ADDED: is_message_queue/1 to test the existence of a message queue.

man/summary.doc

@@ -378,6 +378,7 @@ suggest predicates from a keyword.
 \predicatesummary{is_dict}{2}{Type check for a dict in a class}
 \predicatesummary{is_engine}{1}{Type check for an engine handle}
 \predicatesummary{is_list}{1}{Type check for a list}
+\predicatesummary{is_message_queue}{1}{Type check for a message queue}
 \predicatesummary{is_most_general_term}{1}{Type check for general term}
 \predicatesummary{is_object}{1}{WASM: Test JavaScript object}
 \predicatesummary{is_object}{2}{WASM: Test JavaScript object and class}

man/threads.doc

@@ -567,6 +567,12 @@ permission error is raised if \arg{Queue} refers to (the default queue
 of) a thread. Other threads that are waiting for \arg{Queue} using
 thread_get_message/2 receive an existence error.
 
+    \predicate[semidet]{is_message_queue}{1}{@Term}
+True if \arg{Term} refers to an existing message queue.  This predicate
+can not block and has no error conditions.  Note that message queues may
+be destroyed asynchronously by another thread and \jargon{anonymous}
+message queues may be garbage collected asynchronously.
+
     \predicate[det]{thread_get_message}{2}{+Queue, ?Term}
 As thread_get_message/1, operating on a given queue. It is allowed (but
 not advised) to get messages from the queue of other threads.  This

src/pl-thread.c

@@ -5478,6 +5478,28 @@ PRED_IMPL("message_queue_destroy", 1, message_queue_destroy, 0)
 }
 
 
+static
+PRED_IMPL("is_message_queue", 1, is_message_queue, 0)
+{ message_queue *q;
+  PL_blob_t *type;
+  void *data;
+  atom_t id;
+
+  if ( PL_get_blob(A1, &data, NULL, &type) && type == &message_queue_blob )
+  { mqref *ref = data;
+
+    q = ref->queue;
+    return (q && !q->destroyed);
+  }
+
+  if ( queueTable && PL_get_atom(A1, &id) )
+  { if ( (q = lookupHTableWP(queueTable, id)) )
+      return !q->destroyed;
+  }
+
+  return false;
+}
+
 		 /*******************************
 		 *    MESSAGE QUEUE PROPERTY	*
 		 *******************************/
@@ -8410,6 +8432,7 @@ BeginPredDefs(thread)
   PRED_DEF("message_queue_create",   2,	message_queue_create2, PL_FA_ISO)
   PRED_DEF("message_queue_property", 2,	message_property,      NDET|PL_FA_ISO)
   PRED_DEF("message_queue_set",      2, message_queue_set,     0)
+  PRED_DEF("is_message_queue",       1, is_message_queue,      0)
 
   PRED_DEF("thread_send_message",    2,	thread_send_message,   PL_FA_ISO)
   PRED_DEF("thread_send_message",    3,	thread_send_message,   0)