🐛 [BUG] - TopicMemorySubscriber enters RUNNING state when inappropriate

[chriseichmann]

Description

According to the code documentation, the TopicMemorySubscriber should never return RUNNING:

ros2_ros_bt_py/ros_bt_py/ros_bt_py/ros_nodes/topic.py

Line 216 in fa46148

This node never returns RUNNING.

But the Node returns RUNNING (at least) for the first tick after _do_reset() was called, because _do_reset() deletes the Memory of the last received message and no new message is received in the minuscule time until _do_tick() is called again.
In my case, the Node is part of a Sequence and gets reset every time a subsequent Node fails, leading to the node returning RUNNING on half the ticks it receives.

From this, some points of discussion arise:

1. The documentation is incorrect.
2. I (and probably many others) would assume a TopicMemorySubscriber would store received messages more persistently.
3. Deleting the memory on _do_reset() defeats the purpose of latching messages on the publisher side. The latched message is lost and the tick will not succeed until a new message is published.

I didn't come up with a clean way to solve the issues.
One solution would be to no longer clear out the memory on _do_reset(), solving 2. and 3. but might introduce new problems.
Point 3. might also be fixed by adding logic to handle latched and non-latched messages differently on _do_reset().
I'm interested in hearing your thoughts on theses or other possible solutions. I will probably write a custom Node now, which does not delete messages on reset.

Reproduction steps

topic_memory_subscriber_debugging_yaml.zip

1. Load the bt from topic_memory_subscriber_debugging_yaml.zip
2. Run `ros2 topic pub -r 10 --qos-depth 1 --qos-durability transient_local --qos-reliability reliable /bt_testing_bool std_msgs/msg/Bool "{data: false}"` to publish messages in rapid succession
3. Press "Tick Once" a couple of times and observe the trees behavior
4. Replace the command with `ros2 topic pub -r 0.1 --qos-depth 1 --qos-durability transient_local --qos-reliability reliable /bt_testing_bool std_msgs/msg/Bool "{data: false}"` to simulate a publisher with lower publication rate
5. Press "Tick Once" a couple of times and observe the trees behavior

Screenshots

ROS 2 Version

Humble

OS

Ubuntu 22.04