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"""
Python Daemonizing helper
Configurable daemon behaviors:
1.) The current working directory set to the "/" directory.
2.) The current file creation mode mask set to 0.
3.) Close all open files (1024).
4.) Redirect standard I/O streams to "/dev/null".
A failed call to fork() now raises an exception.
References:
1) Advanced Programming in the Unix Environment: W. Richard Stevens
http://www.apuebook.com/apue3e.html
2) The Linux Programming Interface: Michael Kerrisk
http://man7.org/tlpi/index.html
3) Unix Programming Frequently Asked Questions:
http://www.faqs.org/faqs/unix-faq/programmer/faq/
Modified to allow a function to be daemonized and return for
bitbake use by Richard Purdie
"""
__author__ = "Chad J. Schroeder"
__copyright__ = "Copyright (C) 2005 Chad J. Schroeder"
__version__ = "0.2"
# Standard Python modules.
import os # Miscellaneous OS interfaces.
import sys # System-specific parameters and functions.
# Default daemon parameters.
# File mode creation mask of the daemon.
# For BitBake's children, we do want to inherit the parent umask.
UMASK = None
# Default maximum for the number of available file descriptors.
MAXFD = 1024
# The standard I/O file descriptors are redirected to /dev/null by default.
if (hasattr(os, "devnull")):
REDIRECT_TO = os.devnull
else:
REDIRECT_TO = "/dev/null"
def createDaemon(function, logfile):
"""
Detach a process from the controlling terminal and run it in the
background as a daemon, returning control to the caller.
"""
try:
# Fork a child process so the parent can exit. This returns control to
# the command-line or shell. It also guarantees that the child will not
# be a process group leader, since the child receives a new process ID
# and inherits the parent's process group ID. This step is required
# to insure that the next call to os.setsid is successful.
pid = os.fork()
except OSError as e:
raise Exception("%s [%d]" % (e.strerror, e.errno))
if (pid == 0): # The first child.
# To become the session leader of this new session and the process group
# leader of the new process group, we call os.setsid(). The process is
# also guaranteed not to have a controlling terminal.
os.setsid()
# Is ignoring SIGHUP necessary?
#
# It's often suggested that the SIGHUP signal should be ignored before
# the second fork to avoid premature termination of the process. The
# reason is that when the first child terminates, all processes, e.g.
# the second child, in the orphaned group will be sent a SIGHUP.
#
# "However, as part of the session management system, there are exactly
# two cases where SIGHUP is sent on the death of a process:
#
# 1) When the process that dies is the session leader of a session that
# is attached to a terminal device, SIGHUP is sent to all processes
# in the foreground process group of that terminal device.
# 2) When the death of a process causes a process group to become
# orphaned, and one or more processes in the orphaned group are
# stopped, then SIGHUP and SIGCONT are sent to all members of the
# orphaned group." [2]
#
# The first case can be ignored since the child is guaranteed not to have
# a controlling terminal. The second case isn't so easy to dismiss.
# The process group is orphaned when the first child terminates and
# POSIX.1 requires that every STOPPED process in an orphaned process
# group be sent a SIGHUP signal followed by a SIGCONT signal. Since the
# second child is not STOPPED though, we can safely forego ignoring the
# SIGHUP signal. In any case, there are no ill-effects if it is ignored.
#
# import signal # Set handlers for asynchronous events.
# signal.signal(signal.SIGHUP, signal.SIG_IGN)
try:
# Fork a second child and exit immediately to prevent zombies. This
# causes the second child process to be orphaned, making the init
# process responsible for its cleanup. And, since the first child is
# a session leader without a controlling terminal, it's possible for
# it to acquire one by opening a terminal in the future (System V-
# based systems). This second fork guarantees that the child is no
# longer a session leader, preventing the daemon from ever acquiring
# a controlling terminal.
pid = os