Unsafe distributed lock with Redis

[Jack12816]

Hey guys,

I stumbled upon this gem while working on an updated test suite for activejob-locking. When I was using the Redis client/implementation my tests for parallel lock acquisition failed, so the lock was over-acquired. (Eg. 3 time parallel processes, on the same machine or on different machines, could acquire the same lock, each)

See: https://bit.ly/3LAT9cl

The issue is based on the Redis watch command in case there is no such key in the first place. To work around this a user must make sure to set the key (to an empty string) before any process wants to acquire the lock. This can be done at application boot or while deployments.

I set up a demo repository for reproducibility: https://github.com/Jack12816/suo-dlm-redis-bug

The Memcached client/implementation does not suffer from this issue as far as I can tell.

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See Redlock for a canonical Redis DLM algorithm as an alternative, in case you care about the safety guarantee that at any given moment, only one client can hold a lock.

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Proposal: Maybe we could add a hint about this in the readme?