AST-2017-013: chan_skinny: Call pthread_detach when sess threads end

chan_skinny creates a new thread for each new session.  In trying
to be a good cleanup citizen, the threads are joinable and the
unload_module function does a pthread_cancel() and a pthread_join()
on any sessions that are active at that time.  This has an
unintended side effect though. Since you can call pthread_join on a
thread that's already terminated, pthreads keeps the thread's
storage around until you explicitly call pthread_join (or
pthread_detach()).   Since only the module_unload function was
calling pthread_join, and even then only on the ones active at the
tme, the storage for every thread/session ever created sticks
around until asterisk exits.

* A thread can detach itself so the session_destroy() function
  now calls pthread_detach() just before it frees the session
  memory allocation.  The module_unload function still takes care
  of the ones that are still active should the module be unloaded.

ASTERISK-27452
Reported by: Juan Sacco

Change-Id: I9af7268eba14bf76960566f891320f97b974e6dd
(cherry picked from commit 8f5dff543e)

channels/chan_skinny.c

@@ -7426,6 +7426,11 @@ static void destroy_session(struct skinnysession *s)
 	}
 	ast_mutex_unlock(&s->lock);
 	ast_mutex_destroy(&s->lock);
+
+	if (s->t != AST_PTHREADT_NULL) {
+		pthread_detach(s->t);
+	}
+
 	ast_free(s);
 }
 
@@ -7512,11 +7517,6 @@ static void *skinny_session(void *data)
 	int eventmessage = 0;
 	struct pollfd fds[1];
 
-	if (!s) {
-		ast_log(LOG_WARNING, "Bad Skinny Session\n");
-		return 0;
-	}
-
 	ast_log(LOG_NOTICE, "Starting Skinny session from %s\n", ast_inet_ntoa(s->sin.sin_addr));
 
 	pthread_cleanup_push(skinny_session_cleanup, s);
@@ -7682,6 +7682,7 @@ static void *accept_thread(void *ignore)
 		s->keepalive_timeout_sched = -1;
 
 		if (ast_pthread_create(&s->t, NULL, skinny_session, s)) {
+			s->t = AST_PTHREADT_NULL;
 			destroy_session(s);
 		}
 	}