mavlink ulog: advertise ulog_stream topic before subscribing to ulog_stream_ack

This reduces the maximum number of needed file descriptors by 1 when using
ulog streaming.

src/modules/logger/log_writer_mavlink.cpp

@@ -69,6 +69,13 @@ LogWriterMavlink::~LogWriterMavlink()
 
 void LogWriterMavlink::start_log()
 {
+	if (_ulog_stream_pub == nullptr) {
+		memset(&_ulog_stream_data, 0, sizeof(_ulog_stream_data));
+		// advertise before we subscribe: this is a trick to reduce the number of needed file descriptors
+		// (the advertise temporarily opens a file descriptor)
+		_ulog_stream_pub = orb_advertise_queue(ORB_ID(ulog_stream), &_ulog_stream_data, _queue_size);
+	}
+
 	if (_ulog_stream_ack_sub == -1) {
 		_ulog_stream_ack_sub = orb_subscribe(ORB_ID(ulog_stream_ack));
 	}

src/modules/mavlink/mavlink_ulog.cpp

@@ -143,32 +143,34 @@ int MavlinkULog::handle_update(mavlink_channel_t channel)
 	while (updated && !ret && _current_num_msgs < _max_num_messages) {
 		orb_copy(ORB_ID(ulog_stream), _ulog_stream_sub, &_ulog_data);
 
-		if (_ulog_data.flags & ulog_stream_s::FLAGS_NEED_ACK) {
-			_sent_tries = 1;
-			_last_sent_time = hrt_absolute_time();
-			lock();
-			_wait_for_ack_sequence = _ulog_data.sequence;
-			_ack_received = false;
-			unlock();
-
-			mavlink_logging_data_acked_t msg;
-			msg.sequence = _ulog_data.sequence;
-			msg.length = _ulog_data.length;
-			msg.first_message_offset = _ulog_data.first_message_offset;
-			msg.target_system = _target_system;
-			msg.target_component = _target_component;
-			memcpy(msg.data, _ulog_data.data, sizeof(msg.data));
-			mavlink_msg_logging_data_acked_send_struct(channel, &msg);
-
-		} else {
-			mavlink_logging_data_t msg;
-			msg.sequence = _ulog_data.sequence;
-			msg.length = _ulog_data.length;
-			msg.first_message_offset = _ulog_data.first_message_offset;
-			msg.target_system = _target_system;
-			msg.target_component = _target_component;
-			memcpy(msg.data, _ulog_data.data, sizeof(msg.data));
-			mavlink_msg_logging_data_send_struct(channel, &msg);
+		if (_ulog_data.timestamp > 0) {
+			if (_ulog_data.flags & ulog_stream_s::FLAGS_NEED_ACK) {
+				_sent_tries = 1;
+				_last_sent_time = hrt_absolute_time();
+				lock();
+				_wait_for_ack_sequence = _ulog_data.sequence;
+				_ack_received = false;
+				unlock();
+
+				mavlink_logging_data_acked_t msg;
+				msg.sequence = _ulog_data.sequence;
+				msg.length = _ulog_data.length;
+				msg.first_message_offset = _ulog_data.first_message_offset;
+				msg.target_system = _target_system;
+				msg.target_component = _target_component;
+				memcpy(msg.data, _ulog_data.data, sizeof(msg.data));
+				mavlink_msg_logging_data_acked_send_struct(channel, &msg);
+
+			} else {
+				mavlink_logging_data_t msg;
+				msg.sequence = _ulog_data.sequence;
+				msg.length = _ulog_data.length;
+				msg.first_message_offset = _ulog_data.first_message_offset;
+				msg.target_system = _target_system;
+				msg.target_component = _target_component;
+				memcpy(msg.data, _ulog_data.data, sizeof(msg.data));
+				mavlink_msg_logging_data_send_struct(channel, &msg);
+			}
 		}
 
 		++_current_num_msgs;