MessageConsumer::receiveNoWait() may return NULL even if messages are available

Issue

This problem affects C/C++ applications that use the CMS library to consume from an ActiveMQ destination. Consider the following piece of code, which polls a destination for a message and, if none is available, does some other work and then delays for a short time:

 MessageConsumer *consumer = session->createConsumer (destination);
 Message *m = consumer->receiveNoWait();
 if (m)
   {
   // Process the message
   }
 else
   {
   // Do something else
   sleep (1); // Wait before polling again
   }

If a message is available, and if processing it is quick (milliseconds), then after the first successful call to receiveNoWait() another call will be made very rapidly. This second call can fail -- that is, return NULL even if messages are available. If the actions to be carried out in the event of a message not being available are lengthy (such as a the simple time delay shown above), then the whole application will run very slowly.

The problem can be demonstrated using a Java client in some circumstances, but it seems much less problematic in practice; possibly this is because the actual processing of the message in Java takes a bit longer.