Very simple daemonization code (double-fork, fds close, in/out/err reopen)

dumb-daemonizer.py

#!/usr/bin/python

# based on: http://code.activestate.com/recipes/278731-creating-a-daemon-the-python-way/

from subprocess import call
import os
import sys
import signal
import resource

if hasattr(os, "devnull"):
    DEVNULL = os.devnull
else:
    DEVNULL = "/dev/null"

def setup_daemon(cwd="/", stdin=DEVNULL, stderr=DEVNULL, stdout=DEVNULL, umask=0):
    pid = os.fork()

    if pid == 0:
        os.setsid() # make it session leader
        signal.signal(signal.SIGHUP, signal.SIG_IGN)

        pid = os.fork()
        if pid == 0:
            os.chdir(cwd)
            os.umask(umask) # don't inherit file creation perms from parent
        else:
            os._exit(0)   # no atexit and fd flush/close
    else:
        os._exit(0)	 # no atexit and fd flush/close

    if os.sysconf_names.has_key("SC_OPEN_MAX"):
        MAXFD = os.sysconf("SC_OPEN_MAX")
    else:
        MAXFD = 1024

    maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1]
    if maxfd == resource.RLIM_INFINITY:
        maxfd = MAXFD

    os.closerange(0, maxfd)
    # Redirect the standard I/O file descriptors to the specified file.  Since
    # the daemon has no controlling terminal, most daemons redirect stdin,
    # stdout, and stderr to /dev/null.  This is done to prevent side-effects
    # from reads and writes to the standard I/O file descriptors.

    # This call to open is guaranteed to return the lowest file descriptor,
    # which will be 0 (stdin), since it was closed above.
    os.open(stdin, os.O_RDWR) # standard input (0)
    os.open(stdout, os.O_WRONLY|os.O_TRUNC|os.O_CREAT) # standard output (1)
    os.open(stderr, os.O_WRONLY|os.O_TRUNC|os.O_CREAT) # standard error (2)

if __name__ == "__main__":
    cwd = os.getcwd()

    setup_daemon(
        cwd=cwd,
        stderr="foo.stderr",
        stdout="foo.stdout",
        umask=0113, # equiv to "u-x g-x o-wx"
    )