documented handlerton structure

#define MIN_XID_LIST_SIZE  128

#define MAX_XID_LIST_SIZE  (1024*128)

/*

  handlerton is a singleton structure - one instance per storage engine -

  to provide access to storage engine functionality that works on

  "global" level (unlike handler class that works on per-table basis)

  usually handlerton instance is defined statically in ha_xxx.cc as

  static handlerton { ... } xxx_hton;

  savepoint_*, prepare, recover, and *_by_xid pointers can be 0.

*/

typedef struct

{

  /*

    each storage engine has it's own memory area (actually a pointer)

    in the thd, for storing per-connection information.

    It is accessed as

      thd->ha_data[xxx_hton.slot]

   slot number is initialized by MySQL after xxx_init() is called.

   */

   uint slot;

   /*

     to store per-savepoint data storage engine is provided with an area

     of a requested size (0 is ok here).

     savepoint_offset must be initialized statically to the size of

     the needed memory to store per-savepoint information.

     After xxx_init it is changed to be an offset to savepoint storage

     area and need not be used by storage engine.

     see binlog_hton and binlog_savepoint_set/rollback for an example.

   */

   uint savepoint_offset;

   /*

     handlerton methods:

     close_connection is only called if

     thd->ha_data[xxx_hton.slot] is non-zero, so even if you don't need

     this storage area - set it to something, so that MySQL would know

     this storage engine was accessed in this connection

   */

   int  (*close_connection)(THD *thd);

   /*

     sv points to an uninitialized storage area of requested size

     (see savepoint_offset description)

   */

   int  (*savepoint_set)(THD *thd, void *sv);

   /*

     sv points to a storage area, that was earlier passed

     to the savepoint_set call

   */

   int  (*savepoint_rollback)(THD *thd, void *sv);

   int  (*savepoint_release)(THD *thd, void *sv);

   /*

     'all' is true if it's a real commit, that makes persistent changes

     'all' is false if it's not in fact a commit but an end of the

     statement that is part of the transaction.

     NOTE 'all' is also false in auto-commit mode where 'end of statement'

     and 'real commit' mean the same event.

   */

   int  (*commit)(THD *thd, bool all);

   int  (*rollback)(THD *thd, bool all);

   int  (*prepare)(THD *thd, bool all);