stasis/app: don't lock an app before a call to send

Calling 'app_send' eventually calls the app's message handler. It's possible
for a handler to obtain a lock on another object, and then need/want to lock
the app object. If the caller of 'app_send' locks the app object prior to
calling then there's a potential for a deadlock, if another thread calls
'app_send' without locking.

This patch makes it so 'app_send' is not called with the app object locked in
the section of code doing such.

ASTERISK-28423 #close

Change-Id: I6767c6d0933c7db1b984018966eefca4c0638a27
(cherry picked from commit 737bd8365e)

res/stasis/app.c

@@ -1126,8 +1126,23 @@ void app_update(struct stasis_app *app, stasis_app_cb handler, void *data)
 			"timestamp", ast_json_timeval(ast_tvnow(), NULL),
 			"application", app->name);
 		if (msg) {
+			/*
+			 * The app must be unlocked before calling 'send' since a handler may
+			 * subsequently attempt to grab the app lock after first obtaining a
+			 * lock for another object, thus causing a deadlock.
+			 */
+			ao2_unlock(app);
 			app_send(app, msg);
+			ao2_lock(app);
 			ast_json_unref(msg);
+			if (!app->handler) {
+				/*
+				 * If the handler disappeared then the app was deactivated. In that
+				 * case don't replace. Re-activation will reset the handler later.
+				 */
+				ao2_unlock(app);
+				return;
+			}
 		}
 	} else {
 		ast_verb(1, "Activating Stasis app '%s'\n", app->name);