CODEWITHSUNDEEP
mysqlmariadblocking
Ralf Jahr
Ralf Jahr

Reputation: 331

Force-release a user-level lock in MySQL/MAriaDB

My session management (Zebra Session) uses user-level locks to avoid race conditions between two requests in the same session. To start the session, GET_LOCK is used. After closing the session, RELEASE_LOCK is used.

MariaDB [planner_20201026]> select GET_LOCK('session_ebe210e9b39f1ad3a409763be60efebff587aaaa', '5');
+-------------------------------------------------------------------+
| GET_LOCK('session_ebe210e9b39f1ad3a409763be60efebff587aaaa', '5') |
+-------------------------------------------------------------------+
|                                                                 1 |
+-------------------------------------------------------------------+
1 row in set (0.000 sec)

MariaDB [planner_20201026]> select  RELEASE_LOCK('session_ebe210e9b39f1ad3a409763be60efebff587aaa');
+-----------------------------------------------------------------+
| RELEASE_LOCK('session_ebe210e9b39f1ad3a409763be60efebff587aaa') |
+-----------------------------------------------------------------+
|                                                            NULL |
+-----------------------------------------------------------------+
1 row in set (0.000 sec)

Now I am in a situation because of a reason which I do not know yet where the lock was not released properly. GET_LOCK finishes because of the timeout, RELEASE_LOCK tells me that it cannot release the lock because it was (according to the documentation) established by another thread:

MariaDB [xyz]> select GET_LOCK('session_ebe210e9b39f1ad3a409763be60efebff587ac8b', '5');
+-------------------------------------------------------------------+
| GET_LOCK('session_ebe210e9b39f1ad3a409763be60efebff587ac8b', '5') |
+-------------------------------------------------------------------+
|                                                                 0 |
+-------------------------------------------------------------------+
1 row in set (5.015 sec)

MariaDB [xyz]> select  RELEASE_LOCK('session_ebe210e9b39f1ad3a409763be60efebff587ac8b');
+------------------------------------------------------------------+
| RELEASE_LOCK('session_ebe210e9b39f1ad3a409763be60efebff587ac8b') |
+------------------------------------------------------------------+
|                                                                0 |
+------------------------------------------------------------------+
1 row in set (0.000 sec)

The session is now more or less blocked/useless/doomed, each request takes TIMEOUT seconds extra.

Is there any chance how I can clear that lock, especially after a timeout?

Upvotes: 2

Views: 2083

Answers (1)

Bill Karwin
Bill Karwin

Reputation: 562368

You can only use RELEASE_LOCK() to release a lock acquired in the same thread. A thread has no privilege to force another thread to give up its lock.

That would be a pretty useless locking system if you could acquire a lock but any other thread could unilaterally force you to release it!

One way you could work around this is to call IS_USED_LOCK() to tell you which thread holds the lock. It returns the integer thread id of the holder, or NULL if the lock is not held by anyone.

Then if you have SUPER privilege, your thread can KILL that other thread, and this will force it to release its lock (as well as disconnecting that client). But that's a pretty rude thing to do.

I have a feeling this is an XY Problem. You are searching for a solution to force locks held by other threads to be released, but this is a bad solution because it doesn't solve your real problem.

The real problem is:

Now I am in a situation because of a reason which I do not know yet where the lock was not released properly.

You need to think harder about this and design a system where you do not lose track of who has acquired the lock.

Hint: GET_LOCK(name, 0) may help. This returns immediately (that is, with zero seconds of timeout). If the lock can be acquired, it is acquired, and the return value of GET_LOCK is 1. If it was already held by another thread, the GET_LOCK still returns immediately, but with a return value of 0, telling you that it could not be acquired.

Upvotes: 2

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