fix: avoid race condition when reconnecting

[martinslota]

Background

We have been seeing occasional Connection is aborted errors when pausing a Bull queue that is backed by Redis in cluster mode. The investigation led to this fix in ioredis. After deploying that fix, the errors became a lot more rare, but they still pop up from time to time - sometimes days apart.

The debug logs from ioredis led me to believe that the issue may be around how bull re-establishes the blocking connection when pause() is called. Specifically, the promise returned by redisClientDisconnect may get resolved before this disconnect() call is executed, causing ioredis to abort its already ongoing connection attempt.

Attempts to reliably reproduce the bug have led to this repository where ioredis is intentionally modified to slightly delay the resolution of the promise returned from the Redis cluster client's quit() method in order to provoke the bug into occurring. All that is then necessary is to create a queue and repeatedly pause/resume it.

Change

Avoids calling client.disconnect() within redisClientDisconnect if the promise is already resolved at that point.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[martinslota]

I'd love to keep this pull request alive because the bug it fixes is affecting our production deployments and it generates quite a bit of noise.

@manast I apologize for cold-tagging you here - I'd appreciate any pointers or suggestions regarding how I could take this pull request further.