ping_pong rand() is not seeded so device ids are likely the same across nodes

[corytodd]

Issue template

• Hardware description: docker only
• RTOS: N/A
• Installation type: a containerized version of https://micro.ros.org/docs/tutorials/core/first_application_linux/
• Version or commit hash: b86db30
  • this patch to fix the stack corruption demo: fix stack corruption in ping_pong demo #96

Steps to reproduce the issue

Build this repo on your target

  cd /ros2_ws
  . /opt/ros/humble/setup.bash
  . install/local_setup.bash
  colcon build --packages-select micro_ros_demos_rclc
  /ros2_ws/install/micro_ros_demos_rclc/lib/micro_ros_demos_rclc/ping_pong

Expected behavior

I expect to see ping/pong messages

Actual behavior

You will see only Ping or only Pong messages

Additional information

Running on Ubuntu 24.04 host, docker compose driving a pair of containers to pubsub.

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In my testing, seeding random with a nanosecond clock is sufficient to generate different device ids. I'd file a PR but I'd like to see if this was intended or maybe just an artifact of launching two container simultaneously

[David-LP99]

Hi @corytodd, thanks for raising the issue.

I have been checking and this is actually a bug depending on the rmw used, rmw_init(...) from rmw_microxrcedds already sets srand(uxr_nanos()); because it needs unique XRCE client keys, and that "fixes" this issue under RMW_IMPLEMENTATION=rmw_microxrcedds.

At the moment, many of these demos are meant to work with rmw_microxrcedds, including the ping_pong one, so that is not a problem under that assumption. However, I agree that this is an undocumented issue under other rmw and it would be interesting to update this demo solving this issue in a rmw_* independent manner and removing the dependency on rmw_microxrcedds.

That said, I would be more than happy to support on the topic if you decide to continue and open a PR.