In 0-8..0-9-1 protocols, message acknowledgement (BasicAck) is async by design. There is no BasicAckOk. This presents a problem for JMS CLIENT_ACK mode. When Message#acknowledge() returns the spec requires that the Broker has processed the ack and the client won't see the message(s) again.
Currently calling Message#acknowledge() merely puts the command in buffer to be put on the wire later by the IoSender. There is no assurance that the Broker has even received the command let alone process it successfully.
The client should be changed to sync() once before returning the the client to give assurance that the Broker has received and processed the commands.
For the 0-8..0-9-1 code paths acknowledgement of messages in CLIENT_ACK mode might not happen immediately in Message#acknowledge. It should be updated to acknowledge messages synchronously. There will be a system property to allow the old behaviour to be restored if so desired.