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.

Answer 2

Fist thing; there is a send_signal() method on the Popen object. If you want to send a signal to one you've launched, use this method to send it.

Second thing; a deeper problem with the way you're setting up communication with your subprocess and then, um, not communicating with it. You cannot safely tell the subprocess to send its output to subprocess.PIPE and then not read from the pipes. UNIX pipes are buffered (typically a 4K buffer?), and if the subprocess fills up the buffer and the process on the other end of the pipe doesn't read the buffered data, the subprocess will pend (locking up, from an observer's perspective) on its next write to the pipe. So, the usual pattern when using subprocess.PIPE is to call communicate() on the Popen object.

It is not mandatory to use subprocess.PIPE if you want data back from the subprocess. A cool trick is to use the tempfile.TemporaryFile class to make an unnamed temp file (really it opens a file and immediately deletes the inode from the file system, so you have access to the file but no-one else can open one. You can do something like:

with tempfile.TemporaryFile() as iofile:
    p = Popen(cmd, stdout=iofile, stderr=iofile)
    while True:
        if p.poll() is not None:
            break
        else:
            time.sleep(0.1) # without some sleep, this polling is VERY busy...

Then you can read the contents of your temporary file (seek to the beginning of it before you do, to be sure you're at the beginning) when you know the subprocess has exited, instead of using pipes. The pipe buffering problem won't be a problem if the subprocess's output is going to a file (temporary or not).

Here is a riff on your code sample that I think does what you want. The signal handler just repeats the signals being trapped by the parent process (in this example, SIGINT and SIGTERM) to all current subprocesses (there should only ever be one in this program) and sets a module-level flag saying to shutdown at the next opportunity. Since I'm using subprocess.PIPE I/O, I call communicate() on the Popen object.

#!/usr/bin/env python

from subprocess import Popen, PIPE
import signal
import sys

current_subprocs = set()
shutdown = False

def handle_signal(signum, frame):
    # send signal recieved to subprocesses
    global shutdown
    shutdown = True
    for proc in current_subprocs:
        if proc.poll() is None:
            proc.send_signal(signum)


signal.signal(signal.SIGINT, handle_signal)
signal.signal(signal.SIGTERM, handle_signal)

for _ in range(10):
    if shutdown:
        break
    cmd = ["sleep", "2"]
    p = Popen(cmd, stdout=PIPE, stderr=PIPE)
    current_subprocs.add(p)
    out, err = p.communicate()
    current_subprocs.remove(p)
    print "subproc returncode", p.returncode

And calling it (with a Ctrl-C in the third 2 second interval):

% python /tmp/proctest.py 
subproc returncode 0
subproc returncode 0
^Csubproc returncode -2

Answer 3

This hack will work, but it's ugly...

Change the command to this:

success_flag = '/tmp/success.flag'
cmd = [ 'script', '-q', '-c', "sleep 2 && touch " + success_flag, '/dev/null']

And put

if os.path.isfile( success_flag ) :
    os.remove( success_flag )
else :
    return

at the end of the for loop

Answer 4

If you have no python processing to do after your process is spawned (like in your example), then the easiest way is to use os.execvp instead of the subprocess module. Your subprocess is going to completely replace your python process, and will be the one catching SIGINT directly.

Answer 5

I found a -e switch in the script man page:

-e      Return the exit code of the child process. Uses the same format
         as bash termination on signal termination exit code is 128+n.

Not too sure what the 128+n is all about but it seems to return 130 for ctrl-c. So modifying your cmd to be

cmd = [ 'script', '-e', '-q', '-c', "sleep 2 && (exit 1)", '/dev/null']

and putting

if p.returncode == 130:
    break

at the end of the for loop seems to do what you want.