specific descriptor varies among plugin types and is determined by the requirements of what the plugin needs to do. The server plugin interface also enables plugins to expose status and system variables. These variables become visible through the SHOW STATUS and SHOW VARIABLES statements and the corresponding INFORMATION_SCHEMA tables.

For client-side plugins, the architecture is a bit different. Each plugin must have a descriptor, but there is no division into separate general and type-specific descriptors. Instead, the descriptor begins with a fixed set of members common to all client plugin types, and the common members are followed by any additional members required to implement the specific plugin type.

You can write plugins in C or C++ (or another language that can use C calling conventions). Plugins are loaded and unloaded dynamically, so your operating system must support dynamic loading and you must have compiled the calling application dynamically (not statically). For server plugins, this means that mysqld must be compiled dynamically.

A server plugin contains code that becomes part of the running server, so when you write the plugin, you are bound by any and all constraints that otherwise apply to writing server code. For example, you may have problems if you attempt to use functions from the libstdc++ library. These constraints may change in future versions of the server, so it is possible that server upgrades will require revisions to plugins originally written for older servers. For information about these constraints, see Section 2.9.4, “MySQL Source-Configuration Options”, and Section 2.9.5, “Dealing with Problems Compiling MySQL”.

Client plugin writers should avoid dependencies on what symbols the calling application has because you cannot be sure what applications will use the plugin.

27.2.4.1 Overview of Plugin Writing

The following procedure provides an overview of the steps needed to create a plugin library. The next sections provide additional details on setting plugin data structures and writing specific types of plugins.

  1. In the plugin source file, include the header files that the plugin library needs. The plugin.h file is required, and the library might require other files as well. For example:

    #include <stdlib.h>
    #include <ctype.h>
    #include <mysql/plugin.h>
    
  2. Set up the descriptor information for the plugin library file. For server plugins, write the library descriptor, which must contain the general plugin descriptor for each server plugin in the file. For more information, see Section 27.2.4.2.1, “Server Plugin Library and Plugin Descriptors”. In addition, set up the type-specific descriptor for each server plugin in the library. Each plugin's general descriptor points to its type-specific descriptor.

    For client plugins, write the client descriptor. For more information, see Section 27.2.4.2.3, “Client Plugin Descriptors”.

  3. Write the plugin interface functions for each plugin. For example, each plugin's general plugin descriptor points to the initialization and deinitialization functions that the server should invoke when it loads and unloads the plugin. The plugin's type-specific description may also point to interface functions.

  4. For server plugins, set up the status and system variables, if there are any.

  5. Compile the plugin library as a shared library and install it in the plugin directory. For more information, see Section 27.2.4.3, “Compiling and Installing Plugin Libraries”.

  6. For server plugins, register the plugin with the server. For more information, see Section 6.5.2, “Installing and Uninstalling Plugins”.

  7. Test the plugin to verify that it works properly.

27.2.4.2 Plugin Data Structures

A plugin library file includes descriptor information to indicate what plugins it contains.

If the plugin library contains any server plugins, it must include the following descriptor information:

  • A library descriptor indicates the general server plugin API version number used by the library and contains a general plugin descriptor for each server plugin in the library. To provide the framework for this descriptor, invoke two macros from the plugin.h header file:

    mysql_declare_plugin(name)
     ... one or more server plugin descriptors here ...
    mysql_declare_plugin_end;
    

    The macros expand to provide a declaration for the API version automatically. You must provide the plugin descriptors.

  • Within the library descriptor, each general server plugin is described by a st_mysql_plugin structure. This plugin descriptor structure contains information that is common to every type of server plugin: A value that indicates the plugin type; the plugin name, author, description, and license type; pointers to the initialization and deinitialization functions that the server invokes when it loads and unloads the plugin, and pointers to any status or system variables the plugin implements.

  • Each general server plugin descriptor within the library descriptor also contains a pointer to a type-specific plugin descriptor. The structure of the type-specific descriptors varies from one plugin type to another because each type of plugin can have its own API. A type-specific plugin descriptor contains a type-specific API version number and pointers to the functions that are needed to implement that plugin type. For example, a full-text parser plugin has initialization and deinitialization functions, and a main parsing function. The server invokes these functions when it uses the plugin to parse text.

The plugin library also contains the interface functions that are referenced by the general and type-specific descriptors for each plugin in the library.

If the plugin library contains a client plugin, it must include a descriptor for the plugin. The descriptor begins with a fixed set of members common to all client plugins, followed by any members specific to the plugin type. To provide the descriptor framework, invoke two macros from the client_plugin.h header file:

mysql_declare_client_plugin(plugin_type)
   ... members common to all client plugins ...
   ... type-specific extra members ...
mysql_end_client_plugin;

The plugin library also contains any interface functions referenced by the client descriptor.

The mysql_declare_plugin() and mysql_declare_client_plugin() macros differ somewhat in how they can be invoked, which has implications for the contents of plugin libraries. The following guidelines summarize the rules:

  • mysql_declare_plugin() and mysql_declare_client_plugin() can both be used in the same source file, which means that a plugin library can contain both server and client plugins. However, each of mysql_declare_plugin() and mysql_declare_client_plugin() can be used at most once.

  • mysql_declare_plugin() permits multiple server plugin declarations, so a plugin library can contain multiple server plugins.

  • mysql_declare_client_plugin() permits only a single client plugin declaration. To create multiple client plugins, separate plugin libraries must be used.

When a client program looks for a client plugin that is in a plugin library and not built into libmysqlclient, it looks for a file with a base name that is the same as the plugin name. For example, if a program needs to use a client authentication plugin named auth_xxx on a system that uses .so as the library suffix, it looks in the file named auth_xxx.so. (On OS X, the program looks first for auth_xxx.dylib, then for auth_xxx.so.) For this reason, if a plugin library contains a client plugin, the library must have the same base name as that plugin.

The same is not true for a library that contains server plugins. The --plugin-load option and the INSTALL PLUGIN statement provide the library file name explicitly, so there need be no explicit relationship between the library name and the name of any server plugins it contains.

27.2.4.2.1 Server Plugin Library and Plugin Descriptors

Every plugin library that contains server plugins must include a library descriptor that contains the general plugin descriptor for each server plugin in the file. This section discusses how to write the library and general descriptors for server plugins.

The library descriptor must define two symbols:

  • _mysql_plugin_interface_version_ specifies the version number of the general plugin framework. This is given by the MYSQL_PLUGIN_INTERFACE_VERSION symbol, which is defined in the plugin.h file.

  • _mysql_plugin_declarations_ defines an array of plugin declarations, terminated by a declaration with all members set to 0. Each declaration is an instance of the st_mysql_plugin structure (also defined in plugin.h). There must be one of these for each server plugin in the library.

If the server does not find those two symbols in a library, it does not accept it as a legal plugin library and rejects it with an error. This prevents use of a library for plugin purposes unless it was built specifically as a plugin library.

The conventional way to define the two required symbols is by using the mysql_declare_plugin() and mysql_declare_plugin_end macros from the plugin.h file:

mysql_declare_plugin(name)
 ... one or more server plugin descriptors here ...
mysql_declare_plugin_end;

Each server plugin must have a general descriptor that provides information to the server plugin API. The general descriptor has the same structure for all plugin types. The st_mysql_plugin structure in the plugin.h file defines this descriptor:

struct st_mysql_plugin
{
  int type;             /* the plugin type (a MYSQL_XXX_PLUGIN value)   */
  void *info;           /* pointer to type-specific plugin descriptor   */
  const char *name;     /* plugin name                                  */
  const char *author;   /* plugin author (for I_S.PLUGINS)              */
  const char *descr;    /* general descriptive text (for I_S.PLUGINS)   */
  int license;          /* the plugin license (PLUGIN_LICENSE_XXX)      */
  int (*init)(void *);  /* the function to invoke when plugin is loaded */
  int (*deinit)(void *);/* the function to invoke when plugin is unloaded */
  unsigned int version; /* plugin version (for I_S.PLUGINS)             */
  struct st_mysql_show_var *status_vars;
  struct st_mysql_sys_var **system_vars;
  void * __reserved1;   /* reserved for dependency checking             */
  unsigned long flags;  /* flags for plugin */
};

The st_mysql_plugin descriptor structure members are used as follows. char * members should be specified as null-terminated strings.

  • type: The plugin type. This must be one of the plugin-type values from plugin.h:

    /*
      The allowable types of plugins
    */
    #define MYSQL_UDF_PLUGIN             0  /* User-defined function        */
    #define MYSQL_STORAGE_ENGINE_PLUGIN  1  /* Storage Engine               */
    #define MYSQL_FTPARSER_PLUGIN        2  /* Full-text parser plugin      */
    #define MYSQL_DAEMON_PLUGIN          3  /* The daemon/raw plugin type */
    #define MYSQL_INFORMATION_SCHEMA_PLUGIN  4  /* The I_S plugin type */
    #define MYSQL_AUDIT_PLUGIN           5  /* The Audit plugin type        */
    #define MYSQL_REPLICATION_PLUGIN     6  /* The replication plugin type */
    #define MYSQL_AUTHENTICATION_PLUGIN  7  /* The authentication plugin type */
    ...
    

    For example, for a full-text parser plugin, the type value is MYSQL_FTPARSER_PLUGIN.

  • info: A pointer to the type-specific descriptor for the plugin. This descriptor's structure depends on the particular type of plugin, unlike that of the general plugin descriptor structure. For version-control purposes, the first member of the type-specific descriptor for every plugin type is expected to be the interface version for the type. This enables the server to check the type-specific version for every plugin no matter its type. Following the version number, the descriptor includes any other members needed, such as callback functions and other information needed by the server to invoke the plugin properly. Later sections on writing particular types of server plugins describe the structure of their type-specific descriptors.

  • name: A string that gives the plugin name. This is the name that will be listed in the mysql.plugin table and by which you refer to the plugin in SQL statements such as INSTALL PLUGIN and UNINSTALL PLUGIN, or with the --plugin-load option. The name is also visible in the INFORMATION_SCHEMA.PLUGINS table or the output from SHOW PLUGINS.

    The plugin name should not begin with the name of any server option. If it does, the server will fail to initialize it. For example, the server has a --socket option, so you should not use a plugin name such as socket, socket_plugin, and so forth.

  • author: A string naming the plugin author. This can be whatever you like.

  • desc: A string that provides a general description of the plugin. This can be whatever you like.

  • license: The plugin license type. The value can be one of PLUGIN_LICENSE_PROPRIETARY, PLUGIN_LICENSE_GPL, or PLUGIN_LICENSE_BSD.

  • init: A once-only initialization function, or NULL if there is no such function. The server executes this function when it loads the plugin, which happens for INSTALL PLUGIN or, for plugins listed in the mysql.plugin table, at server startup. The function takes one argument that points to the internal structure used to identify the plugin. It returns zero for success and nonzero for failure.

  • deinit: A once-only deinitialization function, or NULL if there is no such function. The server executes this function when it unloads the plugin, which happens for UNINSTALL PLUGIN or, for plugins listed in the mysql.plugin table, at server shutdown. The function takes one argument that points to the internal structure used to identify the plugin It returns zero for success and nonzero for failure.

  • version: The plugin version number. When the plugin is installed, this value can be retrieved from the INFORMATION_SCHEMA.PLUGINS table. The value includes major and minor numbers. If you write the value as a hex constant, the format is 0xMMNN, where MM and NN are the major and minor numbers, respectively. For example, 0x0302 represents version 3.2.

  • status_vars: A pointer to a structure for status variables associated with the plugin, or NULL if there are no such variables. When the plugin is installed, these variables are displayed in the output of the SHOW STATUS statement.

    The status_vars member, if not NULL, points to an array of st_mysql_show_var structures that describe status variables. See Section 27.2.4.2.2, “Server Plugin Status and System Variables”.

  • system_vars: A pointer to a structure for system variables associated with the plugin, or NULL if there are no such variables. These options and system variables can be used to help initialize variables within the plugin.

    The system_vars member, if not NULL, points to an array of st_mysql_sys_var structures that describe system variables. See Section 27.2.4.2.2, “Server Plugin Status and System Variables”.

  • __reserved1: A placeholder for the future. It should be set to NULL.

  • flags: Plugin flags. Individual bits correspond to different flags. The value should be set to the OR of the applicable flags. These flags are available:

    #define PLUGIN_OPT_NO_INSTALL   1UL   /* Not dynamically loadable */
    #define PLUGIN_OPT_NO_UNINSTALL 2UL   /* Not dynamically unloadable */
    

    PLUGIN_OPT_NO_INSTALL indicates that the plugin cannot be loaded at runtime with the INSTALL PLUGIN statement. This is appropriate for plugins that must be loaded at server startup with the --plugin-load option. PLUGIN_OPT_NO_UNINSTALL indicates that the plugin cannot be unloaded at runtime with the UNINSTALL PLUGIN statement.

The server invokes the init and deinit functions in the general plugin descriptor only when loading and unloading the plugin. They have nothing to do with use of the plugin such as happens when an SQL statement causes the plugin to be invoked.

For example, the descriptor information for a library that contains a single full-text parser plugin named simple_parser looks like this:

mysql_declare_plugin(ftexample)
{
  MYSQL_FTPARSER_PLUGIN,      /* type                            */
  &simple_parser_descriptor,  /* descriptor                      */
  "simple_parser",            /* name                            */
  "Oracle Corporation",       /* author                          */
  "Simple Full-Text Parser",  /* description                     */
  PLUGIN_LICENSE_GPL,         /* plugin license                  */
  simple_parser_plugin_init,  /* init function (when loaded)     */
  simple_parser_plugin_deinit,/* deinit function (when unloaded) */
  0x0001,                     /* version                         */
  simple_status,              /* status variables                */
  simple_system_variables,    /* system variables                */
  NULL,
  0
}
mysql_declare_plugin_end;

For a full-text parser plugin, the type must be MYSQL_FTPARSER_PLUGIN. This is the value that identifies the plugin as being legal for use in a WITH PARSER clause when creating a FULLTEXT index. (No other plugin type is legal for this clause.)

plugin.h defines the mysql_declare_plugin() and mysql_declare_plugin_end macros like this:

#ifndef MYSQL_DYNAMIC_PLUGIN
#define __MYSQL_DECLARE_PLUGIN(NAME, VERSION, PSIZE, DECLS) \
MYSQL_PLUGIN_EXPORT int VERSION= MYSQL_PLUGIN_INTERFACE_VERSION; \
MYSQL_PLUGIN_EXPORT int PSIZE= sizeof(struct st_mysql_plugin); \
MYSQL_PLUGIN_EXPORT struct st_mysql_plugin DECLS[]= {
#else
#define __MYSQL_DECLARE_PLUGIN(NAME, VERSION, PSIZE, DECLS) \
MYSQL_PLUGIN_EXPORT int _mysql_plugin_interface_version_= MYSQL_PLUGIN_INTERFACE_VERSION; \
MYSQL_PLUGIN_EXPORT int _mysql_sizeof_struct_st_plugin_= sizeof(struct st_mysql_plugin); \
MYSQL_PLUGIN_EXPORT struct st_mysql_plugin _mysql_plugin_declarations_[]= {
#endif

#define mysql_declare_plugin(NAME) \
__MYSQL_DECLARE_PLUGIN(NAME, \
                 builtin_ ## NAME ## _plugin_interface_version, \
                 builtin_ ## NAME ## _sizeof_struct_st_plugin, \
                 builtin_ ## NAME ## _plugin)

#define mysql_declare_plugin_end ,{0,0,0,0,0,0,0,0,0,0,0,0,0}}
Note

Those declarations define the _mysql_plugin_interface_version_ symbol only if the MYSQL_DYNAMIC_PLUGIN symbol is defined. This means that -DMYSQL_DYNAMIC_PLUGIN must be provided as part of the compilation command to build the plugin as a shared library.

When the macros are used as just shown, they expand to the following code, which defines both of the required symbols (_mysql_plugin_interface_version_ and _mysql_plugin_declarations_):

int _mysql_plugin_interface_version_= MYSQL_PLUGIN_INTERFACE_VERSION;
int _mysql_sizeof_struct_st_plugin_= sizeof(struct st_mysql_plugin);
struct st_mysql_plugin _mysql_plugin_declarations_[]= {
{
  MYSQL_FTPARSER_PLUGIN,      /* type                            */
  &simple_parser_descriptor,  /* descriptor                      */
  "simple_parser",            /* name                            */
  "Oracle Corporation",       /* author                          */
  "Simple Full-Text Parser",  /* description                     */
  PLUGIN_LICENSE_GPL,         /* plugin license                  */
  simple_parser_plugin_init,  /* init function (when loaded)     */
  simple_parser_plugin_deinit,/* deinit function (when unloaded) */
  0x0001,                     /* version                         */
  simple_status,              /* status variables                */
  simple_system_variables,    /* system variables                */
  NULL,
  0
}
  ,{0,0,0,0,0,0,0,0,0,0,0,0}}
};

The preceding example declares a single plugin in the general descriptor, but it is possible to declare multiple plugins. List the declarations one after the other between mysql_declare_plugin() and mysql_declare_plugin_end, separated by commas.

MySQL server plugins can be written in C or C++ (or another language that can use C calling conventions). If you write a C++ plugin, one C++ feature that you should not use is nonconstant variables to initialize global structures. Members of structures such as the st_mysql_plugin structure should be initialized only with constant variables. The simple_parser descriptor shown earlier is permissible in a C++ plugin because it satisfies that requirement:

mysql_declare_plugin(ftexample)
{
  MYSQL_FTPARSER_PLUGIN,      /* type                            */
  &simple_parser_descriptor,  /* descriptor                      */
  "simple_parser",            /* name                            */
  "Oracle Corporation",       /* author                          */
  "Simple Full-Text Parser",  /* description                     */
  PLUGIN_LICENSE_GPL,         /* plugin license                  */
  simple_parser_plugin_init,  /* init function (when loaded)     */
  simple_parser_plugin_deinit,/* deinit function (when unloaded) */
  0x0001,                     /* version                         */
  simple_status,              /* status variables                */
  simple_system_variables,    /* system variables                */
  NULL,
  0
}
mysql_declare_plugin_end;

Here is another valid way to write the general descriptor. It uses constant variables to indicate the plugin name, author, and description:

const char *simple_parser_name = "simple_parser";
const char *simple_parser_author = "Oracle Corporation";
const char *simple_parser_description = "Simple Full-Text Parser";

mysql_declare_plugin(ftexample)
{
  MYSQL_FTPARSER_PLUGIN,      /* type                            */
  &simple_parser_descriptor,  /* descriptor                      */
  simple_parser_name,         /* name                            */
  simple_parser_author,       /* author                          */
  simple_parser_description,  /* description                     */
  PLUGIN_LICENSE_GPL,         /* plugin license                  */
  simple_parser_plugin_init,  /* init function (when loaded)     */
  simple_parser_plugin_deinit,/* deinit function (when unloaded) */
  0x0001,                     /* version                         */
  simple_status,              /* status variables                */
  simple_system_variables,    /* system variables                */
  NULL,
  0
}
mysql_declare_plugin_end;

However, the following general descriptor is invalid. It uses structure members to indicate the plugin name, author, and description, but structures are not considered constant initializers in C++:

typedef struct
{
  const char *name;
  const char *author;
  const char *description;
} plugin_info;

plugin_info parser_info = {
  "simple_parser",
  "Oracle Corporation",
  "Simple Full-Text Parser"
};

mysql_declare_plugin(ftexample)
{
  MYSQL_FTPARSER_PLUGIN,      /* type                            */
  &simple_parser_descriptor,  /* descriptor                      */
  parser_info.name,           /* name                            */
  parser_info.author,         /* author                          */
  parser_info.description,    /* description                     */
  PLUGIN_LICENSE_GPL,         /* plugin license                  */
  simple_parser_plugin_init,  /* init function (when loaded)     */
  simple_parser_plugin_deinit,/* deinit function (when unloaded) */
  0x0001,                     /* version                         */
  simple_status,              /* status variables                */
  simple_system_variables,    /* system variables                */
  NULL,
  0
}
mysql_declare_plugin_end;
27.2.4.2.2 Server Plugin Status and System Variables

The server plugin interface enables plugins to expose status and system variables using the status_vars and system_vars members of the general plugin descriptor.

The status_vars member of the general plugin descriptor, if not 0, points to an array of st_mysql_show_var structures, each of which describes one status variable, followed by a structure with all members set to 0. The st_mysql_show_var structure has this definition:

struct st_mysql_show_var {
  const char *name;
  char *value;
  enum enum_mysql_show_type type;
};

When the plugin is installed, the plugin name and the name value are joined with an underscore to form the name displayed by SHOW STATUS.

The following table shows the permissible status variable type values and what the corresponding variable should be.

Table 27.1 Server Plugin Status Variable Types

Variable TypeMeaning
SHOW_BOOLPointer to a boolean variable
SHOW_INTPointer to an integer variable
SHOW_LONGPointer to a long integer variable
SHOW_LONGLONGPointer to a longlong integer variable
SHOW_CHARA string
SHOW_CHAR_PTRPointer to a string
SHOW_ARRAYPointer to another st_mysql_show_var array
SHOW_FUNCPointer to a function
SHOW_DOUBLEPointer to a double

For the SHOW_FUNC type, the function is called and fills in its out parameter, which then provides information about the variable to be displayed. The function has this signature:

#define SHOW_VAR_FUNC_BUFF_SIZE 1024

typedef int (*mysql_show_var_func) (void *thd,
                                    struct st_mysql_show_var *out,
                                    char *buf);

The system_vars member, if not 0, points to an array of st_mysql_sys_var structures, each of which describes one system variable (which can also be set from the command-line or configuration file), followed by a structure with all members set to 0. The st_mysql_sys_var structure is defined as follows:

struct st_mysql_sys_var {
 int flags;
 const char *name, *comment;
 int (*check)(THD*, struct st_mysql_sys_var *, void*, st_mysql_value*);
 void (*update)(THD*, struct st_mysql_sys_var *, void*, const void*);
};

Additional fields are append as required depending upon the flags.

For convenience, a number of macros are defined that make creating new system variables within a plugin much simpler.

Throughout the macros, the following fields are available:

  • name: An unquoted identifier for the system variable.

  • varname: The identifier for the static variable. Where not available, it is the same as the name field.

  • opt: Additional use flags for the system variable. The following table shows the permissible flags.

    Table 27.2 Server Plugin System Variable Flags

    Flag ValueDescription
    PLUGIN_VAR_READONLYThe system variable is read only
    PLUGIN_VAR_NOSYSVARThe system variable is not user visible at runtime
    PLUGIN_VAR_NOCMDOPTThe system variable is not configurable from the command line
    PLUGIN_VAR_NOCMDARGNo argument is required at the command line (typically used for boolean variables)
    PLUGIN_VAR_RQCMDARGAn argument is required at the command line (this is the default)
    PLUGIN_VAR_OPCMDARGAn argument is optional at the command line
    PLUGIN_VAR_MEMALLOCUsed for string variables; indicates that memory is to be allocated for storage of the string

  • comment: A descriptive comment to be displayed in the server help message. NULL if this variable is to be hidden.

  • check: The check function, NULL for default.

  • update: The update function, NULL for default.

  • default: The variable default value.

  • minimum: The variable minimum value.

  • maximum: The variable maximum value.

  • blocksize: The variable block size. When the value is set, it is rounded to the nearest multiple of blocksize.

A system variable may be accessed either by using the static variable directly or by using the SYSVAR()accessor macro. The SYSVAR() macro is provided for completeness. Usually it should be used only when the code cannot directly access the underlying variable.

For example:

static int my_foo;
static MYSQL_SYSVAR_INT(foo_var, my_foo,
                        PLUGIN_VAR_RQCMDARG, "foo comment",
                        NULL, NULL, 0, 0, INT_MAX, 0);
 ...
   SYSVAR(foo_var)= value;
   value= SYSVAR(foo_var);
   my_foo= value;
   value= my_foo;

Session variables may be accessed only through the THDVAR() accessor macro. For example:

static MYSQL_THDVAR_BOOL(some_flag,
                         PLUGIN_VAR_NOCMDARG, "flag comment",
                         NULL, NULL, FALSE);
 ...
   if (THDVAR(thd, some_flag))
   {
     do_something();
     THDVAR(thd, some_flag)= FALSE;
   }

All global and session system variables must be published to mysqld before use. This is done by constructing a NULL-terminated array of the variables and linking to it in the plugin public interface. For example:

static struct st_mysql_sys_var *my_plugin_vars[]= {
  MYSQL_SYSVAR(foo_var),
  MYSQL_SYSVAR(some_flag),
  NULL
};
mysql_declare_plugin(fooplug)
{
  MYSQL_..._PLUGIN,
  &plugin_data,
  "fooplug",
  "foo author",
  "This does foo!",
  PLUGIN_LICENSE_GPL,
  foo_init,
  foo_fini,
  0x0001,
  NULL,
  my_plugin_vars,
  NULL,
  0
}
mysql_declare_plugin_end;

The following convenience macros enable you to declare different types of system variables:

  • Boolean system variables of type my_bool, which is a 1-byte boolean. (0 = FALSE, 1 = TRUE)

    MYSQL_THDVAR_BOOL(name, opt, comment, check, update, default)
    MYSQL_SYSVAR_BOOL(name, varname, opt, comment, check, update, default)
    
  • String system variables of type char*, which is a pointer to a null-terminated string.

    MYSQL_THDVAR_STR(name, opt, comment, check, update, default)
    MYSQL_SYSVAR_STR(name, varname, opt, comment, check, update, default)
    
  • Integer system variables, of which there are several varieties.

    • An int system variable, which is typically a 4-byte signed word.

