"Dictionary changed size during iteration" in client.PubSub.on_connect()

Hi!

I’m using: celery==4.1.0 redis==2.10.6

On a fairly busy backend, and I’m seeing occasional ‘dictionary changed size during iteration’ (full trace below) errors during times of peak load. My guess is that this happens when PubSub.subscribe() and PubSub.on_connect() are called concurrently (thereby changing self.channels as on_connect() iterates over it.

https://github.com/andymccurdy/redis-py/blob/5109cb4f6b610e8d5949716a16435afbbf35075a/redis/client.py#L2376

RuntimeError: dictionary changed size during iteration File “celery/app/task.py”, line 413, in delay return self.apply_async(args, kwargs) File “celery/app/task.py”, line 536, in apply_async **options File “celery/app/base.py”, line 736, in send_task self.backend.on_task_call(P, task_id) File “celery/backends/redis.py”, line 189, in on_task_call self.result_consumer.consume_from(task_id) File “celery/backends/redis.py”, line 76, in consume_from self._consume_from(task_id) File “celery/backends/redis.py”, line 82, in _consume_from self._pubsub.subscribe(key) File “redis/client.py”, line 2482, in subscribe ret_val = self.execute_command(‘SUBSCRIBE’, *iterkeys(new_channels)) File “redis/client.py”, line 2404, in execute_command self._execute(connection, connection.send_command, *args) File “redis/client.py”, line 2415, in _execute connection.connect() File “redis/connection.py”, line 502, in connect callback(self) File “redis/client.py”, line 2374, in on_connect for k, v in iteritems(self.channels):

[marc1n]

This is probably race condition. PubSub object is not thread safe (as described here). Instance of PubSub class should be used in one thread only or access to it from multi threads must be synchronized.

I think the same rule applies to celery.backends.redis.RedisBackend object.

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