Kill the entire subtree when killing child process

[guidow]

On Windows, killing a process will by default not kill that process's
child processes as well. This can be a problem when we're trying to stop
an assignment.

[opalmer]

I've seen this behavior a couple of times on Windows, usually it's because the process group is not setup correctly or the process was spawned in such a way that breaks the connection between parent and child. I have not seen this behavior on Linux however though it could be possible for grandchild processes.

All that said, could you please make a couple of modifications? sendSignal should do the right thing by default on all platforms, but obviously it won't always do this unfortunately. I think we should give it a chance to work how it's supposed however before trying to handle it ourselves since in most cases it may work properly.

I'm thinking something like this:

@classmethod
def _signal_process(signal, parent, children):
    assert signal in ("KILL", "TERM")
    method = signal.lower()
    if signal == "TERM":
        method = "terminate"

    if parent.is_running():
        getattr(parent, method)()  # needs a try: except Exception block

    for child in children:
        getattr(parent, method)()  # needs a try: except Exception block

def kill(self):
    process = self.psutil_process
    children = process.children(recursive=True)
    self.process.signalProcess("KILL")
    reactor.callLater(2, self._signal_process, parent, children)  # '2' should be configurable

[guidow]

Is this better now?

This still has problems, namely the process killed this way will return a return code of zero, signalling successful execution. In my test case (with Maya), this is not that much of a problem, because we are already parsing the process's output to know when a frame was finished, but it's still weird.

The best solution would probably involve setting up a correct process group when running on Windows, but I have no idea how to do that...

[coveralls]

Coverage decreased (-0.11%) when pulling 3d34763 on guidow:kill_child_processes into 749131a on pyfarm:jobtype_rework.

[opalmer]

Yes thank you, merge when ready.

The best solution would probably involve setting up a correct process group when running on Windows, but I have no idea how to do that...

I've done this before using pywin32 so I'll be happy to add it. Opening up #147 to work on this at a later time since for now the solution you have here is probably enough in the short term.