Commits on Oct 28, 2020

1. FIX: Only attempt to disconnect idle client instances …

   Waiting to acquire the mutex on the `::Redis` objects is likely to cause a deadlock in multi-threaded environments. For example:
   
   - Thread 1 and thread 2 have in-progress redis requests during a failover
   - Thread 1 is the first to raise an exception, triggers RailsFailover::Redis::Handler#disconnect_clients, acquiring the RailsFailover::Redis::Handler mutex
   - Thread 1 loops through all ::Redis instances, acquire their locks, and disconnect them
   - When it reaches the ::Redis instance used by Thread 2, Thread 1 waits for the mutex to become available
   
   - Meanwhile, Thread 2 raises an exception and triggers RailsFailover::Redis::Handler#disconnect_clients
   - Thread 2 waits for the RailsFailover::Redis::Handler mutex to become available
   
   This situation is never resolved, and the threads remained deadlocked.
   
   This commit will skip disconnecting `::Redis` clients which are currently locked by another thread. If the lock is held by another thread, it is fair to assume that a request is in process, it will fail, and the client will automatically disconnect very soon.

   

   davidtaylorhq committed Oct 28, 2020
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