How to check the owner of a user defined lock using the IS_USED_LOCK() function?

IS_USED_LOCK(lock) is a MySQL built-in function that returns the connection id the lock associated with if the lock is in use, NULL otherwise. For example:

SELECT GET_LOCK('MyLock', 60), IS_USED_LOCK('MyLock');
  -- +------------------------+------------------------+
  -- | GET_LOCK('MyLock', 60) | IS_USED_LOCK('MyLock') |
  -- +------------------------+------------------------+
  -- |                      1 |                    307 |
  -- +------------------------+------------------------+

SELECT RELEASE_LOCK('MyLock'), IS_USED_LOCK('MyLock');
  -- +------------------------+------------------------+
  -- | RELEASE_LOCK('MyLock') | IS_USED_LOCK('MyLock') |
  -- +------------------------+------------------------+
  -- |                      1 |                   NULL |
  -- +------------------------+------------------------+

Reference information of the IS_USED_LOCK() function:

IS_USED_LOCK(lock): boolean
  Returns the connection id the lock associated with if the lock is in use,
  NULL otherwise.

Arguments, return value and availability:
  lock: Required. The lock name to be examined.
  boolean: Return value. Connection id the lock associated with 
    if the lock is in use, NULL otherwise.
  Available since MySQL 4.

⇒ ISNULL() - Detecting NULL Value

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⇑ MySQL Functions for Miscellaneous Purposes

⇑⇑ MySQL Function References