Implement a ROS 2 (Lyrical Luth) action server and action client in Python (rclpy) with feedback and goal handling

Documented steps

1. Prerequisite: source /opt/ros/lyrical/setup.bash and have the custom_action_interfaces package with the Fibonacci.action interface already built and sourced (from the Creating-an-Action tutorial).
2. Create fibonacci_action_server.py. class FibonacciActionServer(Node): super().__init__('fibonacci_action_server'), then self._action_server = ActionServer(self, Fibonacci, 'fibonacci', self.execute_callback).
3. Define execute_callback(self, goal_handle). It MUST return a result message of the action's result type.
4. Call goal_handle.succeed() inside execute_callback before returning the result. If you do not explicitly set the terminal state, ROS 2 assumes the goal was ABORTED.
5. Build the sequence in a loop and assign it to result.sequence before `return result`.
6. Run the server: `python3 fibonacci_action_server.py`. In another terminal send a goal: `ros2 action send_goal fibonacci custom_action_interfaces/action/Fibonacci "{order: 5}"`.
7. Add feedback: inside the loop build a Fibonacci.Feedback(), update its sequence field, call goal_handle.publish_feedback(feedback_msg), then sleep. Test with `ros2 action send_goal --feedback fibonacci custom_action_interfaces/action/Fibonacci "{order: 5}"`.
8. Create fibonacci_action_client.py. class FibonacciActionClient(Node): super().__init__('fibonacci_action_client'), self._action_client = ActionClient(self, Fibonacci, 'fibonacci').
9. send_goal(self, order) calls self._action_client.wait_for_server(), then self._action_client.send_goal_async(goal_msg) and returns that future - a future to a GOAL HANDLE, not to the result.
10. Register goal_response_callback on that future: goal_handle = future.result(); if not goal_handle.accepted: log rejection and return early.
11. If accepted, call goal_handle.get_result_async() and register get_result_callback on the new future; read future.result().result.sequence there and then shut down.
12. For client-side feedback pass feedback_callback=self.feedback_callback to send_goal_async(...) and read feedback_msg.feedback.sequence in the callback.
13. Start the server first, then run the client in a second terminal; observe accepted-goal, feedback and result logs.
14. Official doc: https://docs.ros.org/en/lyrical/Tutorials/Intermediate/Writing-an-Action-Server-Client/Py.html