MySQL Essentials
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The store_lock() function is called before any reading or writing is performed.

Before adding the lock into the table lock handler mysqld calls store lock with the requested locks. Store lock can modify the lock level, for example change blocking write lock to non-blocking, ignore the lock (if we don't want to use MySQL table locks at all) or add locks for many tables (like we do when we are using a MERGE handler).

Berkeley DB, for example, downgrades blocking table TL_WRITE locks to non-blocking TL_WRITE_ALLOW_WRITE locks (which signals that we are doing WRITEs, but we are still allowing other readers and writers).

When releasing locks, store_lock() is also called. In this case, one usually doesn't have to do anything.

If the argument of store_lock is TL_IGNORE, it means that MySQL requests the handler to store the same lock level as the last time.

The potential lock types are defined in includes/thr_lock.h and are copied here:

enum thr_lock_type
{
         TL_IGNORE=-1,
         TL_UNLOCK,			             /* UNLOCK ANY LOCK */
 	  TL_READ,			               /* Read lock */
 	  TL_READ_WITH_SHARED_LOCKS,  
         TL_READ_HIGH_PRIORITY,      /* High prior. than TL_WRITE. Allow concurrent insert */
         TL_READ_NO_INSERT, 		     /* READ, Don't allow concurrent insert */
         TL_WRITE_ALLOW_WRITE, 		   /*	Write lock, but allow other threads to read / write. */
         TL_WRITE_ALLOW_READ,        /*	Write lock, but allow other threads to read / write. */
         TL_WRITE_CONCURRENT_INSERT, /* WRITE lock used by concurrent insert. */
         TL_WRITE_DELAYED,  		     /* Write used by INSERT DELAYED.  Allows READ locks */
         TL_WRITE_LOW_PRIORITY,	     /* WRITE lock that has lower priority than TL_READ */
         TL_WRITE,          		     /* Normal WRITE lock */
         TL_WRITE_ONLY               /* Abort new lock request with an error */
};  

Actual lock handling will vary depending on your locking implementation and you may choose to implement some or none of the requested lock types, substituting your own methods as appropriate. The following is the minimal implementation, for a storage engine that does not need to downgrade locks:

THR_LOCK_DATA **ha_tina::store_lock(THD *thd,
                                     THR_LOCK_DATA **to,
                                     enum thr_lock_type lock_type)
{
   if (lock_type != TL_IGNORE && lock.type == TL_UNLOCK)
     lock.type=lock_type;
   *to++= &lock;
   return to;
}  

See also ha_berkeley::store_lock() and ha_myisammrg::store_lock() for a more complex implementation.