Calling ffmpeg kills script in background only

Question

I've got a python script that calls ffmpeg via subprocess to do some mp3 manipulations. It works fine in the foreground, but if I run it in the background, it gets as far as the ffmpeg command, which itself gets as far as dumping its config into stderr. At this point, everything stops and the parent task is reported as stopped, without raising an exception anywhere. I've tried a few other simple commands in the place of ffmpeg, they execute normally in foreground or background.

This is the minimal example of the problem:

import subprocess

inf = "3HTOSD.mp3"
outf = "out.mp3"

args = [    "ffmpeg",
            "-y",
            "-i",   inf,
            "-ss",  "0",
            "-t",   "20",
            outf
        ]

print "About to do"

result = subprocess.call(args)

print "Done"

I really can't work out why or how a wrapped process can cause the parent to terminate without at least raising an error, and how it only happens in so niche a circumstance. What is going on?

Also, I'm aware that ffmpeg isn't the nicest of packages, but I'm interfacing with something that has using ffmpeg compiled into it, so using it again seems sensible.

Answer 1

It might be related to Linux process in background - “Stopped” in jobs? e.g., using parent.py:

from subprocess import check_call

check_call(["python", "-c", "import sys; sys.stdin.readline()"])

should reproduce the issue: "parent.py script shown as stopped" if you run it in bash as a background job:

$ python parent.py &
[1] 28052
$ jobs
[1]+  Stopped                 python parent.py

If the parent process is in an orphaned process group then it is killed on receiving SIGTTIN signal (a signal to stop).

The solution is to redirect the input:

import os
from subprocess import check_call
try:
    from subprocess import DEVNULL
except ImportError: # Python 2
    DEVNULL = open(os.devnull, 'r+b', 0)

check_call(["python", "-c", "import sys; sys.stdin.readline()"], stdin=DEVNULL)

If you don't need to see ffmpeg stdout/stderr; you could also redirect them to /dev/null:

check_call(ffmpeg_cmd, stdin=DEVNULL, stdout=DEVNULL, stderr=STDOUT)