      MYSQL_THDVAR_INT(name, opt, comment, check, update, default, min, max, blk)
      MYSQL_SYSVAR_INT(name, varname, opt, comment, check, update, default,
                     minimum, maximum, blocksize)
      
    • An unsigned int system variable, which is typically a 4-byte unsigned word.

      MYSQL_THDVAR_UINT(name, opt, comment, check, update, default, min, max, blk)
      MYSQL_SYSVAR_UINT(name, varname, opt, comment, check, update, default,
                      minimum, maximum, blocksize)
      
    • A long system variable, which is typically either a 4- or 8-byte signed word.

      MYSQL_THDVAR_LONG(name, opt, comment, check, update, default, min, max, blk)
      MYSQL_SYSVAR_LONG(name, varname, opt, comment, check, update, default,
                      minimum, maximum, blocksize)
      
    • An unsigned long system variable, which is typically either a 4- or 8-byte unsigned word.

      MYSQL_THDVAR_ULONG(name, opt, comment, check, update, default, min, max, blk)
      MYSQL_SYSVAR_ULONG(name, varname, opt, comment, check, update, default,
                       minimum, maximum, blocksize)
      
    • A long long system variable, which is typically an 8-byte signed word.

      MYSQL_THDVAR_LONGLONG(name, opt, comment, check, update,
                          default, minimum, maximum, blocksize)
      MYSQL_SYSVAR_LONGLONG(name, varname, opt, comment, check, update,
                          default, minimum, maximum, blocksize)
      
    • An unsigned long long system variable, which is typically an 8-byte unsigned word.

      MYSQL_THDVAR_ULONGLONG(name, opt, comment, check, update,
                           default, minimum, maximum, blocksize)
      MYSQL_SYSVAR_ULONGLONG(name, varname, opt, comment, check, update,
                           default, minimum, maximum, blocksize)
      
    • A double system variable, which is typically an 8-byte signed word. These accessor macros were added in MySQL 5.7.2.

      MYSQL_THDVAR_DOUBLE(name, opt, comment, check, update,
                           default, minimum, maximum, blocksize)
      MYSQL_SYSVAR_DOUBLE(name, varname, opt, comment, check, update,
                           default, minimum, maximum, blocksize)
      
    • An unsigned long system variable, which is typically either a 4- or 8-byte unsigned word. The range of possible values is an ordinal of the number of elements in the typelib, starting from 0.

      MYSQL_THDVAR_ENUM(name, opt, comment, check, update, default, typelib)
      MYSQL_SYSVAR_ENUM(name, varname, opt, comment, check, update,
                      default, typelib)
      
    • An unsigned long long system variable, which is typically an 8-byte unsigned word. Each bit represents an element in the typelib.

      MYSQL_THDVAR_SET(name, opt, comment, check, update, default, typelib)
      MYSQL_SYSVAR_SET(name, varname, opt, comment, check, update,
                     default, typelib)
      

Internally, all mutable and plugin system variables are stored in a HASH structure.

Display of the server command-line help text is handled by compiling a DYNAMIC_ARRAY of all variables relevant to command-line options, sorting them, and then iterating through them to display each option.

When a command-line option has been handled, it is then removed from the argv by the handle_option() function (my_getopt.c); in effect, it is consumed.

The server processes command-line options during the plugin installation process, immediately after the plugin has been successfully loaded but before the plugin initialization function has been called

Plugins loaded at runtime do not benefit from any configuration options and must have usable defaults. Once they are installed, they are loaded at mysqld initialization time and configuration options can be set at the command line or within my.cnf.

Plugins should consider the thd parameter to be read only.

27.2.4.2.3 Client Plugin Descriptors

Each client plugin must have a descriptor that provides information to the client plugin API. The descriptor structure begins with a fixed set of members common to all client plugins, followed by any members specific to the plugin type.

The st_mysql_client_plugin structure in the client_plugin.h file defines a generic descriptor that contains the common members:

struct st_mysql_client_plugin
{
  int type;
  unsigned int interface_version;
  const char *name;
  const char *author;
  const char *desc;
  unsigned int version[3];
  const char *license;
  void *mysql_api;
  int (*init)(char *, size_t, int, va_list);
  int (*deinit)();
  int (*options)(const char *option, const void *);
};

The common st_mysql_client_plugin descriptor structure members are used as follows. char * members should be specified as null-terminated strings.

  • type: The plugin type. This must be one of the plugin-type values from client_plugin.h, such as MYSQL_CLIENT_AUTHENTICATION_PLUGIN.

  • interface_version: The plugin interface version. For example, this is MYSQL_CLIENT_AUTHENTICATION_PLUGIN_INTERFACE_VERSION for an authentication plugin.

  • name: A string that gives the plugin name. This is the name by which you refer to the plugin when you call mysql_options() with the MYSQL_DEFAULT_AUTH option or specify the --default-auth option to a MySQL client program.

  • author: A string naming the plugin author. This can be whatever you like.

  • desc: A string that provides a general description of the plugin. This can be whatever you like.

  • version: The plugin version as an array of three integers indicating the major, minor, and teeny versions. For example, {1,2,3} indicates version 1.2.3.

  • license: A string that specifies the license type.

  • mysql_api: For internal use. Specify it as NULL in the plugin descriptor.

  • init: A once-only initialization function, or NULL if there is no such function. The client library executes this function when it loads the plugin. The function returns zero for success and nonzero for failure.

    The init function uses its first two arguments to return an error message if an error occurs. The first argument is a pointer to a char buffer, and the second argument indicates the buffer length. Any message returned by the init function must be null-terminated, so the maximum message length is the buffer length minus one. The next arguments are passed to mysql_load_plugin(). The first indicates how many more arguments there are (0 if none), followed by any remaining arguments.

  • deinit: A once-only deinitialization function, or NULL if there is no such function. The client library executes this function when it unloads the plugin. The function takes no arguments. It returns zero for success and nonzero for failure.

  • options: A function for handling options passed to the plugin, or NULL if there is no such function. The function takes two arguments representing the option name and a pointer to its value. The function returns zero for success and nonzero for failure.

For a given client plugin type, the common descriptor members may be followed by additional members necessary to implement plugins of that type. For example, the st_mysql_client_plugin_AUTHENTICATION structure for authentication plugins has a function at the end that the client library calls to perform authentication.

To declare a plugin, use the mysql_declare_client_plugin() and mysql_end_client_plugin macros:

mysql_declare_client_plugin(plugin_type)
   ... members common to all client plugins ...
   ... type-specific extra members ...
mysql_end_client_plugin;

Do not specify the type or interface_version member explicitly. The mysql_declare_client_plugin() macro uses the plugin_type argument to generate their values automatically. For example, declare an authentication client plugin like this:

mysql_declare_client_plugin(AUTHENTICATION)
  "my_auth_plugin",
  "Author Name",
  "My Client Authentication Plugin",
  {1,0,0},
  "GPL",
  NULL,
  my_auth_init,
  my_auth_deinit,
  my_auth_options,
  my_auth_main
mysql_end_client_plugin;

This declaration uses the AUTHENTICATION argument to set the type and interface_version members to MYSQL_CLIENT_AUTHENTICATION_PLUGIN and MYSQL_CLIENT_AUTHENTICATION_PLUGIN_INTERFACE_VERSION.

Depending on the plugin type, the descriptor may have other members following the common members. For example, for an authentication plugin, there is a function (my_auth_main() in the descriptor just shown) that handles communication with the server. See Section 27.2.4.9, “Writing Authentication Plugins”.

Normally, a client program that supports the use of authentication plugins causes a plugin to be loaded by calling mysql_options() to set the MYSQL_DEFAULT_AUTH and MYSQL_PLUGIN_DIR options:

char *plugin_dir = "path_to_plugin_dir";
char *default_auth = "plugin_name";

/* ... process command-line options ... */

mysql_options(&mysql, MYSQL_PLUGIN_DIR, plugin_dir);
mysql_options(&mysql, MYSQL_DEFAULT_AUTH, default_auth);

Typically, the program will also accept --plugin-dir and --default-auth options that enable users to override the default values.

Should a client program require lower-level plugin management, the client library contains functions that take an st_mysql_client_plugin argument. See Section 26.8.14, “C API Client Plugin Functions”.

27.2.4.3 Compiling and Installing Plugin Libraries

After your plugin is written, you must compile it and install it. The procedure for compiling shared objects varies from system to system. If you build your library using CMake, it should be able to generate the correct compilation commands for your system. If the library is named somepluglib, you should end up with a shared library file that has a name something like somepluglib.so. (The .so file name suffix might differ on your system.)

To use CMake, you'll need to set up the configuration files to enable the plugin to be compiled and installed. Use the plugin examples under the plugin directory of a MySQL source distribution as a guide.

Create CMakeLists.txt, which should look something like this:

MYSQL_ADD_PLUGIN(somepluglib somepluglib.c
  MODULE_ONLY MODULE_OUTPUT_NAME "somepluglib")

When CMake generates the Makefile, it should take care of passing to the compilation command the -DMYSQL_DYNAMIC_PLUGIN flag, and passing to the linker the -lmysqlservices flag, which is needed to link in any functions from services provided through the plugin services interface. See Section 27.3, “MySQL Services for Plugins”.

Run CMake, then run make:

shell> cmake .
shell> make

If you need to specify configuration options to CMake, see Section 2.9.4, “MySQL Source-Configuration Options”, for a list. For example, you might want to specify CMAKE_INSTALL_PREFIX to indicate the MySQL base directory under which the plugin should be installed. You can see what value to use for this option with SHOW VARIABLES:

mysql> SHOW VARIABLES LIKE 'basedir';
+---------------+------------------+
| Variable_name | Value            |
+---------------+------------------+
| base          | /usr/local/mysql |
+---------------+------------------+

The location of the plugin directory where you should install the library is given by the plugin_dir system variable. For example:

mysql> SHOW VARIABLES LIKE 'plugin_dir';
+---------------+-----------------------------------+
| Variable_name | Value                             |
+---------------+-----------------------------------+
| plugin_dir    | /usr/local/mysql/lib/mysql/plugin |
+---------------+-----------------------------------+

To install the plugin library, use make:

shell> make install

Verify that make install installed the plugin library in the proper directory. After installing it, make sure that the library permissions permit it to be executed by the server.

27.2.4.4 Writing Full-Text Parser Plugins

MySQL supports server-side full-text parser plugins with MyISAM and InnoDB. For introductory information about full-text parser plugins, see Full-Text Parser Plugins.

A full-text parser plugin can be used to replace or modify the built-in full-text parser. This section describes how to write a full-text parser plugin named simple_parser. This plugin performs parsing based on simpler rules than those used by the MySQL built-in full-text parser: Words are nonempty runs of whitespace characters.

The instructions use the source code in the plugin/fulltext directory of MySQL source distributions, so change location into that directory. The following procedure describes how the plugin library is created:

  1. To write a full-text parser plugin, include the following header file in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

    #include <mysql/plugin.h>
    

    plugin.h defines the MYSQL_FTPARSER_PLUGIN server plugin type and the data structures needed to declare the plugin.

  2. Set up the library descriptor for the plugin library file.

    This descriptor contains the general plugin descriptor for the server plugin. For a full-text parser plugin, the type must be MYSQL_FTPARSER_PLUGIN. This is the value that identifies the plugin as being legal for use in a WITH PARSER clause when creating a FULLTEXT index. (No other plugin type is legal for this clause.)

    For example, the library descriptor for a library that contains a single full-text parser plugin named simple_parser looks like this:

    mysql_declare_plugin(ftexample)
    {
      MYSQL_FTPARSER_PLUGIN,      /* type                            */
      &simple_parser_descriptor,  /* descriptor                      */
      "simple_parser",            /* name                            */
      "Oracle Corporation",       /* author                          */
      "Simple Full-Text Parser",  /* description                     */
      PLUGIN_LICENSE_GPL,         /* plugin license                  */
      simple_parser_plugin_init,  /* init function (when loaded)     */
      simple_parser_plugin_deinit,/* deinit function (when unloaded) */
      0x0001,                     /* version                         */
      simple_status,              /* status variables                */
      simple_system_variables,    /* system variables                */
      NULL,
      0
    }
    mysql_declare_plugin_end;
    

    The name member (simple_parser) indicates the name to use for references to the plugin in statements such as INSTALL PLUGIN or UNINSTALL PLUGIN. This is also the name displayed by SHOW PLUGINS or INFORMATION_SCHEMA.PLUGINS.

    For more information, see Section 27.2.4.2.1, “Server Plugin Library and Plugin Descriptors”.

  3. Set up the type-specific plugin descriptor.

    Each general plugin descriptor in the library descriptor points to a type-specific descriptor. For a full-text parser plugin, the type-specific descriptor is an instance of the st_mysql_ftparser structure in the plugin.h file:

    struct st_mysql_ftparser
    {
      int interface_version;
      int (*parse)(MYSQL_FTPARSER_PARAM *param);
      int (*init)(MYSQL_FTPARSER_PARAM *param);
      int (*deinit)(MYSQL_FTPARSER_PARAM *param);
    };
    

    As shown by the structure definition, the descriptor has an interface version number and contains pointers to three functions.

    The interface version number is specified using a symbol, which is in the form: MYSQL_xxx_INTERFACE_VERSION. For full-text parser plugins, the symbol is MYSQL_FTPARSER_INTERFACE_VERSION. In the source code, you will find the actual interface version number for the full-text parser plugin defined in include/mysql/plugin_ftparser.h. With the introduction of full-text parser plugin support for InnoDB, the interface version number was incremented in MySQL 5.7 from 0x0100 to 0x0101.

    The init and deinit members should point to a function or be set to 0 if the function is not needed. The parse member must point to the function that performs the parsing.

    In the simple_parser declaration, that descriptor is indicated by &simple_parser_descriptor. The descriptor specifies the version number for the full-text plugin interface (as given by MYSQL_FTPARSER_INTERFACE_VERSION), and the plugin's parsing, initialization, and deinitialization functions:

    static struct st_mysql_ftparser simple_parser_descriptor=
    {
      MYSQL_FTPARSER_INTERFACE_VERSION, /* interface version      */
      simple_parser_parse,              /* parsing function       */
      simple_parser_init,               /* parser init function   */
      simple_parser_deinit              /* parser deinit function */
    };
    

    A full-text parser plugin is used in two different contexts, indexing and searching. In both contexts, the server calls the initialization and deinitialization functions at the beginning and end of processing each SQL statement that causes the plugin to be invoked. However, during statement processing, the server calls the main parsing function in context-specific fashion:

    • For indexing, the server calls the parser for each column value to be indexed.

    • For searching, the server calls the parser to parse the search string. The parser might also be called for rows processed by the statement. In natural language mode, there is no need for the server to call the parser. For boolean mode phrase searches or natural language searches with query expansion, the parser is used to parse column values for information that is not in the index. Also, if a boolean mode search is done for a column that has no FULLTEXT index, the built-in parser will be called. (Plugins are associated with specific indexes. If there is no index, no plugin is used.)

    The plugin declaration in the general plugin descriptor has init and deinit members that point initialization and deinitialization functions, and so does the type-specific plugin descriptor to which it points. However, these pairs of functions have different purposes and are invoked for different reasons:

    • For the plugin declaration in the general plugin descriptor, the initialization and deinitialization functions are invoked when the plugin is loaded and unloaded.

    • For the type-specific plugin descriptor, the initialization and deinitialization functions are invoked per SQL statement for which the plugin is used.

    Each interface function named in the plugin descriptor should return zero for success or nonzero for failure, and each of them receives an argument that points to a MYSQL_FTPARSER_PARAM structure containing the parsing context. The structure has this definition:

    typedef struct st_mysql_ftparser_param
    {
      int (*mysql_parse)(struct st_mysql_ftparser_param *,
                         char *doc, int doc_len);
      int (*mysql_add_word)(struct st_mysql_ftparser_param *,
                            char *word, int word_len,
                            MYSQL_FTPARSER_BOOLEAN_INFO *boolean_info);
      void *ftparser_state;
      void *mysql_ftparam;
      struct charset_info_st *cs;
      char *doc;
      int length;
      int flags;
      enum enum_ftparser_mode mode;
    } MYSQL_FTPARSER_PARAM;
    

    The structure members are used as follows:

    • mysql_parse: A pointer to a callback function that invokes the server's built-in parser. Use this callback when the plugin acts as a front end to the built-in parser. That is, when the plugin parsing function is called, it should process the input to extract the text and pass the text to the mysql_parse callback.

      The first parameter for this callback function should be the param value itself:

      param->mysql_parse(param, ...);
      

      A front end plugin can extract text and pass it all at once to the built-in parser, or it can extract and pass text to the built-in parser a piece at a time. However, in this case, the built-in parser treats the pieces of text as though there are implicit word breaks between them.

    • mysql_add_word: A pointer to a callback function that adds a word to a full-text index or to the list of search terms. Use this callback when the parser plugin replaces the built-in parser. That is, when the plugin parsing function is called, it should parse the input into words and invoke the mysql_add_word callback for each word.

      The first parameter for this callback function should be the param value itself:

      param->mysql_add_word(param, ...);
      
    • ftparser_state: This is a generic pointer. The plugin can set it to point to information to be used internally for its own purposes.

    • mysql_ftparam: This is set by the server. It is passed as the first argument to the mysql_parse or mysql_add_word callback.

    • cs: A pointer to information about the character set of the text, or 0 if no information is available.

    • doc: A pointer to the text to be parsed.

    • length: The length of the text to be parsed, in bytes.

    • flags: Parser flags. This is zero if there are no special flags. The only nonzero flag is MYSQL_FTFLAGS_NEED_COPY, which means that mysql_add_word() must save a copy of the word (that is, it cannot use a pointer to the word because the word is in a buffer that will be overwritten.)

      This flag might be set or reset by MySQL before calling the parser plugin, by the parser plugin itself, or by the mysql_parse() function.

    • mode: The parsing mode. This value will be one of the following constants:

      • MYSQL_FTPARSER_SIMPLE_MODE: Parse in fast and simple mode, which is used for indexing and for natural language queries. The parser should pass to the server only those words that should be indexed. If the parser uses length limits or a stopword list to determine which words to ignore, it should not pass such words to the server.

      • MYSQL_FTPARSER_WITH_STOPWORDS: Parse in stopword mode. This is used in boolean searches for phrase matching. The parser should pass all words to the server, even stopwords or words that are outside any normal length limits.

      • MYSQL_FTPARSER_FULL_BOOLEAN_INFO: Parse in boolean mode. This is used for parsing boolean query strings. The parser should recognize not only words but also boolean-mode operators and pass them to the server as tokens using the mysql_add_word callback. To tell the server what kind of token is being passed, the plugin needs to fill in a MYSQL_FTPARSER_BOOLEAN_INFO structure and pass a pointer to it.

    Note

    For MyISAM, the stopword list and ft_min_word_len and ft_max_word_len are checked inside the tokenizer. For InnoDB, the stopword list and equivalent word length variable settings (innodb_ft_min_token_size and innodb_ft_max_token_size) are checked outside of the tokenizer. As a result, InnoDB plugin parsers do not need to check the stopword list, innodb_ft_min_token_size, or innodb_ft_max_token_size. Instead, it is recommended that all words be returned to InnoDB. However, if you want to check stopwords within your plugin parser, use MYSQL_FTPARSER_SIMPLE_MODE, which is for full-text search index and natural language search. For MYSQL_FTPARSER_WITH_STOPWORDS and MYSQL_FTPARSER_FULL_BOOLEAN_INFO modes, it is recommended that all words be returned to InnoDB including stopwords, in case of phrase searches.

    If the parser is called in boolean mode, the param->mode value will be MYSQL_FTPARSER_FULL_BOOLEAN_INFO. The MYSQL_FTPARSER_BOOLEAN_INFO structure that the parser uses for passing token information to the server looks like this:

    typedef struct st_mysql_ftparser_boolean_info
    {
      enum enum_ft_token_type type;
      int yesno;
      int weight_adjust;
      char wasign;
      char trunc;
      int position;
      /* These are parser state and must be removed. */
      char prev;
      char *quot;
    } MYSQL_FTPARSER_BOOLEAN_INFO;
    

    The parser should fill in the structure members as follows:

    • type: The token type. The following table shows the permissible types.

      Table 27.3 Full-Text Parser Token Types

      Token ValueMeaning
      FT_TOKEN_EOFEnd of data
      FT_TOKEN_WORDA regular word
      FT_TOKEN_LEFT_PARENThe beginning of a group or subexpression
      FT_TOKEN_RIGHT_PARENThe end of a group or subexpression
      FT_TOKEN_STOPWORDA stopword

    • yesno: Whether the word must be present for a match to occur. 0 means that the word is optional but increases the match relevance if it is present. Values larger than 0 mean that the word must be present. Values smaller than 0 mean that the word must not be present.

    • weight_adjust: A weighting factor that determines how much a match for the word counts. It can be used to increase or decrease the word's importance in relevance calculations. A value of zero indicates no weight adjustment. Values greater than or less than zero mean higher or lower weight, respectively. The examples at Section 13.9.2, “Boolean Full-Text Searches”, that use the < and > operators illustrate how weighting works.

    • wasign: The sign of the weighting factor. A negative value acts like the ~ boolean-search operator, which causes the word's contribution to the relevance to be negative.

    • trunc: Whether matching should be done as if the boolean-mode * truncation operator had been given.

    • position: Start position of the word in the document, in bytes. Used by InnoDB full-text search. For existing plugins that are called in boolean mode, support must be added for the position member.

    Plugins should not use the prev and quot members of the MYSQL_FTPARSER_BOOLEAN_INFO structure.

    Note

    The plugin parser framework does not support:

    • The @distance boolean operator.

    • A leading plus sign (+) or minus sign (-) boolean operator followed by a space and then a word ('+ apple' or '- apple'). The leading plus or minus sign must be directly adjacent to the word, for example: '+apple' or '-apple'.

    For information about boolean full-text search operators, see Section 13.9.2, “Boolean Full-Text Searches”.

  4. Set up the plugin interface functions.

    The general plugin descriptor in the library descriptor names the initialization and deinitialization functions that the server should invoke when it loads and unloads the plugin. For simple_parser, these functions do nothing but return zero to indicate that they succeeded:

    static int simple_parser_plugin_init(void *arg __attribute__((unused)))
    {
      return(0);
    }
    
    static int simple_parser_plugin_deinit(void *arg __attribute__((unused)))
    {
      return(0);
    }
    

    Because those functions do not actually do anything, you could omit them and specify 0 for each of them in the plugin declaration.

    The type-specific plugin descriptor for simple_parser names the initialization, deinitialization, and parsing functions that the server invokes when the plugin is used. For simple_parser, the initialization and deinitialization functions do nothing:

    static int simple_parser_init(MYSQL_FTPARSER_PARAM *param
                                  __attribute__((unused)))
    {
      return(0);
    }
    
    static int simple_parser_deinit(MYSQL_FTPARSER_PARAM *param
                                    __attribute__((unused)))
    {
      return(0);
    }
    

    Here too, because those functions do nothing, you could omit them and specify 0 for each of them in the plugin descriptor.

    The main parsing function, simple_parser_parse(), acts as a replacement for the built-in full-text parser, so it needs to split text into words and pass each word to the server. The parsing function's first argument is a pointer to a structure that contains the parsing context. This structure has a doc member that points to the text to be parsed, and a length member that indicates how long the text is. The simple parsing done by the plugin considers nonempty runs of whitespace characters to be words, so it identifies words like this:

    static int simple_parser_parse(MYSQL_FTPARSER_PARAM *param)
    {
      char *end, *start, *docend= param->doc + param->length;
    
      for (end= start= param->doc;; end++)
      {
        if (end == docend)
        {
          if (end > start)
            add_word(param, start, end - start);
          break;
        }
        else if (isspace(*end))
        {
          if (end > start)
            add_word(param, start, end - start);
          start= end + 1;
        }
      }
      return(0);
    }
    

    As the parser finds each word, it invokes a function add_word() to pass the word to the server. add_word() is a helper function only; it is not part of the plugin interface. The parser passes the parsing context pointer to add_word(), as well as a pointer to the word and a length value:

    static void add_word(MYSQL_FTPARSER_PARAM *param, char *word, size_t len)
    {
      MYSQL_FTPARSER_BOOLEAN_INFO bool_info=
        { FT_TOKEN_WORD, 0, 0, 0, 0, 0, ' ', 0 };
    
      param->mysql_add_word(param, word, len, &bool_info);
    }
    
    Note

    Prior to MySQL 5.7.3, the sixth MYSQL_FTPARSER_BOOLEAN_INFO member (just before the ' ' member) is not present.

    For boolean-mode parsing, add_word() fills in the members of the bool_info structure as described earlier in the discussion of the st_mysql_ftparser_boolean_info structure.

  5. Set up the status variables. For the simple_parser plugin, the following status variable array sets up one status variable with a value that is static text, and another with a value that is stored in a long integer variable:

    long number_of_calls= 0;
    
    struct st_mysql_show_var simple_status[]=
    {
      {"static", (char *)"just a static text", SHOW_CHAR},
      {"called", (char *)&number_of_calls,     SHOW_LONG},
      {0,0,0}
    };
    

    When the plugin is installed, the plugin name and the name value are joined with an underscore to form the name displayed by SHOW STATUS. For the array just shown, the resulting status variable names are simple_parser_static and simple_parser_called. This convention means that you can easily display the variables for a plugin using its name:

    mysql> SHOW STATUS LIKE 'simple_parser%';
    +----------------------+--------------------+
    | Variable_name        | Value              |
    +----------------------+--------------------+
    | simple_parser_static | just a static text |
    | simple_parser_called | 0                  |
    +----------------------+--------------------+
    
  6. To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable). For the simple_parser plugin, it is compiled and installed when you build MySQL from source. It is also included in binary distributions. The build process produces a shared object library with a name of mypluglib.so (the .so suffix might differ depending on your platform).

