Prevent a potential three-thread deadlock. Also added a comment block

to explicitly state the locking order necessary inside app_queue.

(closes issue #11862)
Reported by: flujan
Patches:
      11862.patch uploaded by putnopvut (license 60)
	  Tested by: flujan



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@103120 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_queue.c

@@ -99,6 +99,20 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 #include "asterisk/astobj2.h"
 #include "asterisk/global_datastores.h"
 
+/* Please read before modifying this file.
+ * There are three locks which are regularly used
+ * throughout this file, the queue list lock, the lock
+ * for each individual queue, and the interface list lock.
+ * Please be extra careful to always lock in the following order
+ * 1) queue list lock
+ * 2) individual queue lock
+ * 3) interface list lock
+ * This order has sort of "evolved" over the lifetime of this
+ * application, but it is now in place this way, so please adhere
+ * to this order!
+ */
+
+
 enum {
 	QUEUE_STRATEGY_RINGALL = 0,
 	QUEUE_STRATEGY_ROUNDROBIN,
@@ -889,15 +903,16 @@ static int remove_from_interfaces(const char *interface)
 {
 	struct member_interface *curint;
 
+	if (interface_exists_global(interface))
+		return 0;
+
 	AST_LIST_LOCK(&interfaces);
 	AST_LIST_TRAVERSE_SAFE_BEGIN(&interfaces, curint, list) {
 		if (!strcasecmp(curint->interface, interface)) {
-			if (!interface_exists_global(interface)) {
 			if (option_debug)
 				ast_log(LOG_DEBUG, "Removing %s from the list of interfaces that make up all of our queue members.\n", interface);
 			AST_LIST_REMOVE_CURRENT(&interfaces, list);
 			free(curint);
-			}
 			break;
 		}
 	}
@@ -4248,9 +4263,9 @@ static int reload_queues(void)
 						continue;
 					}
 
-					remove_from_interfaces(cur->interface);
 					q->membercount--;
 					ao2_unlink(q->members, cur);
+					remove_from_interfaces(cur->interface);
 					ao2_ref(cur, -1);
 				}