Deadlock

[AndreiPashkin]

Here is current release Lua script:

if redis.call("get", KEYS[1]) == ARGV[1] then
    redis.call("del", KEYS[2])
    redis.call("lpush", KEYS[2], 1)
    return redis.call("del", KEYS[1])
else
    return 0
end

And here is acquire Python code:

while busy:
    busy = not self._client.set(self._name, self._id, nx=True, ex=self._expire)
    if busy:
        if timed_out:
            return False
        elif blocking:
            timed_out = not self._client.blpop(self._signal, blpop_timeout)
        else:
            logger.debug("Failed to get %r.", self._name)
            return False

Imagine such situation, with two workers:

• The first one acquired the lock previously.
• The second is blocked by blpop() call and waiting.
• Then worker-1 releases the lock, and Redis starts executing the Lua script and execution point comes to LPUSH and Redis executes LPUSH.
• After that worker-2 wakes up, tries to call SET NX, which doesn't succeeds, so worker-2 calls BLPOP again and becomes blocked.
• Then Redis continues execution of Lua script launched by worker-1 and calls DEL on lock's key, but it doesn't affects worker-2.

In the result - worker-2 is blocked forever.

[ionelmc]

In Redis scripts are atomic. Everything is ran serially. Has this changed recently or are you using some redis-compatible implementation that is multi-threaded?

[AndreiPashkin]

Indeed, I just didn't knew about that.