signal.signal receives sigint/sigterm when child process ends

Question

I have the following simplified example:

#!/usr/bin/python

import logging, logging.handlers
from multiprocessing import Process, Queue
import time, sys, signal

def child_proc(sleepr):

    print 'Child: Started child, waiting'
    time.sleep(sleepr)
    print 'Child: Ended sleep, exiting' 

def cleanup(a, b):
    print 'Main: Got SIGTERM: ' + str(a) + ' : ' + str(b)

print 'Main: Starting process'
child_proc_th = Process(target=child_proc, args=(3,))
child_proc_th.start()

print 'Main: Pausing main thread'
signal.signal(signal.SIGTERM, cleanup)
signal.pause()

print 'Main: Running again after pause'

Why does the signal.pause resume once the child_proc has finished? I only want it to resume if a sigterm is received... Is child_proc sending its exit signal up the stack to the parent process? Or is there another way to do this...?

TIA

Answer 1

signal.pause() blocks until any signal is received. When a child exits it sends SIGCHLD to the parent. This causes signal.pause() to return.

As far as I can tell to achieve what you want you have to signal.pause() in a loop until the correct signal is received.

Something like this:

sigtermreceived = False

def sigterm(signum, frame):
  global sigtermreceived
  sigtermreceived = True

signal.signal(signal.SIGTERM, sigterm)

while not sigtermreceived:
  signal.pause()

Note that there is also Process.join() :

child_proc_th = Process(target=child_proc, args=(3,))
child_proc_th.start()
child_proc_th.join()

This will block until the child process exits. Which is basically the behaviour you have now but without messing with signals.