  7. To use the plugin, register it with the server. For example, to register the plugin at runtime, use this statement (adjust the .so suffix for your platform as necessary):

    INSTALL PLUGIN simple_parser SONAME 'mypluglib.so';
    

    For additional information about plugin loading, see Section 6.5.2, “Installing and Uninstalling Plugins”.

  8. To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement. See Section 6.5.3, “Obtaining Server Plugin Information”.

  9. Test the plugin to verify that it works properly.

    Create a table that contains a string column and associate the parser plugin with a FULLTEXT index on the column:

    mysql> CREATE TABLE t (c VARCHAR(255),
        ->   FULLTEXT (c) WITH PARSER simple_parser
        -> ) ENGINE=MyISAM;
    Query OK, 0 rows affected (0.01 sec)
    

    Insert some text into the table and try some searches. These should verify that the parser plugin treats all nonwhitespace characters as word characters:

    mysql> INSERT INTO t VALUES
        ->   ('latin1_general_cs is a case-sensitive collation'),
        ->   ('I\'d like a case of oranges'),
        ->   ('this is sensitive information'),
        ->   ('another row'),
        ->   ('yet another row');
    Query OK, 5 rows affected (0.02 sec)
    Records: 5  Duplicates: 0  Warnings: 0
    
    mysql> SELECT c FROM t;
    +-------------------------------------------------+
    | c                                               |
    +-------------------------------------------------+
    | latin1_general_cs is a case-sensitive collation |
    | I'd like a case of oranges                      |
    | this is sensitive information                   |
    | another row                                     |
    | yet another row                                 |
    +-------------------------------------------------+
    5 rows in set (0.00 sec)
    
    mysql> SELECT MATCH(c) AGAINST('case') FROM t;
    +--------------------------+
    | MATCH(c) AGAINST('case') |
    +--------------------------+
    |                        0 |
    |          1.2968142032623 |
    |                        0 |
    |                        0 |
    |                        0 |
    +--------------------------+
    5 rows in set (0.00 sec)
    
    mysql> SELECT MATCH(c) AGAINST('sensitive') FROM t;
    +-------------------------------+
    | MATCH(c) AGAINST('sensitive') |
    +-------------------------------+
    |                             0 |
    |                             0 |
    |               1.3253291845322 |
    |                             0 |
    |                             0 |
    +-------------------------------+
    5 rows in set (0.01 sec)
    
    mysql> SELECT MATCH(c) AGAINST('case-sensitive') FROM t;
    +------------------------------------+
    | MATCH(c) AGAINST('case-sensitive') |
    +------------------------------------+
    |                    1.3109166622162 |
    |                                  0 |
    |                                  0 |
    |                                  0 |
    |                                  0 |
    +------------------------------------+
    5 rows in set (0.01 sec)
    
    mysql> SELECT MATCH(c) AGAINST('I\'d') FROM t;
    +--------------------------+
    | MATCH(c) AGAINST('I\'d') |
    +--------------------------+
    |                        0 |
    |          1.2968142032623 |
    |                        0 |
    |                        0 |
    |                        0 |
    +--------------------------+
    5 rows in set (0.01 sec)
    

    Neither case nor insensitive match case-insensitive the way that they would for the built-in parser.

27.2.4.5 Writing Daemon Plugins

A daemon plugin is a simple type of plugin used for code that should be run by the server but that does not communicate with it. This section describes how to write a daemon server plugin, using the example plugin found in the plugin/daemon_example directory of MySQL source distributions. That directory contains the daemon_example.cc source file for a daemon plugin named daemon_example that writes a heartbeat string at regular intervals to a file named mysql-heartbeat.log in the data directory.

To write a daemon plugin, include the following header file in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

#include <mysql/plugin.h>

plugin.h defines the MYSQL_DAEMON_PLUGIN server plugin type and the data structures needed to declare the plugin.

The daemon_example.cc file sets up the library descriptor as follows. The library descriptor includes a single general server plugin descriptor.

mysql_declare_plugin(daemon_example)
{
  MYSQL_DAEMON_PLUGIN,
  &daemon_example_plugin,
  "daemon_example",
  "Brian Aker",
  "Daemon example, creates a heartbeat beat file in mysql-heartbeat.log",
  PLUGIN_LICENSE_GPL,
  daemon_example_plugin_init, /* Plugin Init */
  daemon_example_plugin_deinit, /* Plugin Deinit */
  0x0100 /* 1.0 */,
  NULL,                       /* status variables                */
  NULL,                       /* system variables                */
  NULL,                       /* config options                  */
  0,                          /* flags                           */
}
mysql_declare_plugin_end;

The name member (daemon_example) indicates the name to use for references to the plugin in statements such as INSTALL PLUGIN or UNINSTALL PLUGIN. This is also the name displayed by SHOW PLUGINS or INFORMATION_SCHEMA.PLUGINS.

The second member of the plugin descriptor, daemon_example_plugin, points to the type-specific daemon plugin descriptor. This structure consists only of the type-specific API version number:

struct st_mysql_daemon daemon_example_plugin=
{ MYSQL_DAEMON_INTERFACE_VERSION  };

The type-specific structure has no interface functions. There is no communication between the server and the plugin, except that the server calls the initialization and deinitialization functions from the general plugin descriptor to start and stop the plugin:

  • daemon_example_plugin_init() opens the heartbeat file and spawns a thread that wakes up periodically and writes the next message to the file.

  • daemon_example_plugin_deinit() closes the file and performs other cleanup.

To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable). For the daemon_example plugin, it is compiled and installed when you build MySQL from source. It is also included in binary distributions. The build process produces a shared object library with a name of libdaemon_example.so (the .so suffix might differ depending on your platform).

To use the plugin, register it with the server. For example, to register the plugin at runtime, use this statement (adjust the .so suffix for your platform as necessary):

INSTALL PLUGIN daemon_example SONAME 'libdaemon_example.so';

For additional information about plugin loading, see Section 6.5.2, “Installing and Uninstalling Plugins”.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement. See Section 6.5.3, “Obtaining Server Plugin Information”.

While the plugin is loaded, it writes a heartbeat string at regular intervals to a file named mysql-heartbeat.log in the data directory. This file grows without limit, so after you have satistifed yourself that the plugin operates correctly, unload it:

UNINSTALL PLUGIN daemon_example;

27.2.4.6 Writing INFORMATION_SCHEMA Plugins

This section describes how to write a server-side INFORMATION_SCHEMA table plugin. For example code that implements such plugins, see the sql/sql_show.cc file of a MySQL source distribution. You can also look at the example plugins found in the InnoDB source. See the handler/i_s.cc and handler/ha_innodb.cc files within the InnoDB source tree (in the storage/innobase directory).

To write an INFORMATION_SCHEMA table plugin, include the following header files in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

#include <sql_class.h>
#include <table.h>

These header files are located in the sql directory of MySQL source distributions. They contain C++ structures, so the source file for an INFORMATION_SCHEMA plugin must be compiled as C++ (not C) code.

The source file for the example plugin developed here is named simple_i_s_table.cc. It creates a simple INFORMATION_SCHEMA table named SIMPLE_I_S_TABLE that has two columns named NAME and VALUE. The general descriptor for a plugin library that implements the table looks like this:

mysql_declare_plugin(simple_i_s_library)
{
  MYSQL_INFORMATION_SCHEMA_PLUGIN,
  &simple_table_info,                /* type-specific descriptor */
  "SIMPLE_I_S_TABLE",                /* table name */
  "Author Name",                     /* author */
  "Simple INFORMATION_SCHEMA table", /* description */
  PLUGIN_LICENSE_GPL,                /* license type */
  simple_table_init,                 /* init function */
  NULL,
  0x0100,                            /* version = 1.0 */
  NULL,                              /* no status variables */
  NULL,                              /* no system variables */
  NULL,                              /* no reserved information */
  0                                  /* no flags */
}
mysql_declare_plugin_end;

The name member (SIMPLE_I_S_TABLE) indicates the name to use for references to the plugin in statements such as INSTALL PLUGIN or UNINSTALL PLUGIN. This is also the name displayed by SHOW PLUGINS or INFORMATION_SCHEMA.PLUGINS.

The simple_table_info member of the general descriptor points to the type-specific descriptor, which consists only of the type-specific API version number:

static struct st_mysql_information_schema simple_table_info =
{ MYSQL_INFORMATION_SCHEMA_INTERFACE_VERSION };

The general descriptor points to the initialization and deinitialization functions:

  • The initialization function provides information about the table structure and a function that populates the table.

  • The deinitialization function performs any required cleanup. If no cleanup is needed, this descriptor member can be NULL (as in the example shown).

The initialization function should return 0 for success, 1 if an error occurs. The function receives a generic pointer, which it should interpret as a pointer to the table structure:

static int table_init(void *ptr)
{
  ST_SCHEMA_TABLE *schema_table= (ST_SCHEMA_TABLE*)ptr;

  schema_table->fields_info= simple_table_fields;
  schema_table->fill_table= simple_fill_table;
  return 0;
}

The function should set these two members of the table structure:

  • fields_info: An array of ST_FIELD_INFO structures that contain information about each column.

  • fill_table: A function that populates the table.

The array pointed to by fields_info should contain one element per column of the INFORMATION_SCHEMA plus a terminating element. The following simple_table_fields array for the example plugin indicates that SIMPLE_I_S_TABLE has two columns. NAME is string-valued with a length of 10 and VALUE is integer-valued with a display width of 20. The last structure marks the end of the array.

static ST_FIELD_INFO simple_table_fields[]=
{
  {"NAME", 10, MYSQL_TYPE_STRING, 0, 0 0, 0},
  {"VALUE", 6, MYSQL_TYPE_LONG, 0, MY_I_S_UNSIGNED, 0, 0},
  {0, 0, MYSQL_TYPE_NULL, 0, 0, 0, 0}
};

For more information about the column information structure, see the definition of ST_FIELD_INFO in the table.h header file. The permissible MYSQL_TYPE_xxx type values are those used in the C API; see Section 26.8.5, “C API Data Structures”.

The fill_table member should be set to a function that populates the table and returns 0 for success, 1 if an error occurs. For the example plugin, the simple_fill_table() function looks like this:

static int simple_fill_table(THD *thd, TABLE_LIST *tables, Item *cond)
{
  TABLE *table= tables->table;

  table->field[0]->store("Name 1", 6, system_charset_info);
  table->field[1]->store(1);
  if (schema_table_store_record(thd, table))
    return 1;
  table->field[0]->store("Name 2", 6, system_charset_info);
  table->field[1]->store(2);
  if (schema_table_store_record(thd, table))
    return 1;
  return 0;
}

For each row of the INFORMATION_SCHEMA table, this function initializes each column, then calls schema_table_store_record() to install the row. The store() method arguments depend on the type of value to be stored. For column 0 (NAME, a string), store() takes a pointer to a string, its length, and information about the character set of the string:

store(const char *to, uint length, CHARSET_INFO *cs);

For column 1 (VALUE, an integer), store() takes the value and a flag indicating whether it is unsigned:

store(longlong nr, bool unsigned_value);

For other examples of how to populate INFORMATION_SCHEMA tables, search for instances of schema_table_store_record() in sql_show.cc.

To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable).

To test the plugin, install it:

mysql> INSTALL PLUGIN SIMPLE_I_S_TABLE SONAME 'simple_i_s_table.so';

Verify that the table is present:

mysql> SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES
    -> WHERE TABLE_NAME = 'SIMPLE_I_S_TABLE';
+------------------+
| TABLE_NAME       |
+------------------+
| SIMPLE_I_S_TABLE |
+------------------+

Try to select from it:

mysql> SELECT * FROM INFORMATION_SCHEMA.SIMPLE_I_S_TABLE;
+--------+-------+
| NAME   | VALUE |
+--------+-------+
| Name 1 |     1 |
| Name 2 |     2 |
+--------+-------+

Uninstall it:

mysql> UNINSTALL PLUGIN SIMPLE_I_S_TABLE;

27.2.4.7 Writing Semisynchronous Replication Plugins

This section describes how to write server-side semisynchronous replication plugins, using the example plugins found in the plugin/semisync directory of MySQL source distributions. That directory contains the source files for master and slave plugins named rpl_semi_sync_master and rpl_semi_sync_slave. The information here covers only how to set up the plugin framework. For details about how the plugins implement replication functions, see the source.

To write a semisynchronous replication plugin, include the following header file in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

#include <mysql/plugin.h>

plugin.h defines the MYSQL_REPLICATION_PLUGIN server plugin type and the data structures needed to declare the plugin.

For the master side, semisync_master_plugin.cc contains this general descriptor for a plugin named rpl_semi_sync_master:

mysql_declare_plugin(semi_sync_master)
{
  MYSQL_REPLICATION_PLUGIN,
  &semi_sync_master_plugin,
  "rpl_semi_sync_master",
  "He Zhenxing",
  "Semi-synchronous replication master",
  PLUGIN_LICENSE_GPL,
  semi_sync_master_plugin_init, /* Plugin Init */
  semi_sync_master_plugin_deinit, /* Plugin Deinit */
  0x0100 /* 1.0 */,
  semi_sync_master_status_vars, /* status variables */
  semi_sync_master_system_vars, /* system variables */
  NULL,                         /* config options */
  0,                            /* flags */
}
mysql_declare_plugin_end;

For the slave side, semisync_slave_plugin.cc contains this general descriptor for a plugin named rpl_semi_sync_slave:

mysql_declare_plugin(semi_sync_slave)
{
  MYSQL_REPLICATION_PLUGIN,
  &semi_sync_slave_plugin,
  "rpl_semi_sync_slave",
  "He Zhenxing",
  "Semi-synchronous replication slave",
  PLUGIN_LICENSE_GPL,
  semi_sync_slave_plugin_init, /* Plugin Init */
  semi_sync_slave_plugin_deinit, /* Plugin Deinit */
  0x0100 /* 1.0 */,
  semi_sync_slave_status_vars,  /* status variables */
  semi_sync_slave_system_vars,  /* system variables */
  NULL,                         /* config options */
  0,                            /* flags */
}
mysql_declare_plugin_end;

For both the master and slave plugins, the general descriptor has pointers to the type-specific descriptor, the initialization and deinitialization functions, and to the status and system variables implemented by the plugin. For information about variable setup, see Section 27.2.4.2.2, “Server Plugin Status and System Variables”. The following remarks discuss the type-specific descriptor and the initialization and deinitialization functions for the master plugin but apply similarly to the slave plugin.

The semi_sync_master_plugin member of the master general descriptor points to the type-specific descriptor, which consists only of the type-specific API version number:

struct Mysql_replication semi_sync_master_plugin= {
  MYSQL_REPLICATION_INTERFACE_VERSION
};

The initialization and deinitialization function declarations look like this:

static int semi_sync_master_plugin_init(void *p);
static int semi_sync_master_plugin_deinit(void *p);

The initialization function uses the pointer to register transaction and binary logging observers with the server. After successful initialization, the server takes care of invoking the observers at the appropriate times. (For details on the observers, see the source files.) The deinitialization function cleans up by deregistering the observers. Each function returns 0 for success or 1 if an error occurs.

To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable). For the rpl_semi_sync_master and rpl_semi_sync_slave plugins, they are compiled and installed when you build MySQL from source. They are also included in binary distributions. The build process produces shared object libraries with names of semisync_master.so and semisync_slave.so (the .so suffix might differ depending on your platform).

27.2.4.8 Writing Audit Plugins

This section describes how to write a server-side audit plugin, using the example plugin found in the plugin/audit_null directory of MySQL source distributions. The audit_null.c and audit_null_variables.h source files in that directory implement an audit plugin named NULL_AUDIT.

Note

Changes were made in MySQL 5.7.8 to reimplement query rewrite plugins as audit plugins, and then the audit plugin API itself was extensively revised in 5.7.9 (including the parts for query rewrite plugins). For these reasons, the discussion here describes the audit plugin API as of MySQL 5.7.9. The API prior to 5.7.8 is much like that for MySQL 5.6. To write audit plugins against the older API, see Writing Audit Plugins in MySQL 5.6 Reference Manual.

Note

Other examples of plugins that use the audit plugin API are the query rewrite plugin (see Section 6.5.5, “The Rewriter Query Rewrite Plugin”) and the Version Tokens plugin (see Section 6.5.6, “Version Tokens”).

Within the server, the pluggable audit interface is implemented in the sql_audit.h and sql_audit.cc files in the sql directory of MySQL source distributions. Additionally, several places in the server call the audit interface when an auditable event occurs, so that registered audit plugins can be notified about the event if necessary. To see where such calls occur, search the server source files for invocations of functions with names of the form mysql_audit_xxx(). Audit notification occurs for server operations such as these:

  • Client connect and disconnect events

  • Writing a message to the general query log (if the log is enabled)

  • Writing a message to the error log

  • Sending a query result to a client

To write an audit plugin, include the following header file in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

#include <mysql/plugin_audit.h>

plugin_audit.h includes plugin.h, so you need not include the latter file explicitly. plugin.h defines the MYSQL_AUDIT_PLUGIN server plugin type and the data structures needed to declare the plugin. plugin_audit.h defines data structures specific to audit plugins.

Audit Plugin General Descriptor

An audit plugin, like any MySQL server plugin, has a general plugin descriptor (see Section 27.2.4.2.1, “Server Plugin Library and Plugin Descriptors”) and a type-specific plugin descriptor. In audit_null.c, the general descriptor for audit_null looks like this:

mysql_declare_plugin(audit_null)
{
  MYSQL_AUDIT_PLUGIN,         /* type                            */
  &audit_null_descriptor,     /* descriptor                      */
  "NULL_AUDIT",               /* name                            */
  "Oracle Corp",              /* author                          */
  "Simple NULL Audit",        /* description                     */
  PLUGIN_LICENSE_GPL,
  audit_null_plugin_init,     /* init function (when loaded)     */
  audit_null_plugin_deinit,   /* deinit function (when unloaded) */
  0x0003,                     /* version                         */
  simple_status,              /* status variables                */
  system_variables,           /* system variables                */
  NULL,
  0,
}
mysql_declare_plugin_end;

The first member, MYSQL_AUDIT_PLUGIN, identifies this plugin as an audit plugin.

audit_null_descriptor points to the type-specific plugin descriptor, described later.

The name member (NULL_AUDIT) indicates the name to use for references to the plugin in statements such as INSTALL PLUGIN or UNINSTALL PLUGIN. This is also the name displayed by INFORMATION_SCHEMA.PLUGINS or SHOW PLUGINS.

The audit_null_plugin_init initialization function performs plugin initialization when the plugin is loaded. The audit_null_plugin_deinit function performs cleanup when the plugin is unloaded.

The general plugin descriptor also refers to simple_status and system_variables, structures that expose several status and system variables. When the plugin is enabled, these variables can be inspected using SHOW statements (SHOW STATUS, SHOW VARIABLES) or the appropriate Performance Schema tables.

The simple_status structure declares several status variables with names of the form Audit_null_xxx. NULL_AUDIT increments the Audit_null_called status variable for every notification that it receives. The other status variables are more specific and NULL_AUDIT increments them only for notifications of specific events.

system_variables is an array of system variable elements, each of which is defined using a MYSQL_THDVAR_xxx macro. These system variables have names of the form null_audit_xxx. These variables can be used to communicate with the plugin at runtime.

Audit Plugin Type-Specific Descriptor

The audit_null_descriptor value in the general plugin descriptor points to the type-specific plugin descriptor. For audit plugins, this descriptor has the following structure (defined in plugin_audit.h):

struct st_mysql_audit
{
  int interface_version;
  void (*release_thd)(MYSQL_THD);
  int (*event_notify)(MYSQL_THD, mysql_event_class_t, const void *);
  unsigned long class_mask[MYSQL_AUDIT_CLASS_MASK_SIZE];
};

The type-specific descriptor for audit plugins has these members:

  • interface_version: By convention, type-specific plugin descriptors begin with the interface version for the given plugin type. The server checks interface_version when it loads the plugin to see whether the plugin is compatible with it. For audit plugins, the value of the interface_version member is MYSQL_AUDIT_INTERFACE_VERSION (defined in plugin_audit.h).

  • release_thd: A function that the server calls to inform the plugin that it is being dissociated from its thread context. This should be NULL if there is no such function.

  • event_notify: A function that the server calls to notify the plugin that an auditable event has occurred. This function should not be NULL; that would not make sense because no auditing would occur.

  • class_mask: An array of MYSQL_AUDIT_CLASS_MASK_SIZE elements. Each element specifies a bitmask for a given event class to indicate the subclasses for which the plugin wants notification. (This is how the plugin subscribes to events of interest.) An element should be 0 to ignore events for the corresponding event class.

The server uses the event_notify and release_thd functions together. They are called within the context of a specific thread, and a thread might perform an activity that produces several event notifications. The first time the server calls event_notify for a thread, it creates a binding of the plugin to the thread. The plugin cannot be uninstalled while this binding exists. When no more events for the thread will occur, the server informs the plugin of this by calling the release_thd function, and then destroys the binding. For example, when a client issues a statement, the thread processing the statement might notify audit plugins about the result set produced by the statement and about the statement being logged. After these notifications occur, the server releases the plugin before putting the thread to sleep until the client issues another statement.

This design enables the plugin to allocate resources needed for a given thread in the first call to the event_notify function and release them in the release_thd function:

event_notify function:
  if memory is needed to service the thread
    allocate memory
  ... rest of notification processing ...

release_thd function:
  if memory was allocated
    release memory
  ... rest of release processing ...

That is more efficient than allocating and releasing memory repeatedly in the notification function.

For the NULL_AUDIT audit plugin, the type-specific plugin descriptor looks like this:

static struct st_mysql_audit audit_null_descriptor=
{
  MYSQL_AUDIT_INTERFACE_VERSION,                    /* interface version    */
  NULL,                                             /* release_thd function */
  audit_null_notify,                                /* notify function      */
  { (unsigned long) MYSQL_AUDIT_GENERAL_ALL,
    (unsigned long) MYSQL_AUDIT_CONNECTION_ALL,
    (unsigned long) MYSQL_AUDIT_PARSE_ALL,
    (unsigned long) MYSQL_AUDIT_AUTHORIZATION_ALL,
    (unsigned long) MYSQL_AUDIT_TABLE_ACCESS_ALL,
    (unsigned long) MYSQL_AUDIT_GLOBAL_VARIABLE_ALL,
    (unsigned long) MYSQL_AUDIT_SERVER_STARTUP_ALL,
    (unsigned long) MYSQL_AUDIT_SERVER_SHUTDOWN_ALL,
    (unsigned long) MYSQL_AUDIT_COMMAND_ALL,
    (unsigned long) MYSQL_AUDIT_QUERY_ALL,
    (unsigned long) MYSQL_AUDIT_STORED_PROGRAM_ALL }
};

The server calls audit_null_notify() to pass audit event information to the plugin. There is no release_thd function.

The class_mask member is an array that indicates which event classes the plugin subscribes to. As shown, the array contents subscribe to all subclasses of all event classes that are available. To ignore all notifications for a given event class, specify the corresponding class_mask element as 0.

The number of class_mask elements corresponds to the number of event classes, each of which is listed in the mysql_event_class_t enumeration defined in plugin_audit.h:

typedef enum
{
  MYSQL_AUDIT_GENERAL_CLASS          = 0,
  MYSQL_AUDIT_CONNECTION_CLASS       = 1,
  MYSQL_AUDIT_PARSE_CLASS            = 2,
  MYSQL_AUDIT_AUTHORIZATION_CLASS    = 3,
  MYSQL_AUDIT_TABLE_ACCESS_CLASS     = 4,
  MYSQL_AUDIT_GLOBAL_VARIABLE_CLASS  = 5,
  MYSQL_AUDIT_SERVER_STARTUP_CLASS   = 6,
  MYSQL_AUDIT_SERVER_SHUTDOWN_CLASS  = 7,
  MYSQL_AUDIT_COMMAND_CLASS          = 8,
  MYSQL_AUDIT_QUERY_CLASS            = 9,
  MYSQL_AUDIT_STORED_PROGRAM_CLASS   = 10,
  /* This item must be last in the list. */
  MYSQL_AUDIT_CLASS_MASK_SIZE
} mysql_event_class_t;

For any given event class, plugin_audit.h defines bitmask symbols for individual event subclasses, as well as an xxx_ALL symbol that is the union of the all subclass bitmasks. For example, for MYSQL_AUDIT_CONNECTION_CLASS (the class that covers connect and disconnect events), plugin_audit.h defines these symbols:

typedef enum
{
  /** occurs after authentication phase is completed. */
  MYSQL_AUDIT_CONNECTION_CONNECT          = 1 << 0,
  /** occurs after connection is terminated. */
  MYSQL_AUDIT_CONNECTION_DISCONNECT       = 1 << 1,
  /** occurs after COM_CHANGE_USER RPC is completed. */
  MYSQL_AUDIT_CONNECTION_CHANGE_USER      = 1 << 2,
  /** occurs before authentication. */
  MYSQL_AUDIT_CONNECTION_PRE_AUTHENTICATE = 1 << 3
} mysql_event_connection_subclass_t;

#define MYSQL_AUDIT_CONNECTION_ALL (MYSQL_AUDIT_CONNECTION_CONNECT | \
                                    MYSQL_AUDIT_CONNECTION_DISCONNECT | \
                                    MYSQL_AUDIT_CONNECTION_CHANGE_USER | \
                                    MYSQL_AUDIT_CONNECTION_PRE_AUTHENTICATE)

To subscribe to all subclasses of the connection event class (as the NULL_AUDIT plugin does), a plugin specifies MYSQL_AUDIT_CONNECTION_ALL in the corresponding class_mask element (class_mask[1] in this case). To subscribe to only some subclasses, the plugin sets the class_mask element to the union of the subclasses of interest. For example, to subscribe only to the connect and change-user subclasses, the plugin sets class_mask[1] to this value:

MYSQL_AUDIT_CONNECTION_CONNECT | MYSQL_AUDIT_CONNECTION_CHANGE_USER
Audit Plugin Notification Function

Most of the work for an audit plugin occurs in the notification function (the event_notify member of the type-specific plugin descriptor). The server calls this function for each auditable event. Audit plugin notification functions have this prototype:

int (*event_notify)(MYSQL_THD, mysql_event_class_t, const void *);

The second and third parameters of the event_notify function prototype represent the event class and a generic pointer to an event structure. (Events in different classes have different structures. The notification function can use the event class value to determine which event structure applies.) The function processes the event and returns a status indicating whether the server should continue processing the event or terminate it.

