child.py

#!/usr/bin/env python
"""Called from parent.py"""
import sys
import time

try:
    while True:
        time.sleep(1)
except KeyboardInterrupt:  # handle SIGINT
    sys.exit('child exits on KeyboardInterrupt')

parent.py

#!/usr/bin/env python
"""Show that child.wait() hangs in the sighandler.

Example:

  $ python3 parent.py &
  $ kill -USR1 $!
  child exits on KeyboardInterrupt
  From parent 1

The example shows that send_signal() doesn't hang.

If you run `python parent.py --call-wait &` then the parent hangs in the
signal handler after `kill -USR1 $!` command on child.wait() call.

Running `python parent.py --call-waitpid &` and then `kill -USR1 $!`
shows that os.waitpid() does NOT hang in the signal handler but the parent
prints `From parent 0` instead of `From parent 1` -- wrong child's exit status.

"""
import os
import signal
import subprocess
import sys


def sighandler(*args):
    child.send_signal(signal.SIGINT)
    if '--call-wait' in sys.argv:
        child.wait()  # It hangs on Python 3 due to child._waitpid_lock
    if '--call-waitpid' in sys.argv:
        rc = os.waitpid(child.pid, 0)
        raise SystemExit("From parent's signal hander: {}".format(rc))

signal.signal(signal.SIGUSR1, sighandler)
child = subprocess.Popen([sys.executable, 'child.py'])
sys.exit("From parent %d" % child.wait())  # return child's exit status