Emit Xrun in case process-lock cannot be acquired

libs/ardour/audioengine.cc

@@ -260,7 +260,7 @@ AudioEngine::process_callback (pframes_t nframes)
 	if (!tm.locked()) {
 		/* return having done nothing */
 		if (_session) {
-			Xrun();
+			Xrun (); /* EMIT SIGNAL */
 		}
 		/* only JACK requires this (other backends clear the
 		 * output buffers before the process_callback.
@@ -343,6 +343,7 @@ AudioEngine::process_callback (pframes_t nframes)
 				 * to only use try-lock in the process callback.
 				 */
 				if (!tm.try_acquire ()) {
+					Xrun (); /* EMIT SIGNAL */
 					return 0; // XXX or spin?
 				}
 			}