Fix race condition in redis.lock.Lock.release #1031
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The release code wasn't using transaction correctly. It calls `watch`, which puts the pipeline into immediate execution mode, checks the current value of the key, then deletes it. At no stage does it call `multi` to put it back into pipeline mode. As a result, the command stack is empty and so no MULTI/EXEC is done, and the watch has no effect. This leads to a race condition when using expiring locks: - Client 1 acquires the lock, with a timeout - Client 1 calls release() - Client 1 watches the key - Client 1 gets the key, sees that it is still correct - Client 1's timeout expires, so the key disappears - Client 2 acquires the lock - Client 1 deletes the key - Client 3 also acquires the lock Now client 3 has stolen the lock from under client 2.
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Fixed in redis-py 3.0. LuaLock replaces Lock completely. |
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The release code wasn't using transaction correctly. It calls
watch,which puts the pipeline into immediate execution mode, checks the
current value of the key, then deletes it. At no stage does it call
multito put it back into pipeline mode. As a result, the commandstack is empty and so no MULTI/EXEC is done, and the watch has no
effect.
This leads to a race condition when using expiring locks:
Now client 3 has stolen the lock from under client 2.