code: async def _process_ack(self, msg):
          
          pub_ack = protocol.PubAck()
          pub_ack.ParseFromString(msg.data)
  
          # Unblock pending acks queue if required.
          if not self._pending_pub_acks_queue.empty():
              await self._pending_pub_acks_queue.get()
  
          try:
              cb = self._pub_ack_map[pub_ack.guid]
              await cb(pub_ack)
              del self._pub_ack_map[pub_ack.guid]
          except KeyError:
              # Just skip the pub ack
              return
          except:
              # TODO: Check for protocol error
              return
docs: Receives acks from the publishes via the _STAN.acks subscription.