Logging SQL errors plugin.

Sometimes it's useful to collect errors set to clients in a log file, that can later be analyzed. The SQL_ERROR_LOG plugin added to the MariaDB 5.5 allows to do that. It implemented as the MYSQL_AUDIT_PLUGIN, so after installed the plugin receives error notification from the server and stores them to the specified file. The log file can be rotated.

The typical command to enable the SQL errlog plugin:

	install plugin SQL_ERROR_LOG soname 'sql_errlog.so';

or by adding --plugin-load=sql_errlog.so to the server's command line or to the [mysqld] section in your my.cnf file.

and to disable it:

	uninstall plugin SQL_ERROR_LOG;

The SQL_ERROR_LOG plugin declares 5 variables that control it's behaviour:

• @sql_error_log_filename - string value. The name of the logfile.
  rotation of it will be named like @sql_error_log_filename.001

• @sql_error_log_rate - integer value, the rate of logging.
  SET @sql_error_log_rate:=300; means that one of 300 errors will be written to the log.
  If the @sql_error_log_rate is 0 - the logging is disabled.
  The default rate is 1 (each error is logged).

• @sql_error_log_size_limit - integer value, the limitation for the size of the log file.
  After reaching the specified limit, the log file is rotated.
  1M limit set by default.

• @sql_error_log_rotations - integer value, the number of rotations.
  When rotated, the current log file is stored and the new empty one created.
  The @sql_error_log_rotations logs are stored, older are removed.

• @sql_error_log_rotate - boolean value.
  This is the 'write-only' variable. Assigning TRUE to this variable forces the log rotation.