`ros2 topic pub` doesn't publish until after first period has expired

Bug report

Required Info:

• Operating System:
  • Ubuntu 20.04, arm64
• Installation type:
  • Debian packages
• Version or commit hash:
  • ROS Rolling, package version 0.13.0-1
• DDS implementation:
  • CycloneDDS
• Client library (if applicable):
  • rclpy

Steps to reproduce issue

In terminal 1, run:

ros2 topic echo /talker std_msgs/msg/String

In terminal 2, run:

ros2 topic pub -r 0.1 /talker std_msgs/String "data: Hello World"

(that is, publish once every 10 seconds)

Expected behavior

The ros2 topic pub command would publish, then sleep for 10 seconds, then publish, sleep for 10 seconds, etc.

Actual behavior

The ros2 topic pub command sleeps for 10 seconds, then publishes, then sleeps for 10 seconds, then publishes, etc.

Additional information

You could argue that the current way is not wrong, but I think it is unexpected behavior for the user.

[clalancette]

but one use case that i think of is, if frequency is long enough (like 10 seconds), probably waiting that time window until 1st message would be not really useful.

Yeah, that’s exactly where I ran into it. My example above is a very long time period, and thus the initial message doesn’t get published until after the first 10 seconds.

This honestly isn’t a huge deal; I doubt too many people are using a 0.1 Hz rate. I still think it should be fixed, but changing behavior at this point in Galactic is probably a bad idea. I’ll move it to the H-Turtle board instead.

[clalancette]

Yes, we can close this now. Thank you for the implementation!