signals - Making sure a Python script with subprocesses dies on SIGINT

Question

I've got a command that I'm wrapping in script and spawning from a Python script using subprocess.Popen. I'm trying to make sure it dies if the user issues a SIGINT.

I could figure out if the process was interrupted in a least two ways:

A. Die if the wrapped command has a non-zero exit status (doesn't work, because script seems to always return 0)

B. Do something special with SIGINT in the parent Python script rather than simply interrupting the subprocess. I've tried the following:

import sys
import signal
import subprocess

def interrupt_handler(signum, frame):
    print "While there is a 'script' subprocess alive, this handler won't executes"
    sys.exit(1)
signal.signal(signal.SIGINT, interrupt_handler)

for n in range( 10 ):
    print "Going to sleep for 2 second...Ctrl-C to exit the sleep cycles"

    # exit 1 if we make it to the end of our sleep
    cmd = [ 'script', '-q', '-c', "sleep 2 && (exit 1)", '/dev/null']
    p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

    while True:
        if p.poll() != None :
            break
        else :
            pass

    # Exiting on non-zero exit status would suffice
    print "Exit status (script always exits zero, despite what happened to the wrapped command):", p.returncode

I'd like hitting Ctrl-C to exit the python script. What's happening instead is the subprocess dies and the script continues.

Answer 1

The subprocess is by default part of the same process group, and only one can control and receive signals from the terminal, so there are a couple of different solutions.

Setting stdin as a PIPE (in contrast to inheriting from the parent process), this will prevent the child process from receiving signals associated to it.

subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)

Detaching from the parent process group, the child will no longer receive signals

def preexec_function():
    os.setpgrp()

subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, preexec_fn=preexec_function)

Explicitly ignoring signals in the child process

def preexec_function():
    signal.signal(signal.SIGINT, signal.SIG_IGN)

subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, preexec_fn=preexec_function)

This might however be overwritten by the child process.