For NULL_AUDIT, the notification function is audit_null_notify(). This function increments a global event counter (which the plugin exposes as the value of the Audit_null_called status value), and then examines the event class to determine how to process the event structure:

static int audit_null_notify(MYSQL_THD thd __attribute__((unused)),
                             mysql_event_class_t event_class,
                             const void *event)
{
  ...

  number_of_calls++;

  if (event_class == MYSQL_AUDIT_GENERAL_CLASS)
  {
    const struct mysql_event_general *event_general=
                                    (const struct mysql_event_general *)event;
    ...
  }
  else if (event_class == MYSQL_AUDIT_CONNECTION_CLASS)
  {
    const struct mysql_event_connection *event_connection=
                                (const struct mysql_event_connection *) event;
    ...

  }
  else if (event_class == MYSQL_AUDIT_PARSE_CLASS)
  {
    const struct mysql_event_parse *event_parse =
                                      (const struct mysql_event_parse *)event;
    ...
  }
  ...
}

The notification function interprets the event argument according to the value of event_class. The event argument is a generic pointer to the event record, the structure of which differs per event class. (The plugin_audit.h file contains the structures that define the contents of each event class.) For each class, audit_null_notify() casts the event to the appropriate class-specific structure and then checks its subclass to determine which subclass counter to increment. For example, the code to handle events in the connection-event class looks like this:

else if (event_class == MYSQL_AUDIT_CONNECTION_CLASS)
{
  const struct mysql_event_connection *event_connection=
                              (const struct mysql_event_connection *) event;

  switch (event_connection->event_subclass)
  {
  case MYSQL_AUDIT_CONNECTION_CONNECT:
    number_of_calls_connection_connect++;
    break;
  case MYSQL_AUDIT_CONNECTION_DISCONNECT:
    number_of_calls_connection_disconnect++;
    break;
  case MYSQL_AUDIT_CONNECTION_CHANGE_USER:
    number_of_calls_connection_change_user++;
    break;
  case MYSQL_AUDIT_CONNECTION_PRE_AUTHENTICATE:
    number_of_calls_connection_pre_authenticate++;
      break;
  default:
    break;
  }
}
Note

The general event class (MYSQL_AUDIT_GENERAL_CLASS) is deprecated as of MySQL 5.7.9 and will be removed in a future MySQL release. To reduce plugin overhead, it is preferable to subscribe only to the more specific event classes of interest.

For some event classes, the NULL_AUDIT plugin performs other processing in addition to incrementing a counter. In any case, when the notification function finishes processing the event, it should return a status indicating whether the server should continue processing the event or terminate it.

Audit Plugin Error Handling

Audit plugin notification functions can report a status value for the current event two ways:

  • Use the notification function return value. In this case, the function returns zero if the server should continue processing the event, or nonzero if the server should terminate the event.

  • Call the my_message() function to set the error state before returning from the notification function. In this case, the notification function return value is ignored and the server terminates event processing with an error. The my_message() arguments indicate which error to report, and its message. For example:

    my_message(ER_AUDIT_API_ABORT, "This is my error message.", MYF(0));
    

    Some events cannot be aborted. A nonzero return value is not taken into consideration and the my_message() error call must follow an is_error() check. For example:

    if (!thd->get_stmt_da()->is_error())
    {
      my_message(ER_AUDIT_API_ABORT, "This is my error message.", MYF(0));
    }
    

Some events cannot be terminated:

  • MYSQL_AUDIT_CONNECTION_DISCONNECT: The server cannot prevent a client from disconnecting.

  • MYSQL_AUDIT_COMMAND_END: This event provides the status of a command that has finished executing, so there is no purpose to terminating it.

If an audit plugin returns nonzero status for a nonterminable event, the server ignores the status and continues processing the event. As of MySQL 5.7.9, that is also true if an audit plugin uses the my_message() function to terminate a nonterminable event.

Audit Plugin Usage

To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable). For the NULL_AUDIT plugin, it is compiled and installed when you build MySQL from source. It is also included in binary distributions. The build process produces a shared object library with a name of adt_null.so (the .so suffix might differ depending on your platform).

To register the plugin at runtime, use this statement (adjust the .so suffix for your platform as necessary):

INSTALL PLUGIN NULL_AUDIT SONAME 'adt_null.so';

For additional information about plugin loading, see Section 6.5.2, “Installing and Uninstalling Plugins”.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement. See Section 6.5.3, “Obtaining Server Plugin Information”.

While the audit plugin is installed, it exposes status variables that indicate the events for which the plugin has been called:

mysql> SHOW STATUS LIKE 'Audit_null%';
+----------------------------------------+--------+
| Variable_name                          | Value  |
+----------------------------------------+--------+
| Audit_null_authorization_column        | 0      |
| Audit_null_authorization_db            | 0      |
| Audit_null_authorization_procedure     | 0      |
| Audit_null_authorization_proxy         | 0      |
| Audit_null_authorization_table         | 0      |
| Audit_null_authorization_user          | 0      |
| Audit_null_called                      | 185547 |
| Audit_null_command_end                 | 20999  |
| Audit_null_command_start               | 21001  |
| Audit_null_connection_change_user      | 0      |
| Audit_null_connection_connect          | 5823   |
| Audit_null_connection_disconnect       | 5818   |
| Audit_null_connection_pre_authenticate | 5823   |
| Audit_null_general_error               | 1      |
| Audit_null_general_log                 | 26559  |
| Audit_null_general_result              | 19922  |
| Audit_null_general_status              | 21000  |
| Audit_null_global_variable_get         | 0      |
| Audit_null_global_variable_set         | 0      |
| Audit_null_parse_postparse             | 14648  |
| Audit_null_parse_preparse              | 14648  |
| Audit_null_query_nested_start          | 6      |
| Audit_null_query_nested_status_end     | 6      |
| Audit_null_query_start                 | 14648  |
| Audit_null_query_status_end            | 14647  |
| Audit_null_server_shutdown             | 0      |
| Audit_null_server_startup              | 1      |
| Audit_null_table_access_delete         | 104    |
| Audit_null_table_access_insert         | 2839   |
| Audit_null_table_access_read           | 97842  |
| Audit_null_table_access_update         | 278    |
+----------------------------------------+--------+

Audit_null_called counts all events, and the other variables count instances of specific event subclasses. For example, the preceding SHOW STATUS statement causes the server to send a result to the client and to write a message to the general query log if that log is enabled. Thus, a client that issues the statement repeatedly causes Audit_null_called, Audit_null_general_result, and Audit_null_general_log to be incremented each time. (Before MySQL 5.7.5, notification of events for the general query log occur only if the general query log is enabled. As of 5.7.5, notifications occur whether or not that log is enabled.)

The status variables values are aggregated across all sessions. There are no counters for individual sessions.

NULL_AUDIT exposes several system variables that enable communication with the plugin at runtime:

mysql> SHOW VARIABLES LIKE 'null_audit%';
+------------------------------------+-------+
| Variable_name                      | Value |
+------------------------------------+-------+
| null_audit_abort_message           |       |
| null_audit_abort_value             | 1     |
| null_audit_event_order_check       |       |
| null_audit_event_order_check_exact | 1     |
| null_audit_event_order_started     | 0     |
| null_audit_event_record            |       |
| null_audit_event_record_def        |       |
+------------------------------------+-------+

To check the order of audit API calls, set the null_audit_event_order_check variable to the expected event order. For example:

SET null_audit_event_order_check =
      'MYSQL_AUDIT_CONNECTION_PRE_AUTHENTICATE;;;'
      'MYSQL_AUDIT_GENERAL_LOG;;;'
      'MYSQL_AUDIT_CONNECTION_CONNECT;;';

The statement takes advantage of the SQL syntax that concatenates adjacent strings into a single string.

The format of the value is:

'event_name;event_data;command' [';event_name;event_data;command'] ...

After the event order is matched, the null_audit_event_order_check value is replaced with a value of EVENT-ORDER-OK.

Specifying a command value of ABORT_RET makes it possible to abort the audit API call on the specified event. The following example aborts INSERT statement execution when its MYSQL_AUDIT_QUERY_STATUS_END event occurs:

SET null_audit_event_order_check =
   'MYSQL_AUDIT_COMMAND_START;command_id="3";;'
   'MYSQL_AUDIT_GENERAL_LOG;;;'
   'MYSQL_AUDIT_QUERY_START;;;'
   'MYSQL_AUDIT_QUERY_STATUS_END;;ABORT_RET';

After the audit plugin matches the preceding sequence, it aborts event processing and sends an error message to the client:

ERROR 3164 (HY000): Aborted by Audit API ('MYSQL_AUDIT_QUERY_STATUS_END';1).

Returning a nonzero value from the audit API notification routine is the standard way to abort event execution. It is also possible to specify a custom error code by setting the null_audit_abort_value variable to the value that the notification routine should return:

SET null_audit_abort_value = 123;

Aborting a sequence results in a standard message with the custom error code. Suppose that you set audit log system variables like this:

SET null_audit_abort_value = 123;
SET null_audit_event_order_check =
    'MYSQL_AUDIT_COMMAND_START;command_id="3";;'
    'MYSQL_AUDIT_GENERAL_LOG;;;'
    'MYSQL_AUDIT_QUERY_START;;ABORT_RET';

Then execution of SELECT 1 results in this error:

ERROR 3164 (HY000): Aborted by Audit API ('MYSQL_AUDIT_QUERY_START';123).

An event can be also aborted with a custom message, specified by setting the null_audit_abort_message variable: Suppose that you set audit log system variables like this:

SET null_audit_abort_message = 'Custom error text.';
SET null_audit_event_order_check =
    'MYSQL_AUDIT_COMMAND_START;command_id="3";;'
    'MYSQL_AUDIT_GENERAL_LOG;;;'
    'MYSQL_AUDIT_QUERY_START;;ABORT_RET';

Then aborting a sequence results in the following error:

ERROR 3164 (HY000): Custom error text.

For test-creation purposes, it is possible to record events that pass through the plugin. Recording starts by specifying start and end events in the null_audit_event_record_def variable:

SET null_audit_event_record_def =
    'MYSQL_AUDIT_COMMAND_START;MYSQL_AUDIT_COMMAND_END';

Statement execution results in storing the events that occur in the null_audit_event_record variable.

To disable the plugin after testing it, use this statement to unload it:

UNINSTALL PLUGIN NULL_AUDIT;

27.2.4.9 Writing Authentication Plugins

MySQL supports pluggable authentication, in which plugins are invoked to authenticate client connections. Authentication plugins enable the use of authentication methods other than the built-in method of passwords stored in the mysql.user table. For example, plugins can be written to access external authentication methods. Also, authentication plugins can support the proxy user capability, such that the connecting user is a proxy for another user and is treated, for purposes of access control, as having the privileges of a different user. For more information, see Section 7.3.8, “Pluggable Authentication”, and Section 7.3.9, “Proxy Users”.

An authentication plugin can be written for the server side or the client side. Server-side plugins use the same plugin API that is used for the other server plugin types such as full-text parser or audit plugins (although with a different type-specific descriptor). Client-side plugins use the client plugin API.

Several header files contain information relevant to authentication plugins:

  • plugin.h: Defines the MYSQL_AUTHENTICATION_PLUGIN server plugin type.

  • client_plugin.h: Defines the API for client plugins. This includes the client plugin descriptor and function prototypes for client plugin C API calls (see Section 26.8.14, “C API Client Plugin Functions”).

  • plugin_auth.h: Defines the part of the server plugin API specific to authentication plugins. This includes the type-specific descriptor for server-side authentication plugins and the MYSQL_SERVER_AUTH_INFO structure.

  • plugin_auth_common.h: Contains common elements of client and server authentication plugins. This includes return value definitions and the MYSQL_PLUGIN_VIO structure.

To write an authentication plugin, include the following header files in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

  • For a source file that implements a server authentication plugin, include this file:

    #include <mysql/plugin_auth.h>
    
  • For a source file that implements a client authentication plugin, or both client and server plugins, include these files:

    #include <mysql/plugin_auth.h>
    #include <mysql/client_plugin.h>
    #include <mysql.h>
    

plugin_auth.h includes plugin.h and plugin_auth_common.h, so you need not include the latter files explicitly.

This section describes how to write a pair of simple server and client authentication plugins that work together.

Warning

These plugins accept any non-empty password and the password is sent in clear text. This is insecure, so the plugins should not be used in production environments.

The server-side and client-side plugins developed here both are named auth_simple. As described in Section 27.2.4.2, “Plugin Data Structures”, the plugin library file must have the same base name as the client plugin, so the source file name is auth_simple.c and produces a library named auth_simple.so (assuming that your system uses .so as the suffix for library files).

In MySQL source distributions, authentication plugin source is located in the plugin/auth directory and can be examined as a guide to writing other authentication plugins. Also, to see how the built-in authentication plugins are implemented, see sql/sql_acl.cc for plugins that are built in to the MySQL server and sql-common/client.c for plugins that are built in to the libmysqlclient client library. (For the built-in client plugins, note that the auth_plugin_t structures used there differ from the structures used with the usual client plugin declaration macros. In particular, the first two members are provided explicitly, not by declaration macros.)

27.2.4.9.1 Writing the Server-Side Authentication Plugin

Declare the server-side plugin with the usual general descriptor format that is used for all server plugin types (see Section 27.2.4.2.1, “Server Plugin Library and Plugin Descriptors”). For the auth_simple plugin, the descriptor looks like this:

mysql_declare_plugin(auth_simple)
{
  MYSQL_AUTHENTICATION_PLUGIN,
  &auth_simple_handler,                 /* type-specific descriptor */
  "auth_simple",                        /* plugin name */
  "Author Name",                        /* author */
  "Any-password authentication plugin", /* description */
  PLUGIN_LICENSE_GPL,                   /* license type */
  NULL,                                 /* no init function */
  NULL,                                 /* no deinit function */
  0x0100,                               /* version = 1.0 */
  NULL,                                 /* no status variables */
  NULL,                                 /* no system variables */
  NULL,                                 /* no reserved information */
  0                                     /* no flags */
}
mysql_declare_plugin_end;

The name member (auth_simple) indicates the name to use for references to the plugin in statements such as INSTALL PLUGIN or UNINSTALL PLUGIN. This is also the name displayed by SHOW PLUGINS or INFORMATION_SCHEMA.PLUGINS.

The auth_simple_handler member of the general descriptor points to the type-specific descriptor. For an authentication plugin, the type-specific descriptor is an instance of the st_mysql_auth structure (defined in plugin_auth.h):

struct st_mysql_auth
{
  int interface_version;
  const char *client_auth_plugin;
  int (*authenticate_user)(MYSQL_PLUGIN_VIO *vio, MYSQL_SERVER_AUTH_INFO *info);
  int (*generate_authentication_string)(char *outbuf,
      unsigned int *outbuflen, const char *inbuf, unsigned int inbuflen);
  int (*validate_authentication_string)(char* const inbuf, unsigned int buflen);
  int (*set_salt)(const char *password, unsigned int password_len,
                  unsigned char* salt, unsigned char *salt_len);
  const unsigned long authentication_flags;
};

The st_mysql_auth structure has these members:

  • interface_version: The type-specific API version number, always MYSQL_AUTHENTICATION_INTERFACE_VERSION

  • client_auth_plugin: The client plugin name

  • authenticate_user: A pointer to the main plugin function that communicates with the client

  • generate_authentication_string: A pointer to a plugin function that generates a password digest from an authentication string (added in MySQL 5.7.6)

  • validate_authentication_string: A pointer to a plugin function that validates a password digest (added in MySQL 5.7.6)

  • set_salt: A pointer to a plugin function that converts a scrambled password to binary form (added in MySQL 5.7.6)

  • authentication_flags: A flags word (added in MySQL 5.7.8)

The client_auth_plugin member should indicate the name of the client plugin if a specific plugin is required. A value of NULL means any plugin. In the latter case, whatever plugin the client uses will do. This is useful if the server plugin does not care about the client plugin or what user name or password it sends. For example, this might be true if the server plugin authenticates only local clients and uses some property of the operating system rather than the information sent by the client plugin.

For auth_simple, the type-specific descriptor looks like this:

static struct st_mysql_auth auth_simple_handler =
{
  MYSQL_AUTHENTICATION_INTERFACE_VERSION,
  "auth_simple",             /* required client-side plugin name */
  auth_simple_server         /* server-side plugin main function */
  generate_auth_string_hash, /* generate digest from password string */
  validate_auth_string_hash, /* validate password digest */
  set_salt,                  /* generate password salt value */
  AUTH_FLAG_PRIVILEGED_USER_FOR_PASSWORD_CHANGE
};

The main function, auth_simple_server(), takes two arguments representing an I/O structure and a MYSQL_SERVER_AUTH_INFO structure. The structure definition, found in plugin_auth.h, looks like this:

typedef struct st_mysql_server_auth_info
{
  char *user_name;
  unsigned int user_name_length;
  const char *auth_string;
  unsigned long auth_string_length;
  char authenticated_as[MYSQL_USERNAME_LENGTH+1];
  char external_user[512];
  int  password_used;
  const char *host_or_ip;
  unsigned int host_or_ip_length;
} MYSQL_SERVER_AUTH_INFO;

The character set for string members is UTF-8. If there is a _length member associated with a string, it indicates the string length in bytes. Strings are also null-terminated.

When an authentication plugin is invoked by the server, it should interpret the MYSQL_SERVER_AUTH_INFO structure members as follows. Some of these are used to set the value of SQL functions or system variables within the client session, as indicated.

  • user_name: The user name sent by the client. The value becomes the USER() function value.

  • user_name_length: The length of user_name in bytes.

  • auth_string: The value of the authentication_string column of the mysql.user table row for the matching account name (that is, the row that matches the client user name and host name and that the server uses to determine how to authenticate the client).

    Suppose that you create an account using the following statement:

    CREATE USER 'my_user'@'localhost'
      IDENTIFIED WITH my_plugin AS 'my_auth_string';
    

    When my_user connects from the local host, the server invokes my_plugin and passes 'my_auth_string' to it as the auth_string value.

  • auth_string_length: The length of auth_string in bytes.

  • authenticated_as: The server sets this to the user name (the value of user_name). The plugin can alter it to indicate that the client should have the privileges of a different user. For example, if the plugin supports proxy users, the initial value is the name of the connecting (proxy) user, and the plugin can change this member to the proxied user name. The server then treats the proxy user as having the privileges of the proxied user (assuming that the other conditions for proxy user support are satisfied; see Section 27.2.4.9.4, “Implementing Proxy User Support in Authentication Plugins”). The value is represented as a string at most MYSQL_USER_NAME_LENGTH bytes long, plus a terminating null. The value becomes the CURRENT_USER() function value.

  • external_user: The server sets this to the empty string (null terminated). Its value becomes the external_user system variable value. If the plugin wants that system variable to have a different value, it should set this member accordingly; for example, to the connecting user name. The value is represented as a string at most 511 bytes long, plus a terminating null.

  • password_used: This member applies when authentication fails. The plugin can set it or ignore it. The value is used to construct the failure error message of Authentication fails. Password used: %s. The value of password_used determines how %s is handled, as shown in the following table.

    password_used%s Handling
    0NO
    1YES
    2There will be no %s
  • host_or_ip: The name of the client host if it can be resolved, or the IP address otherwise.

  • host_or_ip_length: The length of host_or_ip in bytes.

The auth_simple main function, auth_simple_server(), reads the password (a null-terminated string) from the client and succeeds if the password is nonempty (first byte not null):

static int auth_simple_server (MYSQL_PLUGIN_VIO *vio,
                               MYSQL_SERVER_AUTH_INFO *info)
{
  unsigned char *pkt;
  int pkt_len;

  /* read the password as null-terminated string, fail on error */
  if ((pkt_len= vio->read_packet(vio, &pkt)) < 0)
    return CR_ERROR;

  /* fail on empty password */
  if (!pkt_len || *pkt == '\0')
  {
    info->password_used= PASSWORD_USED_NO;
    return CR_ERROR;
  }

  /* accept any nonempty password */
  info->password_used= PASSWORD_USED_YES;

  return CR_OK;
}

The main function should return one of the error codes shown in the following table.

Error CodeMeaning
CR_OKSuccess
CR_OK_HANDSHAKE_COMPLETEDo not send a status packet back to client
CR_ERRORError
CR_AUTH_USER_CREDENTIALSAuthentication failure
CR_AUTH_HANDSHAKEAuthentication handshake failure
CR_AUTH_PLUGIN_ERRORInternal plugin error

For an example of how the handshake works, see the plugin/auth/dialog.c source file.

The server counts plugin errors in the Performance Schema host_cache table.

auth_simple_server() is so basic that it does not use the authentication information structure except to set the member that indicates whether a password was received.

A plugin that supports proxy users must return to the server the name of the proxied user (the MySQL user whose privileges the client user should get). To do this, the plugin must set the info->authenticated_as member to the proxied user name. For information about proxying, see Section 7.3.9, “Proxy Users”, and Section 27.2.4.9.4, “Implementing Proxy User Support in Authentication Plugins”.

The generate_authentication_string member of the plugin descriptor takes the password and generates a password hash (digest) from it:

  • The first two arguments are pointers to the output buffer and its maximum length in bytes. The function should write the password hash to the output buffer and reset the length to the actual hash length.

  • The second two arguments indicate the password input buffer and its length in bytes.

  • The function returns 0 for success, 1 if an error occurred.

For the auth_simple plugin, the generate_auth_string_hash() function implements the generate_authentication_string member. It just makes a copy of the password, unless it is too long to fit in the output buffer.

int generate_auth_string_hash(char *outbuf, unsigned int *buflen,
                              const char *inbuf, unsigned int inbuflen)
{
  /*
    fail if buffer specified by server cannot be copied to output buffer
  */
  if (*buflen < inbuflen)
    return 1;   /* error */
  strncpy(outbuf, inbuf, inbuflen);
  *buflen= strlen(inbuf);
  return 0;     /* success */
}

The validate_authentication_string member of the plugin descriptor validates a password hash:

  • The arguments are a pointer to the password hash and its length in bytes.

  • The function returns 0 for success, 1 if the password hash cannot be validated.

For the auth_simple plugin, the validate_auth_string_hash() function implements the validate_authentication_string member. It returns success unconditionally:

int validate_auth_string_hash(char* const inbuf  __attribute__((unused)),
                              unsigned int buflen  __attribute__((unused)))
{
  return 0;     /* success */
}

The set_salt member of the plugin descriptor is used only by the mysql_native_password plugin (see Section 7.5.1.1, “The Native Authentication Plugin”). For other authentication plugins, you can use this trivial implementation:

int set_salt(const char* password __attribute__((unused)),
             unsigned int password_len __attribute__((unused)),
             unsigned char* salt __attribute__((unused)),
             unsigned char* salt_len)
{
  *salt_len= 0;
  return 0;     /* success */
}

The authentication_flags member of the plugin descriptor contains flags that affect plugin operation. The permitted flags are:

  • AUTH_FLAG_PRIVILEGED_USER_FOR_PASSWORD_CHANGE: Credential changes are a privileged operation. If this flag is set, the server requires that the user has the global CREATE USER privilege or the UPDATE privilege for the mysql database.

  • AUTH_FLAG_USES_INTERNAL_STORAGE: Whether the plugin uses internal storage (in the authentication_string column of mysql.user rows). If this flag is not set, attempts to set the password using SET PASSWORD fail and the server produces a warning.

27.2.4.9.2 Writing the Client-Side Authentication Plugin

Declare the client-side plugin descriptor with the mysql_declare_client_plugin() and mysql_end_client_plugin macros (see Section 27.2.4.2.3, “Client Plugin Descriptors”). For the auth_simple plugin, the descriptor looks like this:

mysql_declare_client_plugin(AUTHENTICATION)
  "auth_simple",                        /* plugin name */
  "Author Name",                        /* author */
  "Any-password authentication plugin", /* description */
  {1,0,0},                              /* version = 1.0.0 */
  "GPL",                                /* license type */
  NULL,                                 /* for internal use */
  NULL,                                 /* no init function */
  NULL,                                 /* no deinit function */
  NULL,                                 /* no option-handling function */
  auth_simple_client                    /* main function */
mysql_end_client_plugin;

The descriptor members from the plugin name through the option-handling function are common to all client plugin types. (For descriptions, see Section 27.2.4.2.3, “Client Plugin Descriptors”.) Following the common members, the descriptor has an additional member specific to authentication plugins. This is the main function, which handles communication with the server. The function takes two arguments representing an I/O structure and a connection handler. For our simple any-password plugin, the main function does nothing but write to the server the password provided by the user:

static int auth_simple_client (MYSQL_PLUGIN_VIO *vio, MYSQL *mysql)
{
  int res;

  /* send password as null-terminated string in clear text */
  res= vio->write_packet(vio, (const unsigned char *) mysql->passwd,
                         strlen(mysql->passwd) + 1);

  return res ? CR_ERROR : CR_OK;
}

The main function should return one of the error codes shown in the following table.

