mqtt_client doesn't create generic publisher at runtime change of message type

[vishantyadav-eic]

when we change the message type at runtime no logs are seen from mqtt_client node, after restart the client it throws an error: "[mqtt_client-1] [ERROR] [1702010603.069871187] [mqtt_client]: Failed to create generic subscriber: could not create subscription: invalid allocator, at ./src/rcl/subscription.c:218"
After restarting the mqtt_client again it works successfully but the issue is I have to restart mqtt_client twice.
ROS Distro: humble, using Docker

we want to change the message at runtime without restarting the mqtt_client , How we can achieve the same ?

[lreiher]

I can reproduce the issue. In my case, the mqtt_client errors as soon as the message type changes, no restart needed.

ros2 launch mqtt_client standalone.launch.ros2.xml
# ---
ros2 topic pub /ping/ros std_msgs/msg/String "{data: \"Hello MQTT\"}"
# ---
ros2 topic pub /ping/ros std_msgs/msg/Int32 "{data: 42}"

[INFO] [launch]: All log files can be found below /home/dockeruser/.ros/log/2024-01-08-23-32-51-683641-I2200044-linux-1
[INFO] [launch]: Default logging verbosity is set to INFO
[INFO] [mqtt_client-1]: process started with pid [78]
[mqtt_client-1] [WARN] [1704753171.855153747] [mqtt_client]: Parameter 'broker.tls.enabled' not set, defaulting to '0'
[mqtt_client-1] [WARN] [1704753171.855233754] [mqtt_client]: Parameter 'client.id' not set, defaulting to ''
[mqtt_client-1] [WARN] [1704753171.855244754] [mqtt_client]: Client buffer can not be enabled when client ID is empty
[mqtt_client-1] [WARN] [1704753171.855253483] [mqtt_client]: Parameter 'client.clean_session' not set, defaulting to '1'
[mqtt_client-1] [WARN] [1704753171.855268283] [mqtt_client]: Parameter 'client.keep_alive_interval' not set, defaulting to '60.000000'
[mqtt_client-1] [WARN] [1704753171.855276323] [mqtt_client]: Parameter 'client.max_inflight' not set, defaulting to '65535'
[mqtt_client-1] [INFO] [1704753171.855294562] [mqtt_client]: Bridging ROS topic '/ping/ros' to MQTT topic 'pingpong/ros'
[mqtt_client-1] [INFO] [1704753171.855314551] [mqtt_client]: Bridging MQTT topic 'pingpong/ros' to ROS topic '/pong/ros'
[mqtt_client-1] [INFO] [1704753171.856069647] [mqtt_client]: Connecting to broker at 'tcp://localhost:1883' ...
[mqtt_client-1] [INFO] [1704753171.856856371] [mqtt_client]: Connected to broker at 'tcp://localhost:1883'
[mqtt_client-1] [INFO] [1704753171.856943619] [mqtt_client]: Subscribed MQTT topic 'mqtt_client/ros_msg_type/pingpong/ros'
[mqtt_client-1] [INFO] [1704753200.867931836] [mqtt_client]: Subscribed ROS topic '/ping/ros' of type 'std_msgs/msg/String'
[mqtt_client-1] [INFO] [1704753201.363448881] [mqtt_client]: ROS publisher message type on topic '/pong/ros' set to 'std_msgs/msg/String'
[mqtt_client-1]
[mqtt_client-1] >>> [rcutils|error_handling.c:108] rcutils_set_error_state()
[mqtt_client-1] This error state is being overwritten:
[mqtt_client-1]
[mqtt_client-1]   'create_subscription() called for existing topic name rt/ping/ros with incompatible type std_msgs::msg::dds_::Int32_, at ./src/subscription.cpp:146, at ./src/rcl/subscription.c:108'
[mqtt_client-1]
[mqtt_client-1] with this new error message:
[mqtt_client-1]
[mqtt_client-1]   'invalid allocator, at ./src/rcl/subscription.c:218'
[mqtt_client-1]
[mqtt_client-1] rcutils_reset_error() should be called after error handling to avoid this.
[mqtt_client-1] <<<
[mqtt_client-1] invalid allocator, at ./src/rcl/subscription.c:218
[mqtt_client-1] [ERROR] [1704753223.868018734] [mqtt_client]: Failed to create generic subscriber: could not create subscription: invalid allocator, at ./src/rcl/subscription.c:218

I'm afraid that switching the message type is currently not supported in ROS 2. Feel free to open a pull request if you decide to fix the issue in source code yourself.