Feature Request: LaunchService wrapper for easy non-blocking launch and shutdown

[KKSTB]

Feature request

Feature description

LaunchService has puzzled many people as to how to properly launch a LaunchDescription without blocking the main thread:
#210
https://answers.ros.org/question/321118/ros2-nodes-occasionally-dying-using-launchservice-in-a-subprocess/
#126

So I propose either adding a start() function to LaunchService, or a new wrapper class, that spawns a daemon process to run the async launch loop, like this:

import asyncio
import multiprocessing

from launch import LaunchDescription, LaunchService


class Ros2LaunchParent:
    def start(self, launch_description: LaunchDescription):
        self._stop_event = multiprocessing.Event()
        self._process = multiprocessing.Process(target=self._run_process, args=(self._stop_event, launch_description), daemon=True)
        self._process.start()

    def shutdown(self):
        self._stop_event.set()
        self._process.join()

    def _run_process(self, stop_event, launch_description):
        loop = asyncio.get_event_loop()
        launch_service = LaunchService()
        launch_service.include_launch_description(launch_description)
        launch_task = loop.create_task(launch_service.run_async())
        loop.run_until_complete(loop.run_in_executor(None, stop_event.wait))
        if not launch_task.done():
            asyncio.ensure_future(launch_service.shutdown(), loop=loop)
            loop.run_until_complete(launch_task)

Implementation considerations

Besides launching a LaunchDescription, it would be better if there is another mode of launching an individual node and get its PID to operate the process, just like ROS1 does.

[Rezenders]

HI @KKSTB, I am trying to replicate what you described in this issue, but for me this code is still blocking. Can you help me out?

This is what I have so far:

    def start_node_process(self, launch_description: LaunchDescription):
        self._stop_event = multiprocessing.Event()
        self._process = multiprocessing.Process(
            target=self._run_process,
            args=(self._stop_event, launch_description),
            daemon=True
        )
        self._process.start()

    def _run_process(self, stop_event, launch_description):
        loop = asyncio.get_event_loop()
        launch_service = LaunchService()
        launch_service.include_launch_description(launch_description)
        launch_task = loop.create_task(launch_service.run_async())
        loop.run_until_complete(loop.run_in_executor(None, stop_event.wait))
        if not launch_task.done():
            asyncio.ensure_future(launch_service.shutdown(), loop=loop)
            loop.run_until_complete(launch_task)

    def start_ros_node(self, node_dict):
        node = launch_ros.actions.Node(**node_dict)
        self.start_node_process(LaunchDescription([node]))

However, when I call start_ros_node it still blocks and never returns

Thanks in advance!

[KKSTB]

Hi @Rezenders. I used it in a large project to launch many other python launch files without problem. Maybe you can try the followings to see what's wrong:

1. Just like what I do, use launch description that includes a python launch file that launches something e.g. a talker node
2. Just like what you do, use launch description to launch a node, but launches a talker node