logger: re-use subscribed topic id's, only set them once on first use

Will be necessary when using multiple backends in parallel.

src/modules/logger/logger.cpp

@@ -1086,7 +1086,6 @@ void Logger::start_log_file()
 
 	/* print logging path, important to find log file later */
 	mavlink_log_info(&_mavlink_log_pub, "[logger] file: %s", file_name);
-	_next_topic_id = 0;
 
 	_writer.start_log_file(file_name);
 	_writer.set_need_reliable_transfer(true);
@@ -1142,9 +1141,12 @@ void Logger::write_add_logged_msg(LoggerSubscription &subscription, int instance
 {
 	ulog_message_add_logged_s msg;
 
+	if (subscription.msg_ids[instance] == (uint16_t) - 1) {
+		subscription.msg_ids[instance] = _next_topic_id++;
+	}
+
+	msg.msg_id = subscription.msg_ids[instance];
 	msg.multi_id = instance;
-	subscription.msg_ids[instance] = _next_topic_id;
-	msg.msg_id = _next_topic_id;
 
 	int message_name_len = strlen(subscription.metadata->o_name);
 
@@ -1157,8 +1159,6 @@ void Logger::write_add_logged_msg(LoggerSubscription &subscription, int instance
 	_writer.set_need_reliable_transfer(true);
 	write_message(&msg, msg_size);
 	_writer.set_need_reliable_transfer(prev_reliable);
-
-	++_next_topic_id;
 }
 
 /* write info message */

src/modules/logger/logger.h

@@ -75,6 +75,10 @@ struct LoggerSubscription {
 		for (int i = 1; i < ORB_MULTI_MAX_INSTANCES; i++) {
 			fd[i] = -1;
 		}
+
+		for (int i = 0; i < ORB_MULTI_MAX_INSTANCES; i++) {
+			msg_ids[i] = (uint16_t) - 1;
+		}
 	}
 };
 
@@ -232,7 +236,7 @@ private:
 	uint32_t					_log_interval;
 	param_t						_log_utc_offset;
 	orb_advert_t					_mavlink_log_pub = nullptr;
-	uint16_t					_next_topic_id; ///< id of next subscribed topic
+	uint16_t					_next_topic_id = 0; ///< id of next subscribed ulog topic
 	char						*_replay_file_name = nullptr;
 };