Handling long actions in rclpy (with cancellation)

edit

Whoops, I looked at the rclpy demos, but forgot about the rclpy examples.

There are several server examples showing how to build long-running action servers that handle cancellation. They work well w/ the client examples, as well as ros2 action send_goal.

In looking at the server code, I see a few patterns:

• execute_callback contains a call to time.sleep() to simulate a long-running action
• execute_callback blocks until the goal has been reached, or a cancellation request has been received
• the trick is to use a MultiThreadedExecutor and a ReentrantCallbackGroup: these allow the server to receive and process cancellation requests while the goal is executing
• in some cases the execute_callback is marked async, but this doesn't matter, since it never yields (never awaits)

Thanks.