Error CodeMeaning
CR_OKSuccess
CR_OK_HANDSHAKE_COMPLETESuccess, client done
CR_ERRORError

CR_OK_HANDSHAKE_COMPLETE indicates that the client has done its part successfully and has read the last packet. A client plugin may return CR_OK_HANDSHAKE_COMPLETE if the number of round trips in the authentication protocol is not known in advance and the plugin must read another packet to determine whether authentication is finished.

27.2.4.9.3 Using the Authentication Plugins

To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable).

Register the server-side plugin with the server. For example, to load the plugin at server startup, use a --plugin-load=auth_simple.so option (adjust the .so suffix for your platform as necessary).

Create a user for whom the server will use the auth_simple plugin for authentication:

mysql> CREATE USER 'x'@'localhost'
    -> IDENTIFIED WITH auth_simple;

Use a client program to connect to the server as user x. The server-side auth_simple plugin communicates with the client program that it should use the client-side auth_simple plugin, and the latter sends the password to the server. The server plugin should reject connections that send an empty password and accept connections that send a nonempty password. Invoke the client program each way to verify this:

shell> mysql --user=x --skip-password
ERROR 1045 (28000): Access denied for user 'x'@'localhost' (using password: NO)

shell> mysql --user=x --password=abc
mysql>

Because the server plugin accepts any nonempty password, it should be considered insecure. After testing the plugin to verify that it works, restart the server without the --plugin-load option so as not to indavertently leave the server running with an insecure authentication plugin loaded. Also, drop the user with DROP USER 'x'@'localhost'.

For additional information about loading and using authentication plugins, see Section 6.5.2, “Installing and Uninstalling Plugins”, and Section 7.3.8, “Pluggable Authentication”.

If you are writing a client program that supports the use of authentication plugins, normally such a program causes a plugin to be loaded by calling mysql_options() to set the MYSQL_DEFAULT_AUTH and MYSQL_PLUGIN_DIR options:

char *plugin_dir = "path_to_plugin_dir";
char *default_auth = "plugin_name";

/* ... process command-line options ... */

mysql_options(&mysql, MYSQL_PLUGIN_DIR, plugin_dir);
mysql_options(&mysql, MYSQL_DEFAULT_AUTH, default_auth);

Typically, the program will also accept --plugin-dir and --default-auth options that enable users to override the default values.

Should a client program require lower-level plugin management, the client library contains functions that take an st_mysql_client_plugin argument. See Section 26.8.14, “C API Client Plugin Functions”.

27.2.4.9.4 Implementing Proxy User Support in Authentication Plugins

One of the capabilities that pluggable authentication makes possible is proxy users (see Section 7.3.9, “Proxy Users”). For a server-side authentication plugin to participate in proxy user support, these conditions must be satisfied:

  • When a connecting client should be treated as a proxy user, the plugin must return a different name in the authenticated_as member of the MYSQL_SERVER_AUTH_INFO structure, to indicate the proxied user name. It may also optionally set the external_user member, to set the value of the external_user system variable.

  • Proxy user accounts must be set up to be authenticated by the plugin. Use the CREATE USER or GRANT statement to associate accounts with plugins.

  • Proxy user accounts must have the PROXY privilege for the proxied accounts. Use the GRANT statement to grant this privilege.

In other words, the only aspect of proxy user support required of the plugin is that it set authenticated_as to the proxied user name. The rest is optional (setting external_user) or done by the DBA using SQL statements.

How does an authentication plugin determine which proxied user to return when the proxy user connects? That depends on the plugin. Typically, the plugin maps clients to proxied users based on the authentication string passed to it by the server. This string comes from the AS part of the IDENTIFIED WITH clause of the CREATE USER statement that specifies use of the plugin for authentication.

The plugin developer determines the syntax rules for the authentication string and implements the plugin according to those rules. Suppose that a plugin takes a comma-separated list of pairs that map external users to MySQL users. For example:

CREATE USER ''@'%.example.com'
  IDENTIFIED WITH my_plugin AS 'extuser1=mysqlusera, extuser2=mysqluserb'
CREATE USER ''@'%.example.org'
  IDENTIFIED WITH my_plugin AS 'extuser1=mysqluserc, extuser2=mysqluserd'

When the server invokes a plugin to authenticate a client, it passes the appropriate authentication string to the plugin. The plugin is responsible to:

  1. Parse the string into its components to determine the mapping to use

  2. Compare the client user name to the mapping

  3. Return the proper MySQL user name

For example, if extuser2 connects from an example.com host, the server passes 'extuser1=mysqlusera, extuser2=mysqluserb' to the plugin, and the plugin should copy mysqluserb into authenticated_as, with a terminating null byte. If extuser2 connects from an example.org host, the server passes 'extuser1=mysqluserc, extuser2=mysqluserd', and the plugin should copy mysqluserd instead.

If there is no match in the mapping, the action depends on the plugin. If a match is required, the plugin likely will return an error. Or the plugin might simply return the client name; in this case, it should not change authenticated_as, and the server will not treat the client as a proxy.

The following example demonstrates how to handle proxy users using a plugin named auth_simple_proxy. Like the auth_simple plugin described earlier, auth_simple_proxy accepts any nonempty password as valid (and thus should not be used in production environments). In addition, it examines the auth_string authentication string member and uses these very simple rules for interpreting it:

  • If the string is empty, the plugin returns the user name as given and no proxying occurs. That is, the plugin leaves the value of authenticated_as unchanged.

  • If the string is nonempty, the plugin treats it as the name of the proxied user and copies it to authenticated_as so that proxying occurs.

For testing, set up one account that is not proxied according to the preceding rules, and one that is. This means that one account has no AS clause, and one includes an AS clause that names the proxied user:

CREATE USER 'plugin_user1'@'localhost'
  IDENTIFIED WITH auth_simple_proxy;
CREATE USER 'plugin_user2'@'localhost'
  IDENTIFIED WITH auth_simple_proxy AS 'proxied_user';

In addition, create an account for the proxied user and grant plugin_user2 the PROXY privilege for it:

CREATE USER 'proxied_user'@'localhost'
  IDENTIFIED BY 'proxied_user_pass';
GRANT PROXY
  ON 'proxied_user'@'localhost'
  TO 'plugin_user2'@'localhost';

Before the server invokes an authentication plugin, it sets authenticated_as to the client user name. To indicate that the user is a proxy, the plugin should set authenticated_as to the proxied user name. For auth_simple_proxy, this means that it must examine the auth_string value, and, if the value is nonempty, copy it to the authenticated_as member to return it as the name of the proxied user. In addition, when proxying occurs, the plugin sets the external_user member to the client user name; this becomes the value of the external_user system variable.

static int auth_simple_proxy_server (MYSQL_PLUGIN_VIO *vio,
                                     MYSQL_SERVER_AUTH_INFO *info)
{
  unsigned char *pkt;
  int pkt_len;

  /* read the password as null-terminated string, fail on error */
  if ((pkt_len= vio->read_packet(vio, &pkt)) < 0)
    return CR_ERROR;

  /* fail on empty password */
  if (!pkt_len || *pkt == '\0')
  {
    info->password_used= PASSWORD_USED_NO;
    return CR_ERROR;
  }

  /* accept any nonempty password */
  info->password_used= PASSWORD_USED_YES;

  /* if authentication string is nonempty, use as proxied user name */
  /* and use client name as external_user value */
  if (info->auth_string_length > 0)
  {
    strcpy (info->authenticated_as, info->auth_string);
    strcpy (info->external_user, info->user_name);
  }

  return CR_OK;
}

After a successful connection, the USER() function should indicate the connecting client user and host name, and CURRENT_USER() should indicate the account whose privileges apply during the session. The latter value should be the connecting user account if no proxying occurs or the proxied account if proxying does occur.

Compile and install the plugin, then test it. First, connect as plugin_user1:

shell> mysql --user=plugin_user1 --password=x

In this case, there should be no proxying:

mysql> SELECT USER(), CURRENT_USER(), @@proxy_user, @@external_user\G
*************************** 1. row ***************************
         USER(): plugin_user1@localhost
 CURRENT_USER(): plugin_user1@localhost
   @@proxy_user: NULL
@@external_user: NULL

Then connect as plugin_user2:

shell> mysql --user=plugin_user2 --password=x

In this case, plugin_user2 should be proxied to proxied_user:

mysql> SELECT USER(), CURRENT_USER(), @@proxy_user, @@external_user\G
*************************** 1. row ***************************
         USER(): plugin_user2@localhost
 CURRENT_USER(): proxied_user@localhost
   @@proxy_user: 'plugin_user2'@'localhost'
@@external_user: 'plugin_user2'@'localhost'

27.2.4.10 Writing Password-Validation Plugins

This section describes how to write a server-side password-validation plugin. The instructions are based on the source code in the plugin/password_validation directory of MySQL source distributions. The validate_password.cc source file in that directory implements the plugin named validate_password.

To write a password-validation plugin, include the following header file in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

#include <mysql/plugin_validate_password.h>

plugin_validate_password.h includes plugin.h, so you need not include the latter file explicitly. plugin.h defines the MYSQL_VALIDATE_PASSWORD_PLUGIN server plugin type and the data structures needed to declare the plugin. plugin_validate_password.h defines data structures specific to password-validation plugins.

A password-validation plugin, like any MySQL server plugin, has a general plugin descriptor (see Section 27.2.4.2.1, “Server Plugin Library and Plugin Descriptors”). In validate_password.cc, the general descriptor for validate_password looks like this:

mysql_declare_plugin(validate_password)
{
  MYSQL_VALIDATE_PASSWORD_PLUGIN,     /*   type                            */
  &validate_password_descriptor,      /*   descriptor                      */
  "validate_password",                /*   name                            */
  "Oracle Corporation",               /*   author                          */
  "check password strength",          /*   description                     */
  PLUGIN_LICENSE_GPL,
  validate_password_init,             /*   init function (when loaded)     */
  validate_password_deinit,           /*   deinit function (when unloaded) */
  0x0100,                             /*   version                         */
  NULL,
  validate_password_system_variables, /*   system variables                */
  NULL,
  0,
}
mysql_declare_plugin_end;

The name member (validate_password) indicates the name to use for references to the plugin in statements such as INSTALL PLUGIN or UNINSTALL PLUGIN. This is also the name displayed by INFORMATION_SCHEMA.PLUGINS or SHOW PLUGINS.

The general descriptor also refers to validate_password_system_variables, a structure that exposes several system variables to the SHOW VARIABLES statement:

static struct st_mysql_sys_var* validate_password_system_variables[]= {
  MYSQL_SYSVAR(length),
  MYSQL_SYSVAR(number_count),
  MYSQL_SYSVAR(mixed_case_count),
  MYSQL_SYSVAR(special_char_count),
  MYSQL_SYSVAR(policy),
  MYSQL_SYSVAR(dictionary_file),
  NULL
};

The validate_password_init initialization function reads the dictionary file if one was specified, and the validate_password_deinit function frees data structures associated with the file.

The validate_password_descriptor value in the general descriptor points to the type-specific descriptor. For password-validation plugins, this descriptor has the following structure:

struct st_mysql_validate_password
{
  int interface_version;
  /*
    This function returns TRUE for passwords which satisfy the password
    policy (as chosen by plugin variable) and FALSE for all other
    password
  */
  int (*validate_password)(mysql_string_handle password);
  /*
    This function returns the password strength (0-100) depending
    upon the policies
  */
  int (*get_password_strength)(mysql_string_handle password);
};

The type-specific descriptor has these members:

  • interface_version: By convention, type-specific plugin descriptors begin with the interface version for the given plugin type. The server checks interface_version when it loads the plugin to see whether the plugin is compatible with it. For password-validation plugins, the value of the interface_version member is MYSQL_VALIDATE_PASSWORD_INTERFACE_VERSION (defined in plugin_validate_password.h).

  • validate_password: A function that the server calls to test whether a password satisfies the current password policy. It returns 1 if the password is okay and 0 otherwise. The argument is the password, passed as a mysql_string_handle value. This data type is implemented by the mysql_string server service. For details, see the string_service.h and string_service.cc source files in the sql directory.

  • get_password_strength: A function that the server calls to assess the strength of a password. It returns a value from 0 (weak) to 100 (strong). The argument is the password, passed as a mysql_string_handle value.

For the validate_password plugin, the type-specific descriptor looks like this:

static struct st_mysql_validate_password validate_password_descriptor=
{
  MYSQL_VALIDATE_PASSWORD_INTERFACE_VERSION,
  validate_password,                         /* validate function          */
  get_password_strength                      /* validate strength function */
};

To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable). For the validate_password plugin, it is compiled and installed when you build MySQL from source. It is also included in binary distributions. The build process produces a shared object library with a name of validate_password.so (the .so suffix might differ depending on your platform).

To register the plugin at runtime, use this statement (adjust the .so suffix for your platform as necessary):

INSTALL PLUGIN validate_password SONAME 'validate_password.so';

For additional information about plugin loading, see Section 6.5.2, “Installing and Uninstalling Plugins”.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement. See Section 6.5.3, “Obtaining Server Plugin Information”.

While the validate_password plugin is installed, it exposes system variables that indicate the password-checking parameters:

mysql> SHOW VARIABLES LIKE 'validate_password%';
+--------------------------------------+--------+
| Variable_name                        | Value  |
+--------------------------------------+--------+
| validate_password_dictionary_file    |        |
| validate_password_length             | 8      |
| validate_password_mixed_case_count   | 1      |
| validate_password_number_count       | 1      |
| validate_password_policy             | MEDIUM |
| validate_password_special_char_count | 1      |
+--------------------------------------+--------+

For descriptions of these variables, see Section 7.5.3.2, “Password Validation Plugin Options and Variables”.

To disable the plugin after testing it, use this statement to unload it:

UNINSTALL PLUGIN validate_password;

27.2.4.11 Writing Protocol Trace Plugins

MySQL supports the use of protocol trace plugins: client-side plugins that implement tracing of communication between a client and the server that takes place using the client/server protocol. This capability can be used in MySQL 5.7.2 and up.

27.2.4.11.1 Using the Test Protocol Trace Plugin

MySQL includes a test protocol trace plugin that serves to illustrate the information available from such plugins, and as a guide to writing other protocol trace plugins. To see how the test plugin works, use a MySQL source distribution; binary distributions are built with the test plugin disabled.

Enable the test protocol trace plugin by configuring MySQL with the WITH_TEST_TRACE_PLUGIN CMake option enabled. This causes the test trace plugin to be built and MySQL client programs to load it, but the plugin has no effect by default. Control the plugin using these environment variables:

  • MYSQL_TEST_TRACE_DEBUG: Set this variable to a value other than 0 to cause the test plugin to produce diagnostic output on stderr.

  • MYSQL_TEST_TRACE_CRASH: Set this variable to a value other than 0 to cause the test plugin to abort the client program if it detects an invalid trace event.

Caution

Diagnostic output from the test protocol trace plugin can disclose passwords and other sensitive information.

Given a MySQL installation built from source with the test plugin enabled, you can see a trace of the communication between the mysql client and the MySQL server as follows:

shell> export MYSQL_TEST_TRACE_DEBUG=1
shqll> mysql
test_trace: Test trace plugin initialized
test_trace: Starting tracing in stage CONNECTING
test_trace: stage: CONNECTING, event: CONNECTING
test_trace: stage: CONNECTING, event: CONNECTED
test_trace: stage: WAIT_FOR_INIT_PACKET, event: READ_PACKET
test_trace: stage: WAIT_FOR_INIT_PACKET, event: PACKET_RECEIVED
test_trace: packet received: 87 bytes
  0A 35 2E 37 2E 33 2D 6D  31 33 2D 64 65 62 75 67   .5.7.3-m13-debug
  2D 6C 6F 67 00 04 00 00  00 2B 7C 4F 55 3F 79 67   -log.....+|OU?yg
test_trace: 004: stage: WAIT_FOR_INIT_PACKET, event: INIT_PACKET_RECEIVED
test_trace: 004: stage: AUTHENTICATE, event: AUTH_PLUGIN
test_trace: 004: Using authentication plugin: mysql_native_password
test_trace: 004: stage: AUTHENTICATE, event: SEND_AUTH_RESPONSE
test_trace: 004: sending packet: 188 bytes
  85 A6 7F 00 00 00 00 01  21 00 00 00 00 00 00 00   .?......!.......
  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
...
mysql> quit
test_trace: 008: stage: READY_FOR_COMMAND, event: SEND_COMMAND
test_trace: 008: QUIT
test_trace: 008: stage: READY_FOR_COMMAND, event: PACKET_SENT
test_trace: 008: packet sent: 0 bytes
test_trace: 008: stage: READY_FOR_COMMAND, event: DISCONNECTED
test_trace: 008: Connection  closed
test_trace: 008: Tracing connection has ended
Bye
test_trace: Test trace plugin de-initialized

To disable trace output, do this:

shell> MYSQL_TEST_TRACE_DEBUG=
27.2.4.11.2 Using Your Own Protocol Trace Plugins
Note

To use your own protocol trace plugins, you must configure MySQL with the WITH_TEST_TRACE_PLUGIN CMake option disabled because only one protocol trace plugin can be loaded at a time and an error occurs for attempts to load a second one. If you have already built MySQL with the test protocol trace plugin enabled to see how it works, you must rebuild MySQL without it before you can use your own plugins.

This section discusses how to write a basic protocol trace plugin named simple_trace. This plugin provides a framework showing how to set up the client plugin descriptor and create the trace-related callback functions. In simple_trace, these functions are rudimentary and do little other than illustrate the arguments required. To see in detail how a trace plugin can make use of trace event information, check the source file for the test protocol trace plugin (test_trace_plugin.cc in the libmysql directory of a MySQL source distribution). However, note that the st_mysql_client_plugin_TRACE structure used there differs from the structures used with the usual client plugin declaration macros. In particular, the first two members are defined explicitly, not implicitly by declaration macros.

Several header files contain information relevant to protocol trace plugins:

  • client_plugin.h: Defines the API for client plugins. This includes the client plugin descriptor and function prototypes for client plugin C API calls (see Section 26.8.14, “C API Client Plugin Functions”).

  • plugin_trace.h: Contains declarations for client-side plugins of type MYSQL_CLIENT_TRACE_PLUGIN. It also contains descriptions of the permitted protocol stages, transitions between stages, and the types of events permitted at each stage.

To write a protocol trace plugin, include the following header files in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

#include <mysql/plugin_trace.h>
#include <mysql.h>

plugin_trace.h includes client_plugin.h, so you need not include the latter file explicitly.

Declare the client-side plugin descriptor with the mysql_declare_client_plugin() and mysql_end_client_plugin macros (see Section 27.2.4.2.3, “Client Plugin Descriptors”). For the simple_trace plugin, the descriptor looks like this:

mysql_declare_client_plugin(TRACE)
  "simple_trace",                 /* plugin name */
  "Author Name",                  /* author */
  "Simple protocol trace plugin", /* description */
  {1,0,0},                        /* version = 1.0.0 */
  "GPL",                          /* license type */
  NULL,                           /* for internal use */
  plugin_init,                    /* initialization function */
  plugin_deinit,                  /* deinitialization function */
  plugin_options,                 /* option-handling function */
  trace_start,                    /* start-trace function */
  trace_stop,                     /* stop-trace function */
  trace_event                     /* event-handling function */
mysql_end_client_plugin;

The descriptor members from the plugin name through the option-handling function are common to all client plugin types. The members following the common members implement trace event handling.

Function members for which the plugin needs no processing can be declared as NULL in the descriptor, in which case you need not write any corresponding function. For illustration purposes and to show the argument syntax, the following discussion implements all functions listed in the descriptor, even though some of them do nothing,

The initialization, deinitialization, and options functions common to all client plugins are declared as follows. For a description of the arguments and return values, see Section 27.2.4.2.3, “Client Plugin Descriptors”.

static int
plugin_init(char *errbuf, size_t errbuf_len, int argc, va_list args)
{
  return 0;
}

static int
plugin_deinit()
{
  return 0;
}

static int
plugin_options(const char *option, const void *value)
{
  return 0;
}

The trace-specific members of the client plugin descriptor are callback functions. The following descriptions provide more detail on how they are used. Each has a first argument that is a pointer to the plugin instance in case your implementation needs to access it.

trace_start(): This function is called at the start of each traced connection (each connection that starts after the plugin is loaded). It is passed the connection handler and the protocol stage at which tracing starts. trace_start() allocates memory needed by the trace_event() function, if any, and returns a pointer to it. If no memory is needed, this function returns NULL.

static void*
trace_start(struct st_mysql_client_plugin_TRACE *self,
            MYSQL *conn,
            enum protocol_stage stage)
{
  struct st_trace_data *plugin_data= malloc(sizeof(struct st_trace_data));

  fprintf(stderr, "Initializing trace: stage %d\n", stage);
  if (plugin_data)
  {
    memset(plugin_data, 0, sizeof(struct st_trace_data));
    fprintf(stderr, "Trace initialized\n");
    return plugin_data;
  }
  fprintf(stderr, "Could not initialize trace\n");
  exit(1);
}

trace_stop(): This function is called when tracing of the connection ends. That usually happens when the connection is closed, but can happen earlier. For example, trace_event() can return a nonzero value at any time and that causes tracing of the connection to terminate. trace_stop() is then called even though the connection has not ended.

trace_stop() is passed the connection handler and a pointer to the memory allocated by trace_start() (NULL if none). If the pointer is non-NULL, trace_stop() should deallocate the memory. This function returns no value.

static void
trace_stop(struct st_mysql_client_plugin_TRACE *self,
           MYSQL *conn,
           void *plugin_data)
{
  fprintf(stderr, "Terminating trace\n");
  if (plugin_data)
    free(plugin_data);
}

trace_event(): This function is called for each event occurrence. It is passed a pointer to the memory allocated by trace_start() (NULL if none), the connection handler, the current protocol stage and event codes, and event data. This function returns 0 to continue tracing, nonzero if tracing should stop.

static int
trace_event(struct st_mysql_client_plugin_TRACE *self,
            void *plugin_data,
            MYSQL *conn,
            enum protocol_stage stage,
            enum trace_event event,
            struct st_trace_event_args args)
{
  fprintf(stderr, "Trace event received: stage %d, event %d\n", stage, event);
  if (event == TRACE_EVENT_DISCONNECTED)
    fprintf(stderr, "Connection closed\n");
  return 0;
}

The tracing framework shuts down tracing of the connection when the connection ends, so trace_event() should return nonzero only if you want to terminate tracing of the connection early. Suppose that you want to trace only connections for a certain MySQL account. After authentication, you can check the user name for the connection and stop tracing if it is not the user in whom you are interested.

For each call to trace_event(), the st_trace_event_args structure contains the event data. It has this definition:

struct st_trace_event_args
{
  const char           *plugin_name;
  int                   cmd;
  const unsigned char  *hdr;
  size_t                hdr_len;
  const unsigned char  *pkt;
  size_t                pkt_len;
};

For different event types, the st_trace_event_args structure contains the information described following. All lengths are in bytes. Unused members are set to 0/NULL.

AUTH_PLUGIN event:

plugin_name  The name of the plugin

SEND_COMMAND event:

cmd          The command code
hdr          Pointer to the command packet header
hdr_len      Length of the header
pkt          Pointer to the command arguments
pkt_len      Length of the arguments

Other SEND_xxx and xxx_RECEIVED events:

pkt          Pointer to the data sent or received
pkt_len      Length of the data

PACKET_SENT event:

pkt_len      Number of bytes sent

To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable).

After the plugin library file is compiled and installed in the plugin directory, you can test it easily by setting the LIBMYSQL_PLUGINS environment variable to the plugin name, which affects any client program that uses that variable. mysql is one such program:

shell> export LIBMYSQL_PLUGINS=simple_trace
shqll> mysql
Initializing trace: stage 0
Trace initialized
Trace event received: stage 0, event 1
Trace event received: stage 0, event 2
...
Welcome to the MySQL monitor.  Commands end with ; or \g.
Trace event received
Trace event received
...
mysql> SELECT 1;
Trace event received: stage 4, event 12
Trace event received: stage 4, event 16
...
Trace event received: stage 8, event 14
Trace event received: stage 8, event 15
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

mysql> quit
Trace event received: stage 4, event 12
Trace event received: stage 4, event 16
Trace event received: stage 4, event 3
Connection closed
Terminating trace
Bye

To stop the trace plugin from being loaded, do this:

shell> LIBMYSQL_PLUGINS=

It is also possible to write client programs that directly load the plugin. You can tell the client where the plugin directory is located by calling mysql_options() to set the MYSQL_PLUGIN_DIR option:

char *plugin_dir = "path_to_plugin_dir";

/* ... process command-line options ... */

mysql_options(&mysql, MYSQL_PLUGIN_DIR, plugin_dir);

Typically, the program will also accept a --plugin-dir option that enables users to override the default value.

Should a client program require lower-level plugin management, the client library contains functions that take an st_mysql_client_plugin argument. See Section 26.8.14, “C API Client Plugin Functions”.

27.2.4.12 Writing Keyring Plugins

MySQL Server supports a keyring service that enables internal server components and plugins to securely store sensitive information for later retrieval. This section describes how to write a server-side keyring plugin that can be used by service functions to perform key-management operations. For general keyring information, see Section 7.5.4, “The MySQL Keyring”.

The instructions here are based on the source code in the plugin/keyring directory of MySQL source distributions. The source files in that directory implement a plugin named keyring_file that uses a file local to the server host for data storage.

To write a keyring plugin, include the following header file in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

#include <mysql/plugin_keyring.h>

plugin_keyring.h includes plugin.h, so you need not include the latter file explicitly. plugin.h defines the MYSQL_KEYRING_PLUGIN server plugin type and the data structures needed to declare the plugin. plugin_keyring.h defines data structures specific to keyring plugins.

