Resolve a race condition in Stasis

Because of the way that topics were handled when publishing, it was
possible to dispatch a message to a subscription after that
subscription had been unsubscribed such that the dispatched message
arrived at the callback after the callback had received its final
message. In callbacks that cleaned up user data, this would often cause
a segfault. This has been resolved by locking the topic during the
entirety of dispatch. To prevent long publishing and topic locking
times, forwarding subscriptions have been made to be standard
subscriptions instead of mailboxless subscriptions which were
dispatched at publishing time.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@383422 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/stasis.c

@@ -131,7 +131,7 @@ static void subscription_dtor(void *obj)
 
 static void send_subscription_change_message(struct stasis_topic *topic, char *uniqueid, char *description);
 
-static struct stasis_subscription *__stasis_subscribe(struct stasis_topic *topic, stasis_subscription_cb callback, void *data, int needs_mailbox)
+struct stasis_subscription *stasis_subscribe(struct stasis_topic *topic, stasis_subscription_cb callback, void *data)
 {
 	RAII_VAR(struct stasis_subscription *, sub, NULL, ao2_cleanup);
 	RAII_VAR(struct ast_uuid *, id, NULL, ast_free);
@@ -148,11 +148,10 @@ static struct stasis_subscription *__stasis_subscribe(struct stasis_topic *topic
 		return NULL;
 	}
 	ast_uuid_to_str(id, uniqueid, sizeof(uniqueid));
-	if (needs_mailbox) {
-		sub->mailbox = ast_threadpool_serializer(uniqueid, pool);
-		if (!sub->mailbox) {
-			return NULL;
-		}
+
+	sub->mailbox = ast_threadpool_serializer(uniqueid, pool);
+	if (!sub->mailbox) {
+		return NULL;
 	}
 
 	sub->uniqueid = ast_strdup(uniqueid);
@@ -170,11 +169,6 @@ static struct stasis_subscription *__stasis_subscribe(struct stasis_topic *topic
 	return sub;
 }
 
-struct stasis_subscription *stasis_subscribe(struct stasis_topic *topic, stasis_subscription_cb callback, void *data)
-{
-	return __stasis_subscribe(topic, callback, data, 1);
-}
-
 struct stasis_subscription *stasis_unsubscribe(struct stasis_subscription *sub)
 {
 	if (sub) {
@@ -338,58 +332,29 @@ static int dispatch_exec(void *data)
 
 void stasis_forward_message(struct stasis_topic *topic, struct stasis_topic *publisher_topic, struct stasis_message *message)
 {
-	struct stasis_subscription **subscribers = NULL;
-	size_t num_subscribers, i;
+	size_t i;
+	SCOPED_AO2LOCK(lock, topic);
 
 	ast_assert(topic != NULL);
 	ast_assert(publisher_topic != NULL);
 	ast_assert(message != NULL);
 
-	/* Copy the subscribers, so we don't have to hold the mutex for long */
-	{
-		SCOPED_AO2LOCK(lock, topic);
-		num_subscribers = topic->num_subscribers_current;
-		subscribers = ast_malloc(num_subscribers * sizeof(*subscribers));
-		if (subscribers) {
-			for (i = 0; i < num_subscribers; ++i) {
-				ao2_ref(topic->subscribers[i], +1);
-				subscribers[i] = topic->subscribers[i];
-			}
-		}
-	}
-
-	if (!subscribers) {
-		ast_log(LOG_ERROR, "Dropping message\n");
-		return;
-	}
-
-	for (i = 0; i < num_subscribers; ++i) {
-		struct stasis_subscription *sub = subscribers[i];
+	for (i = 0; i < topic->num_subscribers_current; ++i) {
+		struct stasis_subscription *sub = topic->subscribers[i];
+		RAII_VAR(struct dispatch *, dispatch, NULL, ao2_cleanup);
 
 		ast_assert(sub != NULL);
 
-		if (sub->mailbox) {
-			RAII_VAR(struct dispatch *, dispatch, NULL, ao2_cleanup);
-
-			dispatch = dispatch_create(publisher_topic, message, sub);
-			if (!dispatch) {
-				ast_log(LOG_DEBUG, "Dropping dispatch\n");
-				break;
-			}
-
-			if (ast_taskprocessor_push(sub->mailbox, dispatch_exec, dispatch) == 0) {
-				dispatch = NULL; /* Ownership transferred to mailbox */
-			}
-		} else {
-			/* No mailbox; dispatch directly */
-			sub->callback(sub->data, sub, sub->topic, message);
+		dispatch = dispatch_create(publisher_topic, message, sub);
+		if (!dispatch) {
+			ast_log(LOG_DEBUG, "Dropping dispatch\n");
+			break;
 		}
-	}
 
-	for (i = 0; i < num_subscribers; ++i) {
-		ao2_cleanup(subscribers[i]);
+		if (ast_taskprocessor_push(sub->mailbox, dispatch_exec, dispatch) == 0) {
+			dispatch = NULL; /* Ownership transferred to mailbox */
+		}
 	}
-	ast_free(subscribers);
 }
 
 void stasis_publish(struct stasis_topic *topic, struct stasis_message *message)
@@ -414,8 +379,8 @@ struct stasis_subscription *stasis_forward_all(struct stasis_topic *from_topic,
 	if (!from_topic || !to_topic) {
 		return NULL;
 	}
-	/* Subscribe without a mailbox, since we're just forwarding messages */
-	sub = __stasis_subscribe(from_topic, stasis_forward_cb, to_topic, 0);
+
+	sub = stasis_subscribe(from_topic, stasis_forward_cb, to_topic);
 	if (sub) {
 		/* hold a ref to to_topic for this forwarding subscription */
 		ao2_ref(to_topic, +1);