Addition of a ``wait_for_node()`` function

[adityapande-1995]

Feature request

Feature description

It would be nice to have a wait_for_node(node_name, timeout) function which would wait for a particular node to show up, and block for a specified time. Further, we can have a set of wait_for_* functions which could wait for parameters, services, topics to show up.

Implementation considerations

This feature was mentioned in ros2/launch_ros#263.
Related feature exists in rcl (ros2/rcl#907) but it hasn't been ported to rclcpp (ros2/rclcpp#1621) due to a design issue.

[aprotyas]

+1 for a function like this, but if I understand correctly, ros2/rcl#907 looks for publisher/subscriber counts on a named topic, right? Which is close enough in spirit, but I guess that isn't the API this issue would want to have wrapped in rclpy.

[adityapande-1995]

Right. The implementation I have in ros2/launch_ros#263 is pretty basic, and definitely not efficient. Not sure where to implement this feature though, maybe in rcl and then wrap it in rclpy and rclcpp

[aprotyas]

maybe in rcl and then wrap it in rclpy and rclcpp

I think that's the way to go because I see utility for all of the wait_for_* methods in the client libraries.

[adityapande-1995]

A basic implementation is added here : https://github.com/ros2/examples/blob/200a16c2290e09a26eaa91f201555887cbb3e379/launch_testing/launch_testing_examples/launch_testing_examples/check_multiple_nodes_launch_test.py#L84

[dhood]

FYI I think this has been closed by #930! That has been released into Iron (not backported into Humble AFAICT)

[clalancette]

Good call. I'll go ahead and close this one out, thanks!