A keyring plugin, like any MySQL server plugin, has a general plugin descriptor (see Section 27.2.4.2.1, “Server Plugin Library and Plugin Descriptors”). In keyring.cc, the general descriptor for keyring_file looks like this:

mysql_declare_plugin(keyring_file)
{
  MYSQL_KEYRING_PLUGIN,     /* type                                     */
  &keyring_descriptor,      /* descriptor                               */
  "keyring_file",           /* name                                     */
  "Oracle Corporation",     /* author                                   */
  "store/fetch authentication data to/from a flat file", /* description */
  PLUGIN_LICENSE_GPL,
  keyring_init,             /* init function (when loaded)              */
  keyring_deinit,           /* deinit function (when unloaded)          */
  0x0100,                   /* version                                  */
  NULL,                     /* status variables                         */
  keyring_system_variables, /* system variables                         */
  NULL,
  0,
}
mysql_declare_plugin_end;

The name member (keyring_file) indicates the plugin name. This is the name displayed by INFORMATION_SCHEMA.PLUGINS or SHOW PLUGINS.

The general descriptor also refers to keyring_system_variables, a structure that exposes a system variable to the SHOW VARIABLES statement:

static struct st_mysql_sys_var *keyring_system_variables[]= {
  MYSQL_SYSVAR(data),
  NULL
};

The keyring_init initialization function creates the data file if it does not exist, then reads it and initializes the keystore. The keyring_deinit function frees data structures associated with the file.

The keyring_descriptor value in the general descriptor points to the type-specific descriptor. For keyring plugins, this descriptor has the following structure:

struct st_mysql_keyring
{
  int interface_version;
  my_bool (*mysql_key_store)(const char *key_id, const char *key_type,
                             const char* user_id, const void *key, size_t key_len);
  my_bool (*mysql_key_fetch)(const char *key_id, char **key_type,
                             const char *user_id, void **key, size_t *key_len);
  my_bool (*mysql_key_remove)(const char *key_id, const char *user_id);
  my_bool (*mysql_key_generate)(const char *key_id, const char *key_type,
                                const char *user_id, size_t key_len);
};

The type-specific descriptor has these members:

  • interface_version: By convention, type-specific plugin descriptors begin with the interface version for the given plugin type. The server checks interface_version when it loads the plugin to see whether the plugin is compatible with it. For keyring plugins, the value of the interface_version member is MYSQL_KEYRING_INTERFACE_VERSION (defined in plugin_keyring.h).

  • mysql_key_store: A function that obfuscates and stores a key in the keyring.

  • mysql_key_fetch: A function that deobfuscates and retrieves a key from the keyring.

  • mysql_key_remove: A function that removes a key from the keyring.

  • mysql_key_generate: A function that generates a new random key and stores it in the keyring.

For the keyring_file plugin, the type-specific descriptor looks like this:

static struct st_mysql_keyring keyring_descriptor=
{
  MYSQL_KEYRING_INTERFACE_VERSION,
  mysql_key_store,
  mysql_key_fetch,
  mysql_key_remove,
  mysql_key_generate
};

The mysql_key_xxx functions implemented by a keyring plugin are analogous to the my_key_xxx functions exposed by the keyring service API. For example, the mysql_key_store plugin function is analogous to the my_key_store keyring service function. For information about the arguments to keyring service functions and how they are used, see Section 27.3.2, “The Keyring Service”.

To compile and install a plugin library file, use the instructions in Section 27.2.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable). For the keyring_file plugin, it is compiled and installed when you build MySQL from source. It is also included in binary distributions. The build process produces a shared object library with a name of keyring_file.so (the .so suffix might differ depending on your platform).

Keyring plugins typically are loaded early during the server startup process so that they are available to built-in plugins and storage engines that might depend on them. For keyring_file, use these lines in the server my.cnf file (adjust the .so suffix for your platform as necessary):

[mysqld]
early-plugin-load=keyring_file.so

For additional information about plugin loading, see Section 6.5.2, “Installing and Uninstalling Plugins”.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement (see Section 6.5.3, “Obtaining Server Plugin Information”). For example:

mysql> SELECT PLUGIN_NAME, PLUGIN_STATUS FROM INFORMATION_SCHEMA.PLUGINS
    -> WHERE PLUGIN_NAME LIKE 'keyring%';
+--------------+---------------+
| PLUGIN_NAME  | PLUGIN_STATUS |
+--------------+---------------+
| keyring_file | ACTIVE        |
+--------------+---------------+

While the keyring_file plugin is installed, it exposes a system variable that indicates the location of the data file it uses for secure information storage:

mysql> SHOW VARIABLES LIKE 'keyring_file%';
+-------------------+----------------------------------+
| Variable_name     | Value                            |
+-------------------+----------------------------------+
| keyring_file_data | /usr/local/mysql/keyring/keyring |
+-------------------+----------------------------------+

For a description of the keyring_file_data variable, see Section 6.1.5, “Server System Variables”.

To disable the plugin after testing it, restart the server without an --early-plugin-load option that names the plugin.

27.3 MySQL Services for Plugins

MySQL server plugins have access to server services. The services interface exposes server functionality that plugins can call. It complements the plugin API and has these characteristics:

  • Services enable plugins to access code inside the server using ordinary function calls. Services are also available to user-defined functions (UDFs).

  • Services are portable and work on multiple platforms.

  • The interface includes a versioning mechanism so that service versions supported by the server can be checked at load time against plugin versions. Versioning protects against incompatibilities between the version of a service that the server provides and the version of the service expected or required by a plugin.

  • For information about plugins for testing plugin services, see Plugins for Testing Plugin Services, in The MySQL Test Framework, Version 2.0.

The plugin services interface differs from the plugin API as follows:

  • The plugin API enables plugins to be used by the server. The calling initiative lies with the server to invoke plugins. This enables plugins to extend server functionality or register to receive notifications about server processing.

  • The plugin services interface enables plugins to call code inside the server. The calling initiative lies with plugins to invoke service functions. This enables functionality already implemented in the server to be used by many plugins; they need not individually implement it themselves.

To determine what services exist and what functions they provide, look in the include/mysql directory of a MySQL source distribution. The relevant files are:

  • plugin.h includes services.h, which is the umbrella header that includes all available service-specific header files.

  • Service-specific headers have names of the form service_xxx.h.

Each service-specific header should contain comments that provide full usage documentation for a given service, including what service functions are available, their calling sequences, and return values.

Current services include the following, and others can be implemented:

  • locking_service: A service that implements locks with three attributes: Lock namespace, lock name, and lock mode. This locking interface is available at two levels: 1) As a C language interface, callable as a plugin service from server plugins or user-defined functions; 2) At the SQL level, as a set of user-defined functions that map onto calls to the service routines. For more information, see Section 27.3.1, “The Locking Service”.

  • my_plugin_log_service: A service that enables plugins to report errors and specify error messages. The server writes the messages to its error log.

  • my_snprintf: A string-formatting service that produces consistent results across platforms.

  • my_thd_scheduler: A service for plugins to select a thread scheduler.

  • mysql_keyring: A service for keyring storage. For more information, see Section 27.3.2, “The Keyring Service”.

  • mysql_password_policy: A service for password validation and strength checking.

  • mysql_string: A service for string manipulation.

  • security_context: A service that enables plugins to examine or manipulate thread security contexts. This service provides setter and getter routines to access attributes of the server Security_context class, which includes attributes such as login user and host, authenticated user and host, and client IP address.

  • thd_alloc: A memory-allocation service.

  • thd_wait: A service for plugins to report when they are going to sleep or stall.

For developers who wish to modify the server to add a new service, see MySQL Internals: MySQL Services for Plugins.

The remainder of this section describes how a plugin uses server functionality that is available as a service. See also the source for the daemon example plugin, which uses the my_snprintf service. Within a MySQL source distribution, that plugin is located in the plugin/daemon_example directory.

To use a service or services from within a plugin, the plugin source file must include the plugin.h header file to access service-related information:

#include <mysql/plugin.h>

This does not represent any additional setup cost. A plugin must include that file anyway because it contains definitions and structures that every plugin needs.

To access a service, a plugin calls service functions like any other function. For example, to format a string into a buffer for printing, call the my_snprintf() function provided by the service of the same name:

char buffer[BUFFER_SIZE];

my_snprintf(buffer, sizeof(buffer), format_string, argument_to_format, ...);

To report an error that the server will write to it error log, first choose an error level. mysql/service_my_plugin_log.h defines these levels:

enum plugin_log_level
{
  MY_ERROR_LEVEL,
  MY_WARNING_LEVEL,
  MY_INFORMATION_LEVEL
};

Then invoke my_plugin_log_message():

int my_plugin_log_message(MYSQL_PLUGIN *plugin, enum plugin_log_level level,
                          const char *format, ...);

For example:

my_plugin_log_message(plugin_ptr, MY_ERROR_LEVEL, "Cannot initialize plugin");

Some services for plugins may be provided by plugins and thus are available only if the service-providing plugin is loaded. Any MySQL component that uses such a service should check whether the service is available.

When you build your plugin, use the -lmysqlservices flag at link time to link in the libmysqlservices library. For example, for CMake, put this in the top-level CMakeLists.txt file:

FIND_LIBRARY(MYSQLSERVICES_LIB mysqlservices
 PATHS "${MYSQL_SRCDIR}/libservices" NO_DEFAULT_PATH)

Put this in the CMakeLists.txt file in the directory containing the plugin source:

# the plugin needs the mysql services library for error logging
TARGET_LINK_LIBRARIES (your_plugin_library_name ${MYSQLSERVICES_LIB})

27.3.1 The Locking Service

Distributions of MySQL 5.7.8 or higher provide a locking interface that is available at two levels:

  • As a C language interface, callable as a plugin service from server plugins or user-defined functions

  • At the SQL level, as a set of user-defined functions that map onto calls to the service routines

For general information about plugin services, see Section 27.3, “MySQL Services for Plugins”. For general information about user-defined functions, see Section 27.4.2, “Adding a New User-Defined Function”.

The locking interface has these characteristics:

  • Locks have three attributes: Lock namespace, lock name, and lock mode:

    • Locks are identified by the combination of namespace and lock name. The namespace enables different applications to use the same lock names without colliding by creating locks in separate namespaces. For example, if applications A and B use namespaces of ns1 and ns2, respectively, each application can use lock names lock1 and lock2 without interfering with the other application.

    • A lock mode is either read or write. Read locks are shared: If a session has a read lock on a given lock identifier, other sessions can acquire a read lock on the same identifier. Write locks are exclusive: If a session has a write lock on a given lock identifier, other sessions cannot acquire a read or write lock on the same identifier.

  • Namespace and lock names must be non-NULL, nonempty, and have a maximum length of 64 characters. A namespace or lock name specified as NULL, the empty string, or a string longer than 64 characters results in an ER_LOCKING_SERVICE_WRONG_NAME error.

  • The locking interface treats namespace and lock names as binary strings, so comparisons are case sensitive.

  • The locking interface provides functions to acquire locks and release locks. No special privilege is required to call these functions. Privilege checking is the responsibility of the calling application.

  • Locks can be waited for if not immediately available. Lock acquisition calls take an integer timeout value that indicates how many seconds to wait to acquire locks before giving up. If the timeout is reached without successful lock acquisition, an ER_LOCKING_SERVICE_TIMEOUT error occurs. If the timeout is 0, there is no waiting and the call produces an error if locks cannot be acquired immediately.

  • The locking interface detects deadlock between lock-acquisition calls in different sessions. In this case, the locking service chooses a caller and terminates its lock-acquisition request with an ER_LOCKING_SERVICE_DEADLOCK error. This error does not cause transactions to roll back. To choose a session in case of deadlock, the locking service prefers sessions that hold read locks over sessions that hold write locks.

  • A session can acquire multiple locks with a single lock-acquisition call. For a given call, lock acquisition is atomic: The call succeeeds if all locks are acquired. If acquisition of any lock fails, the call acquires no locks and fails, typically with an ER_LOCKING_SERVICE_TIMEOUT or ER_LOCKING_SERVICE_DEADLOCK error.

  • A session can acquire multiple locks for the same lock identifier (namespace and lock name combination). These lock instances can be read locks, write locks, or a mix of both.

  • Locks acquired within a session are released explicitly by calling a release-locks function, or implicitly when the session terminates (either normally or abnormally). Locks are not released when transactions commit or roll back.

  • Within a session, all locks for a given namespace when released are released together.

The interface provided by the locking service is distinct from that provided by GET_LOCK() and related SQL functions (see Section 13.20, “Miscellaneous Functions”). For example, GET_LOCK() does not implement namespaces and provides only exclusive locks, not distinct read and write locks.

27.3.1.1 The Locking Service C Interface

This section describes how to use the locking service C language interface. To use the UDF interface instead, see Section 27.3.1.2, “The Locking Service UDF Interface” For general characteristics of the locking service interface, see Section 27.3.1, “The Locking Service”. For general information about plugin services, see Section 27.3, “MySQL Services for Plugins”.

Source files that use the locking service should include this header file:

#include <mysql/service_locking.h>

To acquire one or more locks, call this function:

int mysql_acquire_locking_service_locks(MYSQL_THD opaque_thd,
                                        const char* lock_namespace,
                                        const char**lock_names,
                                        size_t lock_num,
                                        enum enum_locking_service_lock_type lock_type,
                                        unsigned long lock_timeout);

The arguments have these meanings:

  • opaque_thd: A thread handle. If specified as NULL, the handle for the current thread is used.

  • lock_namespace: A null-terminated string that indicates the lock namespace.

  • lock_names: An array of null-terminated strings that provides the names of the locks to acquire.

  • lock_num: The number of names in the lock_names array.

  • lock_type: The lock mode, either LOCKING_SERVICE_READ or LOCKING_SERVICE_WRITE to acquire read locks or write locks, respectively.

  • lock_timeout: An integer number of seconds to wait to acquire the locks before giving up.

To release locks acquired for a given namespace, call this function:

int mysql_release_locking_service_locks(MYSQL_THD opaque_thd,
                                        const char* lock_namespace);

The arguments have these meanings:

  • opaque_thd: A thread handle. If specified as NULL, the handle for the current thread is used.

  • lock_namespace: A null-terminated string that indicates the lock namespace.

Locks acquired or waited for by the locking service can be monitored at the SQL level using the Performance Schema. For details, see Section 27.3.1.2.3, “Locking Service Monitoring”.

27.3.1.2 The Locking Service UDF Interface

This section describes how to use the locking service user-defined function (UDF) interface. To use the C language interface instead, see Section 27.3.1.1, “The Locking Service C Interface” For general characteristics of the locking service interface, see Section 27.3.1, “The Locking Service”. For general information about user-defined functions, see Section 27.4.2, “Adding a New User-Defined Function”.

27.3.1.2.1 Installing or Uninstalling the UDF Locking Interface

The locking service routines described in Section 27.3.1.1, “The Locking Service C Interface” need not be installed because they are built into the server. The same is not true of the user-defined functions (UDFs) that map onto calls to the service routines: The UDFs must be installed before use. This section describes how to do that. For general information about UDF installation, see Section 27.4.2.5, “UDF Compiling and Installing”.

The locking service UDFs are implemented in a plugin library file located in the directory named by the plugin_dir system variable. The file base name is locking_service. The file name suffix differs per platform (for example, .so for Unix and Unix-like systems, .dll for Windows).

To install the locking service UDFs, use the CREATE FUNCTION statement (adjust the .so suffix for your platform as necessary):

CREATE FUNCTION service_get_read_locks RETURNS INT SONAME 'locking_service.so';
CREATE FUNCTION service_get_write_locks RETURNS INT SONAME 'locking_service.so';
CREATE FUNCTION service_release_locks RETURNS INT SONAME 'locking_service.so';

If the UDFs are used on a master replication server, install them on all slave servers as well to avoid replication problems.

Once installed, the UDFs remain installed until uninstalled. To remove them, use the DROP FUNCTION statement:

DROP FUNCTION service_get_read_locks;
DROP FUNCTION service_get_write_locks;
DROP FUNCTION service_release_locks;
27.3.1.2.2 Using the UDF Locking Interface

Before using the locking service UDFs, install them according to the instructions provided at Section 27.3.1.2.1, “Installing or Uninstalling the UDF Locking Interface”.

To acquire one or more read locks, call this function:

mysql> SELECT service_get_read_locks('mynamespace', 'rlock1', 'rlock2', 10);
+---------------------------------------------------------------+
| service_get_read_locks('mynamespace', 'rlock1', 'rlock2', 10) |
+---------------------------------------------------------------+
|                                                             1 |
+---------------------------------------------------------------+

The first argument is the lock namespace. The final argument is an integer timeout indicating how many seconds to wait to acquire the locks before giving up. The arguments in between are the lock names.

For the example just shown, the function acquires locks with lock identifiers (mynamespace, rlock1) and (mynamespace, rlock2).

To acquire write locks rather than read locks, call this function:

mysql> SELECT service_get_write_locks('mynamespace', 'wlock1', 'wlock2', 10);
+----------------------------------------------------------------+
| service_get_write_locks('mynamespace', 'wlock1', 'wlock2', 10) |
+----------------------------------------------------------------+
|                                                              1 |
+----------------------------------------------------------------+

In this case, the lock identifiers are (mynamespace, wlock1) and (mynamespace, wlock2).

To release all locks for a namespace, use this function:

mysql> SELECT service_release_locks('mynamespace');
+--------------------------------------+
| service_release_locks('mynamespace') |
+--------------------------------------+
|                                    1 |
+--------------------------------------+

Each locking function returns nonzero for success. If the function fails, an error occurs. For example, the following error occurs because lock names cannot be empty:

mysql> SELECT service_get_read_locks('mynamespace', '', 10);
ERROR 3131 (42000): Incorrect locking service lock name ''.

A session can acquire multiple locks for the same lock identifier. As long as a different session does not have a write lock for an identifier, the session can acquire any number of read or write locks. Each lock request for the identifier acquires a new lock. The following statements acquire three write locks with the same identifier, then three read locks for the same identifier:

SELECT service_get_write_locks('ns', 'lock1', 'lock1', 'lock1', 0);
SELECT service_get_read_locks('ns', 'lock1', 'lock1', 'lock1', 0);

If you examine the Performance Schema metadata_locks table at this point, you will find that the session holds six distinct locks with the same (ns, lock1) identifier. (For details, see Section 27.3.1.2.3, “Locking Service Monitoring”.)

Because the session holds at least one write lock on (ns, lock1), no other session can acquire a lock for it, either read or write. If the session held only read locks for the identifier, other sessions could acquire read locks for it, but not write locks.

Locks for a single lock-acquisition call are acquired atomically, but atomicity does not hold across calls. Thus, for a statement such as the following, where service_get_write_locks() is called once per row of the result set, atomicity holds for each individual call, but not for the statement as a whole:

SELECT service_get_write_locks('ns', 'lock1', 'lock2', 0) FROM t1 WHERE ... ;
Caution

Because the locking service returns a separate lock for each successful request for a given lock identifier, it is possible for a single statement to acquire a large number of locks. For example:

INSERT INTO ... SELECT service_get_write_locks('ns', t1.col_name, 0) FROM t1;

These types of statements may have certain adverse effects. For example, if the statement fails part way through and rolls back, locks acquired up to the point of failure will still exist. If the intent is for there to be a correspondence between rows inserted and locks acquired, that intent will not be satisfied. Also, if it is important that locks are granted in a certain order, be aware that result set order may differ depending on which execution plan the optimizer chooses. For these reasons, it may be best to limit applications to a single lock-acquisition call per statement.

27.3.1.2.3 Locking Service Monitoring

The locking service is implemented using the MySQL Server metadata locks framework, so you monitor locking service locks acquired or waited for by examining the Performance Schema metadata_locks table.

First, enable the metadata lock instrument:

mysql> UPDATE performance_schema.setup_instruments SET ENABLED = 'YES'
    -> WHERE NAME = 'wait/lock/metadata/sql/mdl';

Then acquire some locks and check the contents of the metadata_locks table:

mysql> SELECT service_get_write_locks('mynamespace', 'lock1', 0);
+----------------------------------------------------+
| service_get_write_locks('mynamespace', 'lock1', 0) |
+----------------------------------------------------+
|                                                  1 |
+----------------------------------------------------+
mysql> SELECT service_get_read_locks('mynamespace', 'lock2', 0);
+---------------------------------------------------+
| service_get_read_locks('mynamespace', 'lock2', 0) |
+---------------------------------------------------+
|                                                 1 |
+---------------------------------------------------+
mysql> SELECT OBJECT_TYPE, OBJECT_SCHEMA, OBJECT_NAME, LOCK_TYPE, LOCK_STATUS
    -> FROM performance_schema.metadata_locks
    -> WHERE OBJECT_TYPE = 'LOCKING SERVICE'\G
*************************** 1. row ***************************
  OBJECT_TYPE: LOCKING SERVICE
OBJECT_SCHEMA: mynamespace
  OBJECT_NAME: lock1
    LOCK_TYPE: EXCLUSIVE
  LOCK_STATUS: GRANTED
*************************** 2. row ***************************
  OBJECT_TYPE: LOCKING SERVICE
OBJECT_SCHEMA: mynamespace
  OBJECT_NAME: lock2
    LOCK_TYPE: SHARED
  LOCK_STATUS: GRANTED

Locking service locks have an OBJECT_TYPE value of LOCKING SERVICE. This is distinct from, for example, locks acquired with the GET_LOCK() function, which have an OBJECT_TYPE of USER LEVEL LOCK.

The lock namespace, name, and mode appear in the OBJECT_SCHEMA, OBJECT_NAME, and LOCK_TYPE columns. Read and write locks have LOCK_TYPE values of SHARED and EXCLUSIVE, respectively.

The LOCK_STATUS value is GRANTED for an acquired lock, PENDING for a lock that is being waited for. You will see PENDING if one session holds a write lock and another session is attempting to acquire a lock having the same identifier.

27.3.1.2.4 Locking Service UDF Interface Reference

The SQL interface to the locking service implements the user-defined functions described in this section. For usage examples, see Section 27.3.1.2.2, “Using the UDF Locking Interface”.

The functions share these characteristics:

  • The return value is nonzero for success. Otherwise, an error occurs.

  • Namespace and lock names must be non-NULL, nonempty, and have a maximum length of 64 characters.

  • Timeout values must be integers indicating how many seconds to wait to acquire locks before giving up with an error. If the timeout is 0, there is no waiting and the function produces an error if locks cannot be acquired immediately.

These locking service UDFs are available:

  • service_get_read_locks(namespace, lock_name[, lock_name] ..., timeout)

    Acquires one or more read (shared) locks in the given namespace using the given lock names, timing out with an error if the locks are not acquired within the given timeout value.

  • service_get_write_locks(namespace, lock_name[, lock_name] ..., timeout)

    Acquires one or more write (exclusive) locks in the given namespace using the given lock names, timing out with an error if the locks are not acquired within the given timeout value.

  • service_release_locks(namespace)

    For the given namespace, releases all locks that were acquired within the current session using service_get_read_locks() and service_get_write_locks().

    It is not an error for there to be no locks in the namespace.

27.3.2 The Keyring Service

MySQL Server supports a keyring service that enables internal server components and plugins to securely store sensitive information for later retrieval. This section describes how to use the keyring service functions to store, retrieve, and remove keys in the MySQL keyring keystore. An SQL interface to the keyring service functions is also available as a set of user-defined functions (UDFs); see Section 7.5.4.5, “General-Purpose Keyring Key-Management Functions”. For general keyring information, see Section 7.5.4, “The MySQL Keyring”.

The keyring service uses whatever underlying keyring plugin is enabled, if any. If no keyring plugin is enabled, keyring service calls fail.

A record in the keystore consists of data (the key itself) and a unique identifier through which the key is accessed. The identifier has two parts:

  • key_id: The key ID or name. key_id values that begin with mysql_ are reserved by MySQL Server.

  • user_id: The session effective user ID. If there is no user context, this value can be NULL. The value need not actually be a user; the meaning depends on the application.

    Functions that implement the keyring UDF interface pass the value of CURRENT_USER() as the user_id value to keyring service functions.

The keyring service functions have these characteristics in common:

  • Each function returns 0 for success, 1 for failure.

  • The key_id and user_id arguments form a unique combination indicating which key in the keyring to use.

  • The key_type argument provides additional information about the key, such as its encryption method or intended use.

  • Keyring service functions treat key IDs, user names, types, and values as binary strings, so comparisons are case sensitive. For example, IDs of MyKey and mykey refer to different keys.

These keyring service functions are available:

  • my_key_fetch()

    Deobfuscates and retrieves a key from the keyring, along with its type. The function allocates the memory for the buffers used to store the returned key and key type. The caller should zero or obfuscate the memory when it is no longer needed, then free it.

    Syntax:

    my_bool my_key_fetch(const char *key_id, const char **key_type,
                         const char* user_id, void **key, size_t *key_len)
    

    Arguments:

    • key_id, user_id: Null-terminated strings that as a pair form a unique identifier indicating which key to fetch.

    • key_type: The address of a buffer pointer. The function stores into it a pointer to a null-terminated string that provides additional information about the key (stored when the key was added).

    • key: The address of a buffer pointer. The function stores into it a pointer to the buffer containing the fetched key data.

    • key_len: The address of a variable into which the function stores the size in bytes of the *key buffer.

    Return values:

    Returns 0 for success, 1 for failure.

  • my_key_generate()

    Generates a new random key of a given type and length and stores it in the keyring. The key has a length of key_len and is associated with the identifier formed from key_id and user_id. The type and length values must be consistent with the values supported by the underlying keyring plugin. See Section 7.5.4.4, “Supported Keyring Types”.

    Syntax:

    my_bool my_key_generate(const char *key_id, const char *key_type,
                            const char *user_id, size_t key_len)
    

    Arguments:

    • key_id, user_id: Null-terminated strings that as a pair form a unique identifier for the key to be generated.

    • key_type: A null-terminated string that provides additional information about the key.

    • key_len: The size in bytes of the key to be generated.

    Return values:

    Returns 0 for success, 1 for failure.

  • my_key_remove()

    Removes a key from the keyring.

    Syntax:

    my_bool my_key_remove(const char *key_id, const char* user_id)
    

    Arguments:

    • key_id, user_id: Null-terminated strings that as a pair form a unique identifier for the key to be removed.

    Return values:

    Returns 0 for success, 1 for failure.

  • my_key_store()

    Obfuscates and stores a key in the keyring.

    Syntax:

    my_bool my_key_store(const char *key_id, const char *key_type,
                         const char* user_id, void *key, size_t key_len)
    

    Arguments:

    • key_id, user_id: Null-terminated strings that as a pair form a unique identifier for the key to be stored.

    • key_type: A null-terminated string that provides additional information about the key.

    • key: The buffer containing the key data to be stored.

    • key_len: The size in bytes of the key buffer.

    Return values:

    Returns 0 for success, 1 for failure.

27.4 Adding New Functions to MySQL

There are three ways to add new functions to MySQL:

Each method of creating compiled functions has advantages and disadvantages:

  • If you write user-defined functions, you must install object files in addition to the server itself. If you compile your function into the server, you need not do that.

  • Native functions require you to modify a source distribution. UDFs do not. You can add UDFs to a binary MySQL distribution. No access to MySQL source is necessary.

  • If you upgrade your MySQL distribution, you can continue to use your previously installed UDFs, unless you upgrade to a newer version for which the UDF interface changes. For native functions, you must repeat your modifications each time you upgrade.

Whichever method you use to add new functions, they can be invoked in SQL statements just like native functions such as ABS() or SOUNDEX().

See Section 10.2.4, “Function Name Parsing and Resolution”, for the rules describing how the server interprets references to different kinds of functions.

The following sections describe features of the UDF interface, provide instructions for writing UDFs, discuss security precautions that MySQL takes to prevent UDF misuse, and describe how to add native MySQL functions.

For example source code that illustrates how to write UDFs, take a look at the sql/udf_example.cc file that is provided in MySQL source distributions.

27.4.1 Features of the User-Defined Function Interface

The MySQL interface for user-defined functions provides the following features and capabilities:

  • Functions can return string, integer, or real values and can accept arguments of those same types.

  • You can define simple functions that operate on a single row at a time, or aggregate functions that operate on groups of rows.

  • Information is provided to functions that enables them to check the number, types, and names of the arguments passed to them.

  • You can tell MySQL to coerce arguments to a given type before passing them to a function.

  • You can indicate that a function returns NULL or that an error occurred.

27.4.2 Adding a New User-Defined Function

For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading. MySQL source distributions include a file sql/udf_example.cc that defines five UDF functions. Consult this file to see how UDF calling conventions work. The include/mysql_com.h header file defines UDF-related symbols and data structures, although you need not include this header file directly; it is included by mysql.h.

A UDF contains code that becomes part of the running server, so when you write a UDF, you are bound by any and all constraints that apply to writing server code. For example, you may have problems if you attempt to use functions from the libstdc++ library. These constraints may change in future versions of the server, so it is possible that server upgrades will require revisions to UDFs that were originally written for older servers. For information about these constraints, see Section 2.9.4, “MySQL Source-Configuration Options”, and Section 2.9.5, “Dealing with Problems Compiling MySQL”.

To be able to use UDFs, you must link mysqld dynamically. If you want to use a UDF that needs to access symbols from mysqld (for example, the metaphone function in sql/udf_example.cc uses default_charset_info), you must link the program with -rdynamic (see man dlopen).

For each function that you want to use in SQL statements, you should define corresponding C (or C++) functions. In the following discussion, the name xxx is used for an example function name. To distinguish between SQL and C/C++ usage, XXX() (uppercase) indicates an SQL function call, and xxx() (lowercase) indicates a C/C++ function call.

Note

When using C++ you can encapsulate your C functions within:

extern "C" { ... }

This ensures that your C++ function names remain readable in the completed UDF.

The following list describes the C/C++ functions that you write to implement the interface for a function named XXX(). The main function, xxx(), is required. In addition, a UDF requires at least one of the other functions described here, for reasons discussed in Section 27.4.2.6, “UDF Security Precautions”.

  • xxx()

    The main function. This is where the function result is computed. The correspondence between the SQL function data type and the return type of your C/C++ function is shown here.

    SQL TypeC/C++ Type
    STRINGchar *
    INTEGERlong long
    REALdouble

    It is also possible to declare a DECIMAL function, but currently the value is returned as a string, so you should write the UDF as though it were a STRING function. ROW functions are not implemented.

  • xxx_init()

    The initialization function for xxx(). If present, it can be used for the following purposes:

    • To check the number of arguments to XXX().

    • To verify that the arguments are of a required type or, alternatively, to tell MySQL to coerce arguments to the required types when the main function is called.

    • To allocate any memory required by the main function.

    • To specify the maximum length of the result.

    • To specify (for REAL functions) the maximum number of decimal places in the result.

    • To specify whether the result can be NULL.

  • xxx_deinit()

    The deinitialization function for xxx(). If present, it should deallocate any memory allocated by the initialization function.

When an SQL statement invokes XXX(), MySQL calls the initialization function xxx_init() to let it perform any required setup, such as argument checking or memory allocation. If xxx_init() returns an error, MySQL aborts the SQL statement with an error message and does not call the main or deinitialization functions. Otherwise, MySQL calls the main function xxx() once for each row. After all rows have been processed, MySQL calls the deinitialization function xxx_deinit() so that it can perform any required cleanup.

For aggregate functions that work like SUM(), you must also provide the following functions:

  • xxx_clear()

    Reset the current aggregate value but do not insert the argument as the initial aggregate value for a new group.

  • xxx_add()

    Add the argument to the current aggregate value.

MySQL handles aggregate UDFs as follows:

  1. Call xxx_init() to let the aggregate function allocate any memory it needs for storing results.

  2. Sort the table according to the GROUP BY expression.

  3. Call xxx_clear() for the first row in each new group.

  4. Call xxx_add() for each row that belongs in the same group.

  5. Call xxx() to get the result for the aggregate when the group changes or after the last row has been processed.

  6. Repeat steps 3 to 5 until all rows has been processed

  7. Call xxx_deinit() to let the UDF free any memory it has allocated.

All functions must be thread-safe. This includes not just the main function, but the initialization and deinitialization functions as well, and also the additional functions required by aggregate functions. A consequence of this requirement is that you are not permitted to allocate any global or static variables that change! If you need memory, you should allocate it in xxx_init() and free it in xxx_deinit().

27.4.2.1 UDF Calling Sequences for Simple Functions

This section describes the different functions that you need to define when you create a simple UDF. Section 27.4.2, “Adding a New User-Defined Function”, describes the order in which MySQL calls these functions.

The main xxx() function should be declared as shown in this section. Note that the return type and parameters differ, depending on whether you declare the SQL function XXX() to return STRING, INTEGER, or REAL in the CREATE FUNCTION statement:

For STRING functions:

char *xxx(UDF_INIT *initid, UDF_ARGS *args,
          char *result, unsigned long *length,
          char *is_null, char *error);

For INTEGER functions:

long long xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

For REAL functions:

double xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

DECIMAL functions return string values and should be declared the same way as STRING functions. ROW functions are not implemented.

The initialization and deinitialization functions are declared like this:

my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);

void xxx_deinit(UDF_INIT *initid);

The initid parameter is passed to all three functions. It points to a UDF_INIT structure that is used to communicate information between functions. The UDF_INIT structure members follow. The initialization function should fill in any members that it wishes to change. (To use the default for a member, leave it unchanged.)

  • my_bool maybe_null

    xxx_init() should set maybe_null to 1 if xxx() can return NULL. The default value is 1 if any of the arguments are declared maybe_null.

  • unsigned int decimals

    The number of decimal digits to the right of the decimal point. The default value is the maximum number of decimal digits in the arguments passed to the main function. For example, if the function is passed 1.34, 1.345, and 1.3, the default would be 3, because 1.345 has 3 decimal digits.

    For arguments that have no fixed number of decimals, the decimals value is set to 31, which is 1 more than the maximum number of decimals permitted for the DECIMAL, FLOAT, and DOUBLE data types. This value is available as the constant NOT_FIXED_DEC in the mysql_com.h header file.

    A decimals value of 31 is used for arguments in cases such as a FLOAT or DOUBLE column declared without an explicit number of decimals (for example, FLOAT rather than FLOAT(10,3)) and for floating-point constants such as 1345E-3. It is also used for string and other nonnumber arguments that might be converted within the function to numeric form.

    The value to which the decimals member is initialized is only a default. It can be changed within the function to reflect the actual calculation performed. The default is determined such that the largest number of decimals of the arguments is used. If the number of decimals is NOT_FIXED_DEC for even one of the arguments, that is the value used for decimals.

  • unsigned int max_length

    The maximum length of the result. The default max_length value differs depending on the result type of the function. For string functions, the default is the length of the longest argument. For integer functions, the default is 21 digits. For real functions, the default is 13 plus the number of decimal digits indicated by initid->decimals. (For numeric functions, the length includes any sign or decimal point characters.)

    If you want to return a blob value, you can set max_length to 65KB or 16MB. This memory is not allocated, but the value is used to decide which data type to use if there is a need to temporarily store the data.

  • char *ptr

    A pointer that the function can use for its own purposes. For example, functions can use initid->ptr to communicate allocated memory among themselves. xxx_init() should allocate the memory and assign it to this pointer:

    initid->ptr = allocated_memory;
    

    In xxx() and xxx_deinit(), refer to initid->ptr to use or deallocate the memory.

  • my_bool const_item

    xxx_init() should set const_item to 1 if xxx() always returns the same value and to 0 otherwise.

27.4.2.2 UDF Calling Sequences for Aggregate Functions

This section describes the different functions that you need to define when you create an aggregate UDF. Section 27.4.2, “Adding a New User-Defined Function”, describes the order in which MySQL calls these functions.

  • xxx_reset()

    This function is called when MySQL finds the first row in a new group. It should reset any internal summary variables and then use the given UDF_ARGS argument as the first value in your internal summary value for the group. Declare xxx_reset() as follows:

    void xxx_reset(UDF_INIT *initid, UDF_ARGS *args,
                   char *is_null, char *error);
    

    xxx_reset() is not needed or used in MySQL 5.7, in which the UDF interface uses xxx_clear() instead. However, you can define both xxx_reset() and xxx_clear() if you want to have your UDF work with older versions of the server. (If you do include both functions, the xxx_reset() function in many cases can be implemented internally by calling xxx_clear() to reset all variables, and then calling xxx_add() to add the UDF_ARGS argument as the first value in the group.)

  • xxx_clear()

    This function is called when MySQL needs to reset the summary results. It is called at the beginning for each new group but can also be called to reset the values for a query where there were no matching rows. Declare xxx_clear() as follows:

    void xxx_clear(UDF_INIT *initid, char *is_null, char *error);
    

    is_null is set to point to CHAR(0) before calling xxx_clear().

    If something went wrong, you can store a value in the variable to which the error argument points. error points to a single-byte variable, not to a string buffer.

    xxx_clear() is required by MySQL 5.7.

  • xxx_add()

    This function is called for all rows that belong to the same group. You should use it to add the value in the UDF_ARGS argument to your internal summary variable.

    void xxx_add(UDF_INIT *initid, UDF_ARGS *args,
                 char *is_null, char *error);
    

The xxx() function for an aggregate UDF should be declared the same way as for a nonaggregate UDF. See Section 27.4.2.1, “UDF Calling Sequences for Simple Functions”.

For an aggregate UDF, MySQL calls the xxx() function after all rows in the group have been processed. You should normally never access its UDF_ARGS argument here but instead return a value based on your internal summary variables.

Return value handling in xxx() should be done the same way as for a nonaggregate UDF. See Section 27.4.2.4, “UDF Return Values and Error Handling”.

The xxx_reset() and xxx_add() functions handle their UDF_ARGS argument the same way as functions for nonaggregate UDFs. See Section 27.4.2.3, “UDF Argument Processing”.

The pointer arguments to is_null and error are the same for all calls to xxx_reset(), xxx_clear(), xxx_add() and xxx(). You can use this to remember that you got an error or whether the xxx() function should return NULL. You should not store a string into *error! error points to a single-byte variable, not to a string buffer.

*is_null is reset for each group (before calling xxx_clear()). *error is never reset.

If *is_null or *error are set when xxx() returns, MySQL returns NULL as the result for the group function.

27.4.2.3 UDF Argument Processing

The args parameter points to a UDF_ARGS structure that has the members listed here:

  • unsigned int arg_count

    The number of arguments. Check this value in the initialization function if you require your function to be called with a particular number of arguments. For example:

    if (args->arg_count != 2)
    {
        strcpy(message,"XXX() requires two arguments");
        return 1;
    }
    

    For other UDF_ARGS member values that are arrays, array references are zero-based. That is, refer to array members using index values from 0 to args->arg_count − 1.

  • enum Item_result *arg_type

    A pointer to an array containing the types for each argument. The possible type values are STRING_RESULT, INT_RESULT, REAL_RESULT, and DECIMAL_RESULT.

    To make sure that arguments are of a given type and return an error if they are not, check the arg_type array in the initialization function. For example:

    if (args->arg_type[0] != STRING_RESULT ||
        args->arg_type[1] != INT_RESULT)
    {
        strcpy(message,"XXX() requires a string and an integer");
        return 1;
    }
    

    Arguments of type DECIMAL_RESULT are passed as strings, so you should handle them the same way as STRING_RESULT values.

    As an alternative to requiring your function's arguments to be of particular types, you can use the initialization function to set the arg_type elements to the types you want. This causes MySQL to coerce arguments to those types for each call to xxx(). For example, to specify that the first two arguments should be coerced to string and integer, respectively, do this in xxx_init():

    args->arg_type[0] = STRING_RESULT;
    args->arg_type[1] = INT_RESULT;
    

    Exact-value decimal arguments such as 1.3 or DECIMAL column values are passed with a type of DECIMAL_RESULT. However, the values are passed as strings. If you want to receive a number, use the initialization function to specify that the argument should be coerced to a REAL_RESULT value:

    args->arg_type[2] = REAL_RESULT;
    
  • char **args

    args->args communicates information to the initialization function about the general nature of the arguments passed to your function. For a constant argument i, args->args[i] points to the argument value. (See later for instructions on how to access the value properly.) For a nonconstant argument, args->args[i] is 0. A constant argument is an expression that uses only constants, such as 3 or 4*7-2 or SIN(3.14). A nonconstant argument is an expression that refers to values that may change from row to row, such as column names or functions that are called with nonconstant arguments.

    For each invocation of the main function, args->args contains the actual arguments that are passed for the row currently being processed.

    If argument i represents NULL, args->args[i] is a null pointer (0). If the argument is not NULL, functions can refer to it as follows:

    • An argument of type STRING_RESULT is given as a string pointer plus a length, to enable handling of binary data or data of arbitrary length. The string contents are available as args->args[i] and the string length is args->lengths[i]. Do not assume that the string is null-terminated.

    • For an argument of type INT_RESULT, you must cast args->args[i] to a long long value:

      long long int_val;
      int_val = *((long long*) args->args[i]);
      
    • For an argument of type REAL_RESULT, you must cast args->args[i] to a double value:

      double    real_val;
      real_val = *((double*) args->args[i]);
      
    • For an argument of type DECIMAL_RESULT, the value is passed as a string and should be handled like a STRING_RESULT value.

    • ROW_RESULT arguments are not implemented.

  • unsigned long *lengths

    For the initialization function, the lengths array indicates the maximum string length for each argument. You should not change these. For each invocation of the main function, lengths contains the actual lengths of any string arguments that are passed for the row currently being processed. For arguments of types INT_RESULT or REAL_RESULT, lengths still contains the maximum length of the argument (as for the initialization function).

  • char *maybe_null

    For the initialization function, the maybe_null array indicates for each argument whether the argument value might be null (0 if no, 1 if yes).

  • char **attributes

    args->attributes communicates information about the names of the UDF arguments. For argument i, the attribute name is available as a string in args->attributes[i] and the attribute length is args->attribute_lengths[i]. Do not assume that the string is null-terminated.

    By default, the name of a UDF argument is the text of the expression used to specify the argument. For UDFs, an argument may also have an optional [AS] alias_name clause, in which case the argument name is alias_name. The attributes value for each argument thus depends on whether an alias was given.

    Suppose that a UDF my_udf() is invoked as follows:

    SELECT my_udf(expr1, expr2 AS alias1, expr3 alias2);
    

    In this case, the attributes and attribute_lengths arrays will have these values:

    args->attributes[0] = "expr1"
    args->attribute_lengths[0] = 5
    
    args->attributes[1] = "alias1"
    args->attribute_lengths[1] = 6
    
    args->attributes[2] = "alias2"
    args->attribute_lengths[2] = 6
    
  • unsigned long *attribute_lengths

    The attribute_lengths array indicates the length of each argument name.

27.4.2.4 UDF Return Values and Error Handling

The initialization function should return 0 if no error occurred and 1 otherwise. If an error occurs, xxx_init() should store a null-terminated error message in the message parameter. The message is returned to the client. The message buffer is MYSQL_ERRMSG_SIZE characters long, but you should try to keep the message to less than 80 characters so that it fits the width of a standard terminal screen.

The return value of the main function xxx() is the function value, for long long and double functions. A string function should return a pointer to the result and set *length to the length (in bytes) of the return value. For example:

memcpy(result, "result string", 13);
*length = 13;

MySQL passes a buffer to the xxx() function using the result parameter. This buffer is sufficiently long to hold 255 characters, which can be multibyte characters. The xxx() function can store the result in this buffer if it fits, in which case the return value should be a pointer to the buffer. If the function stores the result in a different buffer, it should return a pointer to that buffer.

If your string function does not use the supplied buffer (for example, if it needs to return a string longer than 255 characters), you must allocate the space for your own buffer with malloc() in your xxx_init() function or your xxx() function and free it in your xxx_deinit() function. You can store the allocated memory in the ptr slot in the UDF_INIT structure for reuse by future xxx() calls. See Section 27.4.2.1, “UDF Calling Sequences for Simple Functions”.

To indicate a return value of NULL in the main function, set *is_null to 1:

*is_null = 1;

To indicate an error return in the main function, set *error to 1:

*error = 1;

If xxx() sets *error to 1 for any row, the function value is NULL for the current row and for any subsequent rows processed by the statement in which XXX() was invoked. (xxx() is not even called for subsequent rows.)

27.4.2.5 UDF Compiling and Installing

Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file sql/udf_example.cc that is included in MySQL source distributions.

If a UDF will be referred to in statements that will be replicated to slave servers, you must ensure that every slave also has the function available. Otherwise, replication will fail on the slaves when they attempt to invoke the function.

The immediately following instructions are for Unix. Instructions for Windows are given later in this section.

The udf_example.cc file contains the following functions:

  • metaphon() returns a metaphon string of the string argument. This is something like a soundex string, but it is more tuned for English.

  • myfunc_double() returns the sum of the ASCII values of the characters in its arguments, divided by the sum of the length of its arguments.

  • myfunc_int() returns the sum of the length of its arguments.

  • sequence([const int]) returns a sequence starting from the given number or 1 if no number has been given.

  • lookup() returns the IP address for a host name.

  • reverse_lookup() returns the host name for an IP address. The function may be called either with a single string argument of the form 'xxx.xxx.xxx.xxx' or with four numbers.

  • avgcost() returns an average cost. This is an aggregate function.

A dynamically loadable file should be compiled as a sharable library file, using a command something like this:

shell> gcc -shared -o udf_example.so udf_example.cc

If you are using gcc with CMake (which is how MySQL is configured), you should be able to create udf_example.so with a simpler command:

shell> make udf_example

After you compile a shared object containing UDFs, you must install it and tell MySQL about it. Compiling a shared object from udf_example.cc using gcc directly produces a file named udf_example.so. Copy the shared object to the server's plugin directory and name it udf_example.so. This directory is given by the value of the plugin_dir system variable.

On some systems, the ldconfig program that configures the dynamic linker does not recognize a shared object unless its name begins with lib. In this case you should rename a file such as udf_example.so to libudf_example.so.

On Windows, you can compile user-defined functions by using the following procedure:

  1. Obtain a MySQL source distribution. See Section 2.1.2, “How to Get MySQL”.

  2. Obtain the CMake build utility, if necessary, from http://www.cmake.org. (Version 2.6 or later is required).

  3. In the source tree, look in the sql directory. There are files named udf_example.def udf_example.cc there. Copy both files from this directory to your working directory.

  4. Create a CMake makefile (CMakeLists.txt) with these contents:

    PROJECT(udf_example)
    
    # Path for MySQL include directory
    INCLUDE_DIRECTORIES("c:/mysql/include")
    
    ADD_DEFINITIONS("-DHAVE_DLOPEN")
    ADD_LIBRARY(udf_example MODULE udf_example.cc udf_example.def)
    TARGET_LINK_LIBRARIES(udf_example wsock32)
    
  5. Create the VC project and solution files:

    cmake -G "<Generator>"
    

    Invoking cmake --help shows you a list of valid Generators.

  6. Create udf_example.dll:

    devenv udf_example.sln /build Release
    

After the shared library file has been installed, notify mysqld about the new functions with the following statements. If library files have a suffix different from .so on your system, substitute the correct suffix throughout (for example, .dll on Windows).

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME 'udf_example.so';
mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME 'udf_example.so';
mysql> CREATE FUNCTION sequence RETURNS INTEGER SONAME 'udf_example.so';
mysql> CREATE FUNCTION lookup RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE FUNCTION reverse_lookup
    ->        RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE AGGREGATE FUNCTION avgcost
    ->        RETURNS REAL SONAME 'udf_example.so';

Once installed, a function remains installed until it is uninstalled.

To delete functions, use DROP FUNCTION:

mysql> DROP FUNCTION metaphon;
mysql> DROP FUNCTION myfunc_double;
mysql> DROP FUNCTION myfunc_int;
mysql> DROP FUNCTION sequence;
mysql> DROP FUNCTION lookup;
mysql> DROP FUNCTION reverse_lookup;
mysql> DROP FUNCTION avgcost;

The CREATE FUNCTION and DROP FUNCTION statements update the func system table in the mysql database. The function's name, type and shared library name are saved in the table. You must have the INSERT or DELETE privilege for the mysql database to create or drop functions, respectively.

You should not use CREATE FUNCTION to add a function that has previously been created. If you need to reinstall a function, you should remove it with DROP FUNCTION and then reinstall it with CREATE FUNCTION. You would need to do this, for example, if you recompile a new version of your function, so that mysqld gets the new version. Otherwise, the server continues to use the old version.

An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION. All active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable.

27.4.2.6 UDF Security Precautions

MySQL takes several measures to prevent misuse of user-defined functions.

UDF library files cannot be placed in arbitrary directories. They must be located in the server's plugin directory. This directory is given by the value of the plugin_dir system variable.

To use CREATE FUNCTION or DROP FUNCTION, you must have the INSERT or DELETE privilege, respectively, for the mysql database. This is necessary because those statements add and delete rows from the mysql.func table.

UDFs should have at least one symbol defined in addition to the xxx symbol that corresponds to the main xxx() function. These auxiliary symbols correspond to the xxx_init(), xxx_deinit(), xxx_reset(), xxx_clear(), and xxx_add() functions. mysqld also supports an --allow-suspicious-udfs option that controls whether UDFs that have only an xxx symbol can be loaded. By default, the option is off, to prevent attempts at loading functions from shared library files other than those containing legitimate UDFs. If you have older UDFs that contain only the xxx symbol and that cannot be recompiled to include an auxiliary symbol, it may be necessary to specify the --allow-suspicious-udfs option. Otherwise, you should avoid enabling this capability.

27.4.3 Adding a New Native Function

To add a new native MySQL function, use the procedure described here, which requires that you use a source distribution. You cannot add native functions to a binary distribution because it is necessary to modify MySQL source code and compile MySQL from the modified source. If you migrate to another version of MySQL (for example, when a new version is released), you must repeat the procedure with the new version.

If the new native function will be referred to in statements that will be replicated to slave servers, you must ensure that every slave server also has the function available. Otherwise, replication will fail on the slaves when they attempt to invoke the function.

To add a new native function, follow these steps to modify source files in the sql directory:

  1. Create a subclass for the function in item_create.cc:

    • If the function takes a fixed number of arguments, create a subclass of Create_func_arg0, Create_func_arg1, Create_func_arg2, or Create_func_arg3, respectively, depending on whether the function takes zero, one, two, or three arguments. For examples, see the Create_func_uuid, Create_func_abs, Create_func_pow, and Create_func_lpad classes.

    • If the function takes a variable number of arguments, create a subclass of Create_native_func. For an example, see Create_func_concat.

  2. To provide a name by which the function can be referred to in SQL statements, register the name in item_create.cc by adding a line to this array:

    static Native_func_registry func_array[]
    

    You can register several names for the same function. For example, see the lines for "LCASE" and "LOWER", which are aliases for Create_func_lcase.

  3. In item_func.h, declare a class inheriting from Item_num_func or Item_str_func, depending on whether your function returns a number or a string.

  4. In item_func.cc, add one of the following declarations, depending on whether you are defining a numeric or string function:

    double   Item_func_newname::val()
    longlong Item_func_newname::val_int()
    String  *Item_func_newname::Str(String *str)
    

    If you inherit your object from any of the standard items (like Item_num_func), you probably only have to define one of these functions and let the parent object take care of the other functions. For example, the Item_str_func class defines a val() function that executes atof() on the value returned by ::str().

  5. If the function is nondeterministic, include the following statement in the item constructor to indicate that function results should not be cached:

    current_thd->lex->safe_to_cache_query=0;
    

    A function is nondeterministic if, given fixed values for its arguments, it can return different results for different invocations.

  6. You should probably also define the following object function:

    void Item_func_newname::fix_length_and_dec()
    

    This function should at least calculate max_length based on the given arguments. max_length is the maximum number of characters the function may return. This function should also set maybe_null = 0 if the main function cannot return a NULL value. The function can check whether any of the function arguments can return NULL by checking the arguments' maybe_null variable. Look at Item_func_mod::fix_length_and_dec for a typical example of how to do this.

All functions must be thread-safe. In other words, do not use any global or static variables in the functions without protecting them with mutexes.

If you want to return NULL from ::val(), ::val_int(), or ::str(), you should set null_value to 1 and return 0.

For ::str() object functions, there are additional considerations to be aware of:

  • The String *str argument provides a string buffer that may be used to hold the result. (For more information about the String type, take a look at the sql_string.h file.)

  • The ::str() function should return the string that holds the result, or (char*) 0 if the result is NULL.

  • All current string functions try to avoid allocating any memory unless absolutely necessary!

27.5 Debugging and Porting MySQL

This section helps you port MySQL to other operating systems. Do check the list of currently supported operating systems first. See http://www.mysql.com/support/supportedplatforms/database.html. If you have created a new port of MySQL, please let us know so that we can list it here and on our Web site (http://www.mysql.com/), recommending it to other users.

Note

If you create a new port of MySQL, you are free to copy and distribute it under the GPL license, but it does not make you a copyright holder of MySQL.

A working POSIX thread library is needed for the server.

To build MySQL from source, your system must satisfy the tool requirements listed at Section 2.9, “Installing MySQL from Source”.

Important

If you are trying to build MySQL 5.7 with icc on the IA64 platform, and need support for MySQL Cluster, you should first ensure that you are using icc version 9.1.043 or later. (For details, see Bug #21875.)

If you run into problems with a new port, you may have to do some debugging of MySQL! See Section 27.5.1, “Debugging a MySQL Server”.

Note

Before you start debugging mysqld, first get the test program mysys/thr_lock to work. This ensures that your thread installation has even a remote chance to work!

27.5.1 Debugging a MySQL Server

If you are using some functionality that is very new in MySQL, you can try to run mysqld with the --skip-new (which disables all new, potentially unsafe functionality). See Section B.5.3.3, “What to Do If MySQL Keeps Crashing”.

If mysqld does not want to start, verify that you have no my.cnf files that interfere with your setup! You can check your my.cnf arguments with mysqld --print-defaults and avoid using them by starting with mysqld --no-defaults ....

If mysqld starts to eat up CPU or memory or if it hangs, you can use mysqladmin processlist status to find out if someone is executing a query that takes a long time. It may be a good idea to run mysqladmin -i10 processlist status in some window if you are experiencing performance problems or problems when new clients cannot connect.

The command mysqladmin debug dumps some information about locks in use, used memory and query usage to the MySQL log file. This may help solve some problems. This command also provides some useful information even if you have not compiled MySQL for debugging!

If the problem is that some tables are getting slower and slower you should try to optimize the table with OPTIMIZE TABLE or myisamchk. See Chapter 6, MySQL Server Administration. You should also check the slow queries with EXPLAIN.

You should also read the OS-specific section in this manual for problems that may be unique to your environment. See Section 2.1, “General Installation Guidance”.

27.5.1.1 Compiling MySQL for Debugging

If you have some very specific problem, you can always try to debug MySQL. To do this you must configure MySQL with the -DWITH_DEBUG=1 option. You can check whether MySQL was compiled with debugging by doing: mysqld --help. If the --debug flag is listed with the options then you have debugging enabled. mysqladmin ver also lists the mysqld version as mysql ... --debug in this case.

If mysqld stops crashing when you configure it with the -DWITH_DEBUG=1 CMake option, you probably have found a compiler bug or a timing bug within MySQL. In this case, you can try to add -g using the CMAKE_C_FLAGS and CMAKE_CXX_FLAGS CMake options and not use -DWITH_DEBUG=1. If mysqld dies, you can at least attach to it with gdb or use gdb on the core file to find out what happened.

When you configure MySQL for debugging you automatically enable a lot of extra safety check functions that monitor the health of mysqld. If they find something unexpected, an entry is written to stderr, which mysqld_safe directs to the error log! This also means that if you are having some unexpected problems with MySQL and are using a source distribution, the first thing you should do is to configure MySQL for debugging! (The second thing is to send mail to a MySQL mailing list and ask for help. See Section 1.6.1, “MySQL Mailing Lists”. If you believe that you have found a bug, please use the instructions at Section 1.7, “How to Report Bugs or Problems”.

27.5.1.2 Creating Trace Files

If the mysqld server does not start or it crashes easily, you can try to create a trace file to find the problem.

To do this, you must have a mysqld that has been compiled with debugging support. You can check this by executing mysqld -V. If the version number ends with -debug, it is compiled with support for trace files. (On Windows, the debugging server is named mysqld-debug rather than mysqld.)

Start the mysqld server with a trace log in /tmp/mysqld.trace on Unix or \mysqld.trace on Windows:

shell> mysqld --debug

On Windows, you should also use the --standalone flag to not start mysqld as a service. In a console window, use this command:

C:\> mysqld-debug --debug --standalone

After this, you can use the mysql.exe command-line tool in a second console window to reproduce the problem. You can stop the mysqld server with mysqladmin shutdown.

The trace file can become very large! To generate a smaller trace file, you can use debugging options something like this:

mysqld --debug=d,info,error,query,general,where:O,/tmp/mysqld.trace

This only prints information with the most interesting tags to the trace file.

If you make a bug report about this, please only send the lines from the trace file to the appropriate mailing list where something seems to go wrong! If you cannot locate the wrong place, you can open a bug report and upload the trace file to the report, so that a MySQL developer can take a look at it. For instructions, see Section 1.7, “How to Report Bugs or Problems”.

The trace file is made with the DBUG package by Fred Fish. See Section 27.5.3, “The DBUG Package”.

27.5.1.3 Using WER with PDB to create a Windows crashdump

Program Database files (extension pdb) are included in the ZIP Archive Debug Binaries & Test Suite distribution of MySQL. These files provide information for debugging your MySQL installation in the event of a problem. This is a separate download from the standard MSI or Zip file.

Note

As of MySQL 5.7.6, the PDB files are available in a separate file labeled "ZIP Archive Debug Binaries & Test Suite".

The PDB file contains more detailed information about mysqld and other tools that enables more detailed trace and dump files to be created. You can use these with WinDbg or Visual Studio to debug mysqld.

Note

The older Dr. Watson debugging tool was removed in Microsoft Vista, with WinDbg being a common alternative.

For more information on PDB files, see Microsoft Knowledge Base Article 121366. For more information on the debugging options available, see Debugging Tools for Windows.

To use WinDbg, either install the full Windows Driver Kit (WDK) or install the standalone version.

Important

The .exe and .pbd files must be an exact match (both version number and MySQL server edition) or WinDBG will complain while attempting to load the symbols.

  1. To generate a minidump mysqld.dmp, enable the core-file option under the [mysqld] section in my.ini. Restart the MySQL server after making these changes.

  2. Create a directory to store the generated files, such as c:\symbols

  3. Determine the path to your windbg.exe executable using the Find GUI or from the command line, for example: dir /s /b windbg.exe -- a common default is C:\Program Files\Debugging Tools for Windows (x64)\windbg.exe

  4. Launch windbg.exe giving it the paths to mysqld-debug.exe, mysqld.pdb, mysqld.dmp, and the source code. Alternatively, pass in each path from the WinDbg GUI. For example:

    
    windbg.exe -i "C:\mysql-5.7.18-winx64\bin\"^
     -z "C:\mysql-5.7.18-winx64\data\mysqld.dmp"^
     -srcpath "E:\ade\mysql_archives\5.7\5.7.18\mysql-5.7.18"^
     -y "C:\mysql-5.7.18-winx64\bin;SRV*c:\symbols*http://msdl.microsoft.com/download/symbols"^
     -v -n -c "!analyze -vvvvv"
    
    
    Note

    The ^ character and newline are removed by the Windows command line processor, so be sure the spaces remain intact.

27.5.1.4 Debugging mysqld under gdb

On most systems you can also start mysqld from gdb to get more information if mysqld crashes.

With some older gdb versions on Linux you must use run --one-thread if you want to be able to debug mysqld threads. In this case, you can only have one thread active at a time. It is best to upgrade to gdb 5.1 because thread debugging works much better with this version!

NPTL threads (the new thread library on Linux) may cause problems while running mysqld under gdb. Some symptoms are:

  • mysqld hangs during startup (before it writes ready for connections).

  • mysqld crashes during a pthread_mutex_lock() or pthread_mutex_unlock() call.

In this case, you should set the following environment variable in the shell before starting gdb:

LD_ASSUME_KERNEL=2.4.1
export LD_ASSUME_KERNEL

When running mysqld under gdb, you should disable the stack trace with --skip-stack-trace to be able to catch segfaults within gdb.

Use the --gdb option to mysqld to install an interrupt handler for SIGINT (needed to stop mysqld with ^C to set breakpoints) and disable stack tracing and core file handling.

It is very hard to debug MySQL under gdb if you do a lot of new connections the whole time as gdb does not free the memory for old threads. You can avoid this problem by starting mysqld with thread_cache_size set to a value equal to max_connections + 1. In most cases just using --thread_cache_size=5' helps a lot!

If you want to get a core dump on Linux if mysqld dies with a SIGSEGV signal, you can start mysqld with the --core-file option. This core file can be used to make a backtrace that may help you find out why mysqld died:

shell> gdb mysqld core
gdb>   backtrace full
gdb>   quit

See Section B.5.3.3, “What to Do If MySQL Keeps Crashing”.

If you are using gdb 4.17.x or above on Linux, you should install a .gdb file, with the following information, in your current directory:

set print sevenbit off
handle SIGUSR1 nostop noprint
handle SIGUSR2 nostop noprint
handle SIGWAITING nostop noprint
handle SIGLWP nostop noprint
handle SIGPIPE nostop
handle SIGALRM nostop
handle SIGHUP nostop
handle SIGTERM nostop noprint

If you have problems debugging threads with gdb, you should download gdb 5.x and try this instead. The new gdb version has very improved thread handling!

Here is an example how to debug mysqld:

shell> gdb /usr/local/libexec/mysqld
gdb> run
...
backtrace full # Do this when mysqld crashes

Include the preceding output in a bug report, which you can file using the instructions in Section 1.7, “How to Report Bugs or Problems”.

If mysqld hangs, you can try to use some system tools like strace or /usr/proc/bin/pstack to examine where mysqld has hung.

strace /tmp/log libexec/mysqld

If you are using the Perl DBI interface, you can turn on debugging information by using the trace method or by setting the DBI_TRACE environment variable.

27.5.1.5 Using a Stack Trace

On some operating systems, the error log contains a stack trace if mysqld dies unexpectedly. You can use this to find out where (and maybe why) mysqld died. See Section 6.4.2, “The Error Log”. To get a stack trace, you must not compile mysqld with the -fomit-frame-pointer option to gcc. See Section 27.5.1.1, “Compiling MySQL for Debugging”.

A stack trace in the error log looks something like this:

mysqld got signal 11;
Attempting backtrace. You can use the following information
to find out where mysqld died. If you see no messages after
this, something went terribly wrong...

stack_bottom = 0x41fd0110 thread_stack 0x40000
mysqld(my_print_stacktrace+0x32)[0x9da402]
mysqld(handle_segfault+0x28a)[0x6648e9]
/lib/libpthread.so.0[0x7f1a5af000f0]
/lib/libc.so.6(strcmp+0x2)[0x7f1a5a10f0f2]
mysqld(_Z21check_change_passwordP3THDPKcS2_Pcj+0x7c)[0x7412cb]
mysqld(_ZN16set_var_password5checkEP3THD+0xd0)[0x688354]
mysqld(_Z17sql_set_variablesP3THDP4ListI12set_var_baseE+0x68)[0x688494]
mysqld(_Z21mysql_execute_commandP3THD+0x41a0)[0x67a170]
mysqld(_Z11mysql_parseP3THDPKcjPS2_+0x282)[0x67f0ad]
mysqld(_Z16dispatch_command19enum_server_commandP3THDPcj+0xbb7[0x67fdf8]
mysqld(_Z10do_commandP3THD+0x24d)[0x6811b6]
mysqld(handle_one_connection+0x11c)[0x66e05e]

If resolution of function names for the trace fails, the trace contains less information:

mysqld got signal 11;
Attempting backtrace. You can use the following information
to find out where mysqld died. If you see no messages after
this, something went terribly wrong...

stack_bottom = 0x41fd0110 thread_stack 0x40000
[0x9da402]
[0x6648e9]
[0x7f1a5af000f0]
[0x7f1a5a10f0f2]
[0x7412cb]
[0x688354]
[0x688494]
[0x67a170]
[0x67f0ad]
[0x67fdf8]
[0x6811b6]
[0x66e05e]

In the latter case, you can use the resolve_stack_dump utility to determine where mysqld died by using the following procedure:

  1. Copy the numbers from the stack trace to a file, for example mysqld.stack. The numbers should not include the surrounding square brackets:

    0x9da402
    0x6648e9
    0x7f1a5af000f0
    0x7f1a5a10f0f2
    0x7412cb
    0x688354
    0x688494
    0x67a170
    0x67f0ad
    0x67fdf8
    0x6811b6
    0x66e05e
    
  2. Make a symbol file for the mysqld server:

    shell> nm -n libexec/mysqld > /tmp/mysqld.sym
    

    If mysqld is not linked statically, use the following command instead:

    shell> nm -D -n libexec/mysqld > /tmp/mysqld.sym
    

    If you want to decode C++ symbols, use the --demangle, if available, to nm. If your version of nm does not have this option, you will need to use the c++filt command after the stack dump has been produced to demangle the C++ names.

  3. Execute the following command:

    shell> resolve_stack_dump -s /tmp/mysqld.sym -n mysqld.stack
    

    If you were not able to include demangled C++ names in your symbol file, process the resolve_stack_dump output using c++filt:

    shell> resolve_stack_dump -s /tmp/mysqld.sym -n mysqld.stack | c++filt
    

    This prints out where mysqld died. If that does not help you find out why mysqld died, you should create a bug report and include the output from the preceding command with the bug report.

    However, in most cases it does not help us to have just a stack trace to find the reason for the problem. To be able to locate the bug or provide a workaround, in most cases we need to know the statement that killed mysqld and preferably a test case so that we can repeat the problem! See Section 1.7, “How to Report Bugs or Problems”.

Newer versions of glibc stack trace functions also print the address as relative to the object. On glibc-based systems (Linux), the trace for a crash within a plugin looks something like:

plugin/auth/auth_test_plugin.so(+0x9a6)[0x7ff4d11c29a6]

To translate the relative address (+0x9a6) into a file name and line number, use this command:

shell> addr2line -fie auth_test_plugin.so 0x9a6
auth_test_plugin
mysql-trunk/plugin/auth/test_plugin.c:65

The addr2line utility is part of the binutils package on Linux.

On Solaris, the procedure is similar. The Solaris printstack() already prints relative addresses:

plugin/auth/auth_test_plugin.so:0x1510

To translate, use this command:

shell> gaddr2line -fie auth_test_plugin.so 0x1510
mysql-trunk/plugin/auth/test_plugin.c:88

Windows already prints the address, function name and line:

000007FEF07E10A4 auth_test_plugin.dll!auth_test_plugin()[test_plugin.c:72]

27.5.1.6 Using Server Logs to Find Causes of Errors in mysqld

Note that before starting mysqld with the general query log enabled, you should check all your tables with myisamchk. See Chapter 6, MySQL Server Administration.

If mysqld dies or hangs, you should start mysqld with the general query log enabled. See Section 6.4.3, “The General Query Log”. When mysqld dies again, you can examine the end of the log file for the query that killed mysqld.

If you use the default general query log file, the log is stored in the database directory as host_name.log In most cases it is the last query in the log file that killed mysqld, but if possible you should verify this by restarting mysqld and executing the found query from the mysql command-line tools. If this works, you should also test all complicated queries that did not complete.

You can also try the command EXPLAIN on all SELECT statements that takes a long time to ensure that mysqld is using indexes properly. See Section 14.8.2, “EXPLAIN Syntax”.

You can find the queries that take a long time to execute by starting mysqld with the slow query log enabled. See Section 6.4.5, “The Slow Query Log”.

If you find the text mysqld restarted in the error log file (normally named hostname.err) you probably have found a query that causes mysqld to fail. If this happens, you should check all your tables with myisamchk (see Chapter 6, MySQL Server Administration), and test the queries in the MySQL log files to see whether one fails. If you find such a query, try first upgrading to the newest MySQL version. If this does not help and you cannot find anything in the mysql mail archive, you should report the bug to a MySQL mailing list. The mailing lists are described at http://lists.mysql.com/, which also has links to online list archives.

If you have started mysqld with --myisam-recover-options, MySQL automatically checks and tries to repair MyISAM tables if they are marked as 'not closed properly' or 'crashed'. If this happens, MySQL writes an entry in the hostname.err file 'Warning: Checking table ...' which is followed by Warning: Repairing table if the table needs to be repaired. If you get a lot of these errors, without mysqld having died unexpectedly just before, then something is wrong and needs to be investigated further. See Section 6.1.4, “Server Command Options”.

When the server detects MyISAM table corruption, it writes additional information to the error log, such as the name and line number of the source file, and the list of threads accessing the table. Example: Got an error from thread_id=1, mi_dynrec.c:368. This is useful information to include in bug reports.

It is not a good sign if mysqld did die unexpectedly, but in this case, you should not investigate the Checking table... messages, but instead try to find out why mysqld died.

27.5.1.7 Making a Test Case If You Experience Table Corruption

The following procedure applies to MyISAM tables. For information about steps to take when encountering InnoDB table corruption, see Section 1.7, “How to Report Bugs or Problems”.

If you encounter corrupted MyISAM tables or if mysqld always fails after some update statements, you can test whether the issue is reproducible by doing the following:

  1. Stop the MySQL daemon with mysqladmin shutdown.

  2. Make a backup of the tables to guard against the very unlikely case that the repair does something bad.

  3. Check all tables with myisamchk -s database/*.MYI. Repair any corrupted tables with myisamchk -r database/table.MYI.

  4. Make a second backup of the tables.

  5. Remove (or move away) any old log files from the MySQL data directory if you need more space.

  6. Start mysqld with the binary log enabled. If you want to find a statement that crashes mysqld, you should start the server with the general query log enabled as well. See Section 6.4.3, “The General Query Log”, and Section 6.4.4, “The Binary Log”.

  7. When you have gotten a crashed table, stop the mysqld server.

  8. Restore the backup.

  9. Restart the mysqld server without the binary log enabled.

  10. Re-execute the statements with mysqlbinlog binary-log-file | mysql. The binary log is saved in the MySQL database directory with the name hostname-bin.NNNNNN.

  11. If the tables are corrupted again or you can get mysqld to die with the above command, you have found a reproducible bug. FTP the tables and the binary log to our bugs database using the instructions given in Section 1.7, “How to Report Bugs or Problems”. If you are a support customer, you can use the MySQL Customer Support Center (http://www.mysql.com/support/) to alert the MySQL team about the problem and have it fixed as soon as possible.

27.5.2 Debugging a MySQL Client

To be able to debug a MySQL client with the integrated debug package, you should configure MySQL with -DWITH_DEBUG=1. See Section 2.9.4, “MySQL Source-Configuration Options”.

Before running a client, you should set the MYSQL_DEBUG environment variable:

shell> MYSQL_DEBUG=d:t:O,/tmp/client.trace
shell> export MYSQL_DEBUG

This causes clients to generate a trace file in /tmp/client.trace.

If you have problems with your own client code, you should attempt to connect to the server and run your query using a client that is known to work. Do this by running mysql in debugging mode (assuming that you have compiled MySQL with debugging on):

shell> mysql --debug=d:t:O,/tmp/client.trace

This provides useful information in case you mail a bug report. See Section 1.7, “How to Report Bugs or Problems”.

If your client crashes at some 'legal' looking code, you should check that your mysql.h include file matches your MySQL library file. A very common mistake is to use an old mysql.h file from an old MySQL installation with new MySQL library.

27.5.3 The DBUG Package

The MySQL server and most MySQL clients are compiled with the DBUG package originally created by Fred Fish. When you have configured MySQL for debugging, this package makes it possible to get a trace file of what the program is doing. See Section 27.5.1.2, “Creating Trace Files”.

This section summarizes the argument values that you can specify in debug options on the command line for MySQL programs that have been built with debugging support. For more information about programming with the DBUG package, see the DBUG manual in the dbug directory of MySQL source distributions. It's best to use a recent distribution to get the most updated DBUG manual.

The DBUG package can be used by invoking a program with the --debug[=debug_options] or -# [debug_options] option. If you specify the --debug or -# option without a debug_options value, most MySQL programs use a default value. The server default is d:t:i:o,/tmp/mysqld.trace on Unix and d:t:i:O,\mysqld.trace on Windows. The effect of this default is:

  • d: Enable output for all debug macros

  • t: Trace function calls and exits

  • i: Add PID to output lines

  • o,/tmp/mysqld.trace, O,\mysqld.trace: Set the debug output file.

Most client programs use a default debug_options value of d:t:o,/tmp/program_name.trace, regardless of platform.

Here are some example debug control strings as they might be specified on a shell command line:

--debug=d:t
--debug=d:f,main,subr1:F:L:t,20
--debug=d,input,output,files:n
--debug=d:t:i:O,\\mysqld.trace

For mysqld, it is also possible to change DBUG settings at runtime by setting the debug system variable. This variable has global and session values:

mysql> SET GLOBAL debug = 'debug_options';
mysql> SET SESSION debug = 'debug_options';

Changes at runtime require the SUPER privilege, even for the session value.

The debug_options value is a sequence of colon-separated fields:

field_1:field_2:...:field_N

Each field within the value consists of a mandatory flag character, optionally preceded by a + or - character, and optionally followed by a comma-delimited list of modifiers:

[+|-]flag[,modifier,modifier,...,modifier]

The following table describes the permitted flag characters. Unrecognized flag characters are silently ignored.

Flag

Description

d

Enable output from DBUG_XXX macros for the current state. May be followed by a list of keywords, which enables output only for the DBUG macros with that keyword. An empty list of keywords enables output for all macros.

In MySQL, common debug macro keywords to enable are enter, exit, error, warning, info, and loop.

D

Delay after each debugger output line. The argument is the delay, in tenths of seconds, subject to machine capabilities. For example, D,20 specifies a delay of two seconds.

f

Limit debugging, tracing, and profiling to the list of named functions. An empty list enables all functions. The appropriate d or t flags must still be given; this flag only limits their actions if they are enabled.

F

Identify the source file name for each line of debug or trace output.

i

Identify the process with the PID or thread ID for each line of debug or trace output.

L

Identify the source file line number for each line of debug or trace output.

n

Print the current function nesting depth for each line of debug or trace output.

N

Number each line of debug output.

o

Redirect the debugger output stream to the specified file. The default output is stderr.

O

Like o, but the file is really flushed between each write. When needed, the file is closed and reopened between each write.

p

Limit debugger actions to specified processes. A process must be identified with the DBUG_PROCESS macro and match one in the list for debugger actions to occur.

P

Print the current process name for each line of debug or trace output.

r

When pushing a new state, do not inherit the previous state's function nesting level. Useful when the output is to start at the left margin.

S

Do function _sanity(_file_,_line_) at each debugged function until _sanity() returns something that differs from 0.

t

Enable function call/exit trace lines. May be followed by a list (containing only one modifier) giving a numeric maximum trace level, beyond which no output occurs for either debugging or tracing macros. The default is a compile time option.

The leading + or - character and trailing list of modifiers are used for flag characters such as d or f that can enable a debug operation for all applicable modifiers or just some of them:

  • With no leading + or -, the flag value is set to exactly the modifier list as given.

  • With a leading + or -, the modifiers in the list are added to or subtracted from the current modifier list.

The following examples show how this works for the d flag. An empty d list enabled output for all debug macros. A nonempty list enables output only for the macro keywords in the list.

These statements set the d value to the modifier list as given:

mysql> SET debug = 'd';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| d       |
+---------+
mysql> SET debug = 'd,error,warning';
mysql> SELECT @@debug;
+-----------------+
| @@debug         |
+-----------------+
| d,error,warning |
+-----------------+

A leading + or - adds to or subtracts from the current d value:

mysql> SET debug = '+d,loop';
mysql> SELECT @@debug;
+----------------------+
| @@debug              |
+----------------------+
| d,error,warning,loop |
+----------------------+
mysql> SET debug = '-d,error,loop';
mysql> SELECT @@debug;
+-----------+
| @@debug   |
+-----------+
| d,warning |
+-----------+

Adding to all macros enabled results in no change:

mysql> SET debug = 'd';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| d       |
+---------+
mysql> SET debug = '+d,loop';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| d       |
+---------+

Disabling all enabled macros disables the d flag entirely:

mysql> SET debug = 'd,error,loop';
mysql> SELECT @@debug;
+--------------+
| @@debug      |
+--------------+
| d,error,loop |
+--------------+
mysql> SET debug = '-d,error,loop';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
|         |
+---------+
Note

Prior to MySQL 5.7.2, the + and - modifiers were not always handled correctly and could leave a flag value in an incorrect state. Verify your debug-setting sequence in advance or set it without using + or -.