Syntax:
IS_FREE_LOCK(str)

Checks whether the lock named str is free to use (that is, not locked).
Returns 1 if the lock is free (no one is using the lock), 0 if the lock
is in use, and NULL if an error occurs (such as an incorrect argument).

URL: http://dev.mysql.com/doc/refman/5.5/en/miscellaneous-functions.html

