# File lib/mail_room/arbitration/redis.rb, line 25
      def deliver?(uid)
        key = "delivered:#{uid}"

        incr = nil
        client.multi do |c|
          # At this point, `incr` is a future, which will get its value after
          # the MULTI command returns.
          incr = c.incr(key)

          c.expire(key, EXPIRATION)
        end

        # If INCR returns 1, that means the key didn't exist before, which means
        # we are the first mail_room to try to deliver this message, so we get to.
        # If we get any other value, another mail_room already (tried to) deliver
        # the message, so we don't have to anymore.
        incr.value == 1
      end