Allow queue-high and queue-low events to be reset when messages have expired and the MQGET call returns no messages

When messages go to an MQ queue that eventually causes the Queue Depth High Event to be triggered, this enables the Queue Depth Low Event (as expected). These messages might then expire and will eventually be deleted once the queue is browsed. Currently when this happens, the Queue Depth Low Event never gets triggered and, as a result, the Queue Depth High Event never gets enabled.

IBM suggests that this is working as expected. “If an MQGET call completes with MQRC_NO_MSG_AVAILABLE and expired messages on the queue are processed along the way, event messages are not generated and the high/low depth threshold switches are not flipped.” It seems that the queue-high and queue-low thresholds would be reset only following a ‘successful' MQGET call and that, when no messages are returned because they've all expired, this does not class as a successful MQGET call. However, from a logical viewpoint, if messages have expiry set and by the time the MQGET call is issued they've expired, then this would appear to be MQ working as expected and thus it would not be unreasonable to expect the thresholds to be reset in these circumstances.

So, once the queue-high alert has been triggered due to a high-queue depth, this request is to allow the Queue-High event to be re-enabled and the Queue-Low event to be disabled automatically when the queue depth drops from above to below the queue-low threshold as a result of an MQGET call returning no messages due to messages having expiry set and where the time set has expired (which, as such, is not really an 'error' as this a perfectly normal outcome following an MQGET call).