Topic Monitor has problem with hidden topics

[jkaniuka]

Operating system:

• Ubuntu Linux - Jammy Jellyfish (22.04)

ROS 2 distro:

• Humble Hawksbill

Installation type:

• Debian packages

Description:

I had turtlesim_node running and in a separate terminal window I started Topic Monitor RQT plugin. The following error appeared in the terminal window:

[ERROR] [1679441995.440332899] [get_message_class]: Malformed msg message_type: turtlesim/action/RotateAbsolute_FeedbackMessage
TopicInfo.__init__(): topic "/turtle1/rotate_absolute/_action/feedback": can not get message class for type "turtlesim/action/RotateAbsolute_FeedbackMessage"

It appears that RQT has a problem with handling a message published on a hidden topic with underscore before the action word (/turtle1/rotate_absolute/_action/feedback).

Steps to reproduce:

First terminal window: ros2 run turtlesim turtlesim_node
Second terminal window rqt -s rqt_topic

Bug source:

1. This ERROR is thrown in line 235 of message_helpers.py:

    if len(message_info) == 3 and message_info[1] != mode:
        logger.error('Malformed {} message_type: {}'.format(mode, message_type))
        return None

2. The second part of the logical condition is False.
   The code snippet I pasted above is a part of _get_rosidl_class_helper(message_type, mode, logger=None) function which is called in line 330 of message_helpers.py ➡️ class_val = _get_rosidl_class_helper(action_type, ACTION_MODE, logger)
3. The cause of the problem can be seen in line 44 of message_helpers.py ➡️ ACTION_MODE = 'action'. The case of a hidden topic, where there is an underscore before the action word, is simply not covered.