avoid SIGINT on Windows

[wjwwood]

This is required to avoid a (non-fatal, but annoying) traceback on Windows when launch tries to send SIGINT to it's children.

This patch automatically escalates to SIGTERM (which does work), which is what the old launch did, and is also what eventually happens with launch now, but only after waiting five seconds. This removes the wait as well.

Eventually this might be fixed with some combination of creationflags and signal.CTRL_C_EVENT, but that's an ongoing issue with subprocess on Windows.

[wjwwood]

Before:

C:\dev\ros2>ros2 launch demo_nodes_cpp add_two_ints.launch.py
[INFO] [launch]: process[add_two_ints_server.EXE-1]: started with pid [12376]
[INFO] [launch]: process[add_two_ints_client.EXE-2]: started with pid [7220]
[INFO] [add_two_ints_server]: Incoming request
a: 2 b: 3
[INFO] [add_two_ints_client]: Result of add_two_ints: 5
[INFO] [launch]: process[add_two_ints_client.EXE-2]: process has finished cleanly
[INFO] [launch]: sending signal 'SIGINT' to process[add_two_ints_server.EXE-1]
[ERROR] [asyncio]: Task exception was never retrieved
future: <Task finished coro=<LaunchService._process_one_event() done, defined at C:\dev\ros2\install\Lib\site-packages\launch\launch_service.py:176> exception=ValueError('Unsupported signal: 2',)>
Traceback (most recent call last):
  File "C:\dev\ros2\install\Lib\site-packages\launch\launch_service.py", line 178, in _process_one_event
    await self.__process_event(next_event)
  File "C:\dev\ros2\install\Lib\site-packages\launch\launch_service.py", line 197, in __process_event
    visit_all_entities_and_collect_futures(entity, self.__context))
  File "C:\dev\ros2\install\Lib\site-packages\launch\utilities\visit_all_entities_and_collect_futures_impl.py", line 38, in visit_all_entities_and_collect_futures
    sub_entities = entity.visit(context)
  File "C:\dev\ros2\install\Lib\site-packages\launch\action.py", line 39, in visit
    return self.execute(context)  # type: ignore
  File "C:\dev\ros2\install\Lib\site-packages\launch\actions\opaque_function.py", line 72, in execute
    return self.__function(context, *self.__args, **self.__kwargs)
  File "C:\dev\ros2\install\Lib\site-packages\launch\actions\execute_process.py", line 249, in __on_signal_process_event
    self._subprocess_transport.send_signal(typed_event.signal)
  File "c:\python36\lib\asyncio\base_subprocess.py", line 152, in send_signal
    self._proc.send_signal(signal)
  File "c:\python36\lib\subprocess.py", line 1131, in send_signal
    raise ValueError("Unsupported signal: {}".format(sig))
ValueError: Unsupported signal: 2
[ERROR] [launch]: process[add_two_ints_server.EXE-1] failed to terminate '5' seconds after receiving 'SIGINT', escalating to 'SIGTERM'
[INFO] [launch]: sending signal 'SIGTERM' to process[add_two_ints_server.EXE-1]
[ERROR] [launch]: process[add_two_ints_server.EXE-1] process has died [pid 12376, exit code 1, cmd 'C:\dev\ros2\install\lib\demo_nodes_cpp\add_two_ints_server.EXE'].

After:

C:\dev\ros2>ros2 launch demo_nodes_cpp add_two_ints.launch.py
[INFO] [launch]: process[add_two_ints_server.EXE-1]: started with pid [11484]
[INFO] [launch]: process[add_two_ints_client.EXE-2]: started with pid [1344]
[INFO] [add_two_ints_server]: Incoming request
a: 2 b: 3
[INFO] [add_two_ints_client]: Result of add_two_ints: 5
[INFO] [launch]: process[add_two_ints_client.EXE-2]: process has finished cleanly
[WARNING] [launch]: 'SIGINT' sent to process[add_two_ints_server.EXE-1] not supported on Windows, escalating to 'SIGTERM'
[INFO] [launch]: sending signal 'SIGTERM' to process[add_two_ints_server.EXE-1]
[ERROR] [launch]: process[add_two_ints_server.EXE-1] process has died [pid 11484, exit code 1, cmd 'C:\dev\ros2\install\lib\demo_nodes_cpp\add_two_ints_server.EXE'].

[dhood]

For the example counter that ignores sigterms, because we don't cancel the sigterm timer that's already been scheduled, it's going to have a misleading error message printed 5s after sigint which will again say that sigint is being escalated to sigterm. Shall we cancel that timer to prevent the second sigterm and the message?

[nuclearsandwich]

I also tested this on Windows and had the same before and after output as William.

[wjwwood]

We cannot cancel the SIGTERM timer from there because we don't know if SIGINT was sent by the user (using the EmitEvent action) or as part of the escalating shutdown.

I don't think it will be misleading because it will simply send SIGTERM twice, and the first time the warning message will indicate that is because it cannot send SIGINT as requested.

Also, the counter doesn't work right on Windows because you cannot ignore SIGTERM on Windows either. This is because Windows just emulates SIGTERM as a signal and instead just calls a terminate function on the process which cannot be handle that way I think. I'll have to rethink the behavior on Windows w.r.t. to shutting down processes programmatically and likely rewrite the counter.py example, but this is an incremental improvement only. It just avoids the traceback and ensures the process actually gets terminated.

[wjwwood]

I still need a code review so that I can run CI.

[wjwwood]

Thanks for the review @dhood!

CI:

• Linux
• Linux-aarch64
• macOS
• Windows