Understanding MySQL Client Plugin Descriptors: A Deep Dive into 4.4-2.3

MySQL client plugins extend the functionality of MySQL client applications. A key component of every plugin is its descriptor, defined in section 4.4-2.3 of the MySQL documentation. This descriptor provides essential information about the plugin to the MySQL client API. Let’s explore the structure and significance of these descriptors.

The Structure of a MySQL Client Plugin Descriptor

Each client plugin descriptor begins with a common set of members, followed by type-specific members. This common structure, st_mysql_client_plugin, is defined in the client_plugin.h file:

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 *);
};

Let’s break down each member:

• type: Identifies the plugin type (e.g., authentication, logging). Defined in client_plugin.h.
• interface_version: Specifies the plugin interface version. Ensures compatibility between the plugin and the MySQL client library.
• name: The plugin’s name, used to reference it (e.g., with --default-auth).
• author: The plugin author’s name.
• desc: A brief description of the plugin’s functionality.
• version: An array holding the plugin’s major, minor, and patch version numbers.
• license: Indicates the plugin’s licensing terms (e.g., GPL).
• mysql_api: Reserved for internal use; should be set to NULL.
• init: An initialization function executed when the plugin loads. Handles setup and returns an error message if initialization fails.
• deinit: A deinitialization function executed when the plugin unloads. Performs cleanup tasks.
• options: A function to handle plugin-specific options passed from the client.

Declaring a Client Plugin

Plugins are declared using the mysql_declare_client_plugin() and mysql_end_client_plugin macros:

mysql_declare_client_plugin(plugin_type)
  ... common members ...
  ... type-specific members ...
mysql_end_client_plugin;

The plugin_type argument automatically sets the type and interface_version members. For example:

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

This declares an authentication plugin named “my_auth_plugin.” Note the my_auth_main function, a type-specific member for authentication plugins, handling server communication.

Loading Client Plugins

Client programs load plugins using mysql_options() to set MYSQL_DEFAULT_AUTH and MYSQL_PLUGIN_DIR:

char *plugin_dir = "/path/to/plugin/dir";
char *default_auth = "my_auth_plugin";
mysql_options(&mysql, MYSQL_PLUGIN_DIR, plugin_dir);
mysql_options(&mysql, MYSQL_DEFAULT_AUTH, default_auth);

This code snippet directs the client to load the “my_auth_plugin” from the specified directory.

Conclusion

MySQL client plugin descriptors, detailed in section 4.4-2.3, play a vital role in extending MySQL client functionalities. They provide a structured way to define plugin properties and behaviors, enabling seamless integration with the MySQL ecosystem. Understanding their structure and purpose is crucial for developing and utilizing MySQL client plugins effectively.