chore: note it is safe to auto-reconnect

[bf4]

per redis-rb/redis-client#92 (comment)

[byroot]

I assume you mean unsafe if you're using call_once?

[bf4]

ah, comment got out of date with my code. I changed my mind that locking probably isn't idempotent and change the code from call to call_once didn't update the comment.

[bf4]

now the logic and comments are in sync

unlock/lock

# NOTE: is not idempotent and unsafe to retry
conn.call_once(...)

load_scripts/get remaining ttl

# NOTE: is idempotent and safe to retry
conn.call(...)

[bf4]

aside, interesting logic inside sidekiq where it defines redis(&block) https://github.com/sidekiq/sidekiq/blob/v7.0.5/lib/sidekiq/config.rb#L118-L133

    def redis
      raise ArgumentError, "requires a block" unless block_given?
      redis_pool.with do |conn|
        retryable = true
        begin
          yield conn
        rescue RedisClientAdapter::BaseError => ex
          # 2550 Failover can cause the server to become a replica, need
          # to disconnect and reopen the socket to get back to the primary.
          # 4495 Use the same logic if we have a "Not enough replicas" error from the primary
          # 4985 Use the same logic when a blocking command is force-unblocked
          # The same retry logic is also used in client.rb
          if retryable && ex.message =~ /READONLY|NOREPLICAS|UNBLOCKED/
            conn.close
            retryable = false
            retry
          end
          raise
        end
      end
    end

I'm doing some more comparisons since we got a flood of OpenSSL errors for about 10 minutes today which Heroku https://help.heroku.com/70UD7VSR/why-am-i-seeing-syscall-returned-5-errno-0-state-sslv3-tls-write-client-hello-when-trying-to-connect-to-heroku-redis reports as "because the maximum number of clients to Redis has been reached" which appears to be what happened

Nothing this PR would address, I don't think, but worth sharing in the upgrade changes discussion, I think.