How to stop python from propagating signals to subprocesses?

user_178154 0
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I'm using python to manage some simulations. I build the parameters and run the program using:

pipe = open('/dev/null', 'w')
pid = subprocess.Popen(shlex.split(command), stdout=pipe, stderr=pipe)

My code handles different signal. Ctrl+C will stop the simulation, ask if I want to save, and exit gracefully. I have other signal handlers (to force data output for example).

What I want is to send a signal (SIGINT, Ctrl+C) to my python script which will ask the user which signal he wants to send to the program.

The only thing preventing the code to work is that it seems that whatever I do, Ctrl+C will be "forwarded" to the subprocess: the code will catch it to and exit:

try:
  <wait for available slots>
except KeyboardInterrupt:
  print "KeyboardInterrupt catched! All simulations are paused. Please choose the signal to send:"
  print "  0: SIGCONT (Continue simulation)"
  print "  1: SIGINT  (Exit and save)"
  [...]
  answer = raw_input()
  pid.send_signal(signal.SIGCONT)
  if   (answer == "0"):
    print "    --> Continuing simulation..."
  elif (answer == "1"):
    print "    --> Exit and save."
    pid.send_signal(signal.SIGINT)
    [...]

So whatever I do, the program is receiving the SIGINT that I only want my python script to see. How can I do that???

I also tried:

signal.signal(signal.SIGINT, signal.SIG_IGN)
pid = subprocess.Popen(shlex.split(command), stdout=pipe, stderr=pipe)
signal.signal(signal.SIGINT, signal.SIG_DFL)

to run the program but this gives the same result: the program catches the SIGINT.

Thanx!

subprocess signals python
5 Answers

I resolved this problem by creating a helper app that I call instead of creating the child directly. This helper changes its parent group and then spawn the real child process.

import os
import sys

from time import sleep
from subprocess import Popen

POLL_INTERVAL=2

# dettach from parent group (no more inherited signals!)
os.setpgrp()

app = Popen(sys.argv[1:])
while app.poll() is None:
    sleep(POLL_INTERVAL)

exit(app.returncode)

I call this helper in the parent, passing the real child and its parameters as arguments:

Popen(["helper", "child", "arg1", ...])

I have to do this because my child app is not under my control, if it were I could have added the setpgrp there and bypassed the helper altogether.

user_134590
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