Question

I am trying to spawn a django process that lives on after the calling script died. But I need it’s PID.

So I wrote the following code:

def runserver():
  print("START PID: " + str(os.getpid()))
  pid = os.fork()

  if pid == 0:
      #cmd = "/usr/bin/env python manage.py runserver 0.0.0.0:2869"
      print("IN THE CHILD PID: " + str(os.getpid()))
      os.execvp("python", ["", "manage.py", "runserver", "0.0.0.0:2869"])
  else:
      print("PARENT PID: " + str(os.getpid()))
      print("CHILD PID: " + str(pid))
      updatepid("runserver", pid) 

This gives me the following output:

START PID: 13019
PARENT PID: 13019
CHILD PID: 13020
IN THE CHILD PID: 13020

But now when I check the live processes:

> ps aux | grep python | grep -v grep
sandro   13031  0.4  0.3 296080 23756 pts/2    Sl   22:14   0:01 /home/sandro/.virtualenvs/polling/bin/python2.7 manage.py runserver 0.0.0.0:2869

The pid changed! What on earth is going on???

Answer 1

If you see the other PID then there is definitely a new process. You can easily find the place where the new process spawned. Start in django.core.management.commands.runserver and you’ll come to django.utils.autoreload.python_reloader. When python_reloader called first time in a process it goes to restart_with_reloader where you can see this:

exit_code = os.spawnve(os.P_WAIT, sys.executable, args, new_environ)

Thereby, with your script you get two processes: one where runserver is executing and one (spawned) with webserver. Check it:

START PID: 3091
PARENT PID: 3091
CHILD PID: 3092
IN THE CHILD PID: 3092

$ ps ax | grep runserver | grep -v grep
3092 pts/1    S      0:00  runserver 0.0.0.0:2869
3093 pts/1    Sl     0:05 /home/kirill/testenv/bin/python manage.py runserver 0.0.0.0:2869