* with a response, the object reported will be non-null. If the request's completion
* is implied by a barrier, the object reported will be null.
*
* with a response, the object reported will be non-null. If the request's completion
* is implied by a barrier, the object reported will be null.
*
* @param xid Previously-reserved XID
* @param message Message which should be sent out, or null if the reservation
* should be cancelled.
* @param xid Previously-reserved XID
* @param message Message which should be sent out, or null if the reservation
* should be cancelled.