LaunchService.shutdown() fails to terminate run loop

Bug report

Required Info:

• Operating System:
  • Ubuntu 16.04
• Python Version:
• Python 3.5.2
• Installation type:
  • source
• Version or commit hash:
  • 0.6.0
• DDS implementation:
  • Fast-RTPS
• Client library (if applicable):

Steps to reproduce issue

Run the following python code

#!/usr/bin/env python

import sys
import time
import threading
from launch import LaunchDescription
from launch import LaunchService
import launch_ros.actions

def main():
    ld = LaunchDescription([
        launch_ros.actions.Node(
            package='demo_nodes_cpp', node_executable='listener', output='screen'),
            ])
    ls = LaunchService()
    ls.include_launch_description(ld)
    t = threading.Thread(target=ls.run)
    t.start()
    time.sleep(5) # wait for listener to fully start
    ls.shutdown()
    t.join()

if __name__=="__main__":
    main()

Expected behavior

I expect the listener executable to terminate and the python code to exit.

Actual behavior

The python script produces the following output but never terminates.

[INFO] [launch]: process[listener-1]: started with pid [20208]
Starting shutdown
[INFO] [launch]: sending signal 'SIGINT' to process[listener-1]
[ERROR] [launch]: process[listener-1] failed to terminate '5' seconds after receiving 'SIGINT', escalating to 'SIGTERM'
[INFO] [launch]: sending signal 'SIGTERM' to process[listener-1]
[ERROR] [launch]: process[listener-1] failed to terminate '10.0' seconds after receiving 'SIGTERM', escalating to 'SIGKILL'
[INFO] [launch]: sending signal 'SIGKILL' to process[listener-1]

Additional information

The listener executable in fact terminates when the SIGINT is sent and remains as a zombie process.

20208 pts/20   Z+     0:00 [listener] <defunct>

Start with the provided Python reproduction and trace LaunchService.run(), shutdown(), and the listener process lifecycle. Check how the shutdown thread handles a process that has already exited and verify the script's t.join() returns without leaving the listener as a zombie.