+enum lttng_notification_channel_status
+lttng_notification_channel_has_pending_notification(
+ struct lttng_notification_channel *channel,
+ bool *_notification_pending)
+{
+ int ret;
+ enum lttng_notification_channel_status status =
+ LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;
+ fd_set read_fds;
+ struct timeval timeout;
+
+ FD_ZERO(&read_fds);
+ memset(&timeout, 0, sizeof(timeout));
+
+ if (!channel || !_notification_pending) {
+ status = LTTNG_NOTIFICATION_CHANNEL_STATUS_INVALID;
+ goto end;
+ }
+
+ pthread_mutex_lock(&channel->lock);
+
+ if (channel->pending_notifications.count) {
+ *_notification_pending = true;
+ goto end_unlock;
+ }
+
+ if (channel->socket < 0) {
+ status = LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED;
+ goto end_unlock;
+ }
+
+ /*
+ * Check, without blocking, if data is available on the channel's
+ * socket. If there is data available, it is safe to read (blocking)
+ * on the socket for a message from the session daemon.
+ *
+ * Since all commands wait for the session daemon's reply before
+ * releasing the channel's lock, the protocol only allows for
+ * notifications and "notification dropped" messages to come
+ * through. If we receive a different message type, it is
+ * considered a protocol error.
+ *
+ * Note that this function is not guaranteed not to block. This
+ * will block until our peer (the session daemon) has sent a complete
+ * message if we see data available on the socket. If the peer does
+ * not respect the protocol, this may block indefinitely.
+ */
+ FD_SET(channel->socket, &read_fds);
+ do {
+ ret = select(channel->socket + 1, &read_fds, NULL, NULL, &timeout);
+ } while (ret < 0 && errno == EINTR);
+
+ if (ret == 0) {
+ /* No data available. */
+ *_notification_pending = false;
+ goto end_unlock;
+ } else if (ret < 0) {
+ status = LTTNG_NOTIFICATION_CHANNEL_STATUS_ERROR;
+ goto end_unlock;
+ }
+
+ /* Data available on socket. */
+ ret = receive_message(channel);
+ if (ret) {
+ status = LTTNG_NOTIFICATION_CHANNEL_STATUS_ERROR;
+ goto end_unlock;
+ }
+
+ switch (get_current_message_type(channel)) {
+ case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_NOTIFICATION:
+ ret = enqueue_notification_from_current_message(channel);
+ if (ret) {
+ goto end_unlock;
+ }
+ *_notification_pending = true;
+ break;
+ case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_NOTIFICATION_DROPPED:
+ ret = enqueue_dropped_notification(channel);
+ if (ret) {
+ goto end_unlock;
+ }
+ *_notification_pending = true;
+ break;
+ default:
+ /* Protocol error. */
+ status = LTTNG_NOTIFICATION_CHANNEL_STATUS_ERROR;
+ goto end_unlock;
+ }
+
+end_unlock:
+ pthread_mutex_unlock(&channel->lock);
+end:
+ return status;
+}
+