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.