Python-daemon doesn't kill its kids
Question: When using [python-daemon](http://pypi.python.org/pypi/python-daemon/), I'm
creating subprocesses likeso:

    
    
    import multiprocessing
    
    class Worker(multiprocessing.Process):
       def __init__(self, queue):
          self.queue = queue # we wait for things from this in Worker.run()
    
       ...
    
    q = multiprocessing.Queue()
    
    with daemon.DaemonContext():
        for i in xrange(3):
           Worker(q)
    
        while True: # let the Workers do their thing
           q.put(_something_we_wait_for())
    

When I kill the parent daemonic process (i.e. not a Worker) with a Ctrl-C or
SIGTERM, etc., the children don't die. How does one kill the kids?

My first thought is to use [atexit](http://docs.python.org/library/atexit) to
kill all the workers, likeso:

    
    
     with daemon.DaemonContext():
        workers = list()
        for i in xrange(3):
           workers.append(Worker(q))
    
        @atexit.register
        def kill_the_children():
            for w in workers:
                w.terminate()
    
        while True: # let the Workers do their thing
           q.put(_something_we_wait_for())
    

However, the children of daemons are tricky things to handle, and I'd be
obliged for thoughts and input on how this ought to be done.

Thank you.


Answer: Your options are a bit limited. If doing `self.daemon = True` in the
constructor for the `Worker` class does not solve your problem and trying to
catch signals in the Parent (ie, `SIGTERM, SIGINT`) doesn't work, you may have
to try the opposite solution - instead of having the parent kill the children,
you can have the children commit suicide when the parent dies.

The first step is to give the constructor to `Worker` the `PID` of the parent
process (you can do this with `os.getpid()`). Then, instead of just doing
`self.queue.get()` in the worker loop, do something like this:

    
    
    waiting = True
    while waiting:
        # see if Parent is at home
        if os.getppid() != self.parentPID:
            # woe is me! My Parent has died!
            sys.exit() # or whatever you want to do to quit the Worker process
        try:
            # I picked the timeout randomly; use what works
            data = self.queue.get(block=False, timeout=0.1)
            waiting = False
        except queue.Queue.Empty:
            continue # try again
    # now do stuff with data
    

The solution above checks to see if the parent PID is different than what it
originally was (that is, if the child process was adopted by `init` or
`lauchd` because the parent died) - see [reference](http://www.mail-
archive.com/cocoa-dev@lists.apple.com/msg55718.html). However, if that doesn't
work for some reason you can replace it with the following function (adapted
from [here](http://stackoverflow.com/questions/568271/check-if-pid-is-not-in-
use-in-python)):

    
    
    def parentIsAlive(self):
        try:
            # try to call Parent
            os.kill(self.parentPID, 0)
        except OSError:
            # *beeep* oh no! The phone's disconnected!
            return False
        else:
            # *ring* Hi mom!
            return True
    

Now, when the Parent dies (for whatever reason), the child Workers will
spontaneously drop like flies - just as you wanted, you daemon! `:-D`

