Bridging: Use a ref to bridge_channel's channel to prevent crash.

There's a race condition with bridging where a bridge can be torn down
causing the bridge_channel's ast_channel to become NULL when it's still
needed. This particular case happened with attended transfers, but the
crash occurred when trying to publish a stasis message. Now, the
bridge_channel is locked, a ref to the ast_channel is obtained, and that
ref is passed down the chain.

Change-Id: Ic48715c0c041615d17d286790ae3e8c61bb28814

include/asterisk/bridge_channel.h

@@ -193,6 +193,20 @@ struct ast_bridge_channel {
 	} stream_map;
 };
 
+/*!
+ * \brief Get a ref to the bridge_channel's ast_channel
+ *
+ * \param bridge_channel The bridge channel
+ *
+ * \note The returned channel NEEDS to be unref'd once you are done with it. In general, this
+ * function is best used when accessing the bridge_channel chan from outside of a bridging
+ * thread.
+ *
+ * \retval ref'd ast_channel on success
+ * \retval NULL otherwise
+ */
+struct ast_channel *ast_bridge_channel_get_chan(struct ast_bridge_channel *bridge_channel);
+
 /*!
  * \brief Try locking the bridge_channel.
  *

main/bridge.c

@@ -1729,7 +1729,10 @@ int ast_bridge_join(struct ast_bridge *bridge,
 	ast_channel_lock(chan);
 	ast_channel_internal_bridge_channel_set(chan, NULL);
 	ast_channel_unlock(chan);
+	/* Due to a race condition, we lock the bridge channel here for ast_bridge_channel_get_chan */
+	ao2_lock(bridge_channel);
 	bridge_channel->chan = NULL;
+	ao2_unlock(bridge_channel);
 	/* If bridge_channel->swap is not NULL then the join failed. */
 	ao2_t_cleanup(bridge_channel->swap, "Bridge complete: join failed");
 	bridge_channel->swap = NULL;
@@ -1798,7 +1801,10 @@ static void *bridge_channel_ind_thread(void *data)
 	ast_channel_lock(chan);
 	ast_channel_internal_bridge_channel_set(chan, NULL);
 	ast_channel_unlock(chan);
+	/* Lock here for ast_bridge_channel_get_chan */
+	ao2_lock(bridge_channel);
 	bridge_channel->chan = NULL;
+	ao2_unlock(bridge_channel);
 	/* If bridge_channel->swap is not NULL then the join failed. */
 	ao2_t_cleanup(bridge_channel->swap, "Bridge complete: Independent impart join failed");
 	bridge_channel->swap = NULL;
@@ -1899,7 +1905,10 @@ static int bridge_impart_internal(struct ast_bridge *bridge,
 		ast_channel_lock(chan);
 		ast_channel_internal_bridge_channel_set(chan, NULL);
 		ast_channel_unlock(chan);
+		/* Lock here for ast_bridge_channel_get_chan */
+		ao2_lock(bridge_channel);
 		bridge_channel->chan = NULL;
+		ao2_unlock(bridge_channel);
 		ao2_t_cleanup(bridge_channel->swap, "Bridge complete: Impart failed");
 		bridge_channel->swap = NULL;
 		ast_bridge_features_destroy(bridge_channel->features);
@@ -4752,14 +4761,22 @@ enum ast_transfer_result ast_bridge_transfer_attended(struct ast_channel *to_tra
 
 	if (to_transferee_bridge_channel) {
 		/* Take off hold if they are on hold. */
-		ast_bridge_channel_write_unhold(to_transferee_bridge_channel);
+		if (ast_bridge_channel_write_unhold(to_transferee_bridge_channel)) {
+			ast_log(LOG_ERROR, "Transferee channel disappeared during transfer!\n");
+			res = AST_BRIDGE_TRANSFER_FAIL;
+			goto end;
+		}
 	}
 
 	if (to_target_bridge_channel) {
 		const char *target_complete_sound;
 
 		/* Take off hold if they are on hold. */
-		ast_bridge_channel_write_unhold(to_target_bridge_channel);
+		if (ast_bridge_channel_write_unhold(to_target_bridge_channel)) {
+			ast_log(LOG_ERROR, "Target channel disappeared during transfer!\n");
+			res = AST_BRIDGE_TRANSFER_FAIL;
+			goto end;
+		}
 
 		/* Is there a courtesy sound to play to the target? */
 		ast_channel_lock(to_transfer_target);

main/bridge_channel.c

@@ -208,6 +208,17 @@ static void bridge_sync_signal(struct bridge_sync *sync_struct)
 	ast_sem_post(&sync_struct->sem);
 }
 
+struct ast_channel *ast_bridge_channel_get_chan(struct ast_bridge_channel *bridge_channel)
+{
+	struct ast_channel *chan;
+
+	ao2_lock(bridge_channel);
+	chan = ao2_bump(bridge_channel->chan);
+	ao2_unlock(bridge_channel);
+
+	return chan;
+}
+
 void ast_bridge_channel_lock_bridge(struct ast_bridge_channel *bridge_channel)
 {
 	struct ast_bridge *bridge;
@@ -1177,7 +1188,14 @@ int ast_bridge_channel_write_hold(struct ast_bridge_channel *bridge_channel, con
 
 int ast_bridge_channel_write_unhold(struct ast_bridge_channel *bridge_channel)
 {
-	ast_channel_publish_cached_blob(bridge_channel->chan, ast_channel_unhold_type(), NULL);
+	struct ast_channel *chan = ast_bridge_channel_get_chan(bridge_channel);
+
+	if (!chan) {
+		return -1;
+	}
+
+	ast_channel_publish_cached_blob(chan, ast_channel_unhold_type(), NULL);
+	ao2_ref(chan, -1);
 
 	return ast_bridge_channel_write_control_data(bridge_channel, AST_CONTROL_UNHOLD, NULL, 0);
 }