How to kill a forked child and its jackd subprocess in python

Question

I am trying to implement a tray icon, that lets me control a process (jackd). Most notably

I want to read the processes stdout and stderr for logging purposes
I want to be able to kill the process from a menu entry of the main program

The killing is causing me headaches. I believe, when I read stdout and stderr I must do this in an extra thread/process to keep the icon responsive. So I used fork() to create a child process and called setsid() on the child. That child process then invokes jackd using the subprocess module

from subprocess import PIPE, STDOUT, Popen
def runAndLog(self, cmd):
    po = Popen(cmd, stdout=PIPE, stderr=STDOUT)
    line = po.stdout.readline()
    while line:
       print(line.rstrip())
       line = po.stdout.readline()
 self.runAndLog(["jackd","-R", "-P80", "-t5000", "-dfirewire", "-r44100", "-p256", "-n3"])

Now I was hoping that I could get my forked child process and jackd into the same process group, so I could kill both in one go, but to my surprise Popen created a new process group and a new session:

  PID  PPID  PGID   SID COMMAND
 5790  5788  5788 31258 python (parent, main)
 5791  5790  5791  5791 python (forked child after setsid - ok)
 5804  5791  5804  5804 jackd  (grandchild created by Popen)

So, killing the child won't kill the grandchild and I don't see any clean way of making the grandchild's PID (5804) known to main in order to kill it explicitly. I could probably use some signal trickery to have the dying child kill the grandchild, but I hesitate to do this.

I also cannot use setpgid on the jackd process to force it into the child's process group, because its SID is different from my child's SID.

I believe I either

have to convince Popen not to create a new session, or
use a module with less magic than subprocess, but which still alows me to read stdout and stderr.

but I don't know how to do either. Admittedly my knowledge about process groups is somewhat limited.

/home/martin >python --version
Python 2.7.1

Update Oct 12

I found that the new session isn't created by python but by jackd itself. When I replace jackd with xterm, everything is fine.

  PID  PPID  PGID   SID COMMAND
12239  4024 12239  4024 python (parent, main)
12244 12239 12244 12244 python (forked child after setsid - ok)
12249 12244 12244 12244 xterm  (grandchild has same PGID and SID - ok)

I also found this

these jack days calls setsid() to get itself established as a new process group leader

That only leaves the options to

make jack's PID known to main and kill it explicitly, or
have the dying child kill kill jackd

Any advice on that?

.

That's pretty much what I'm doing. However, I found that the problem is really jackd itself and not python. I'll update my question accordingly. — Martin Drautzburg, Oct 15 '17 at 08:49
Why don't you use just a thread, and control `jack` directly? Why do you make a separate child process to control the actual command at all? What are the supposed benefits of making it so much more complex? — Sergey Vasilyev, Oct 15 '17 at 10:35
The forked child is used to capture the output of jackd. I don't think I can do this in the toplevel process without blocking in. — Martin Drautzburg, Oct 16 '17 at 12:41
You should be able, as long as it is done not in the main thread. The main thread should continue doing its UI stuff, incl. responding to the checks from the OS. This is a common pattern in such apps. — Sergey Vasilyev, Oct 16 '17 at 14:08

score 1 · Accepted Answer · answered Oct 15 '17 at 10:47

Since your grandchild sets its own session and sid, you cannot expect group-kill to work properly. You have three options:

Iterate over the whole tree of children processes, and kill them all starting from the parents (so that they would not restart the children). Ugly and overcomplicated solution.
Use a thread in the main (root) process to control the jack child directly. The intermediate child process is probably not necessary here, as it gives no benefits. This is the most simple solution possible.
If you still have to have the intermediate child process, the only way is to catch the termination signals sent to the child, and transmit them to the grandchild:

child.py:

import signal
import subprocess

# The signal handler for the child process, which passes it further to the grandchild.
def handler(signum, frame):
    print('Killing self, killing the children')

    os.kill(proc.pid, signal.SIGTERM)

    # wait for the process to die gracefully for N secs.
    time.sleep(5)

    os.kill(proc.pid, signal.SIGKILL)

cmd = ["jackd","-R", "-P80", "-t5000", "-dfirewire", "-r44100", "-p256", "-n3"]
po = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

signal.signal(signal.SIGTERM, handler)

for line in po.stdout:
    print(line.rstrip())

Please note that the signal handler is installed before the process communication begins (line reading).

Whenever your root process will be killing the child process "gracefully" (e.g., with SIGTERM), the signal will be passed to the grandchild, and it will be terminated/killed too. You can even extend the logic of killing and waiting and force-killing it.

However, remember that if the child process is killed with SIGKILL, there will be no chances to catch this signal, and the grandchild will remain running orphan. Because SIGKILL is not catchable by design.

How to kill a forked child and its jackd subprocess in python

1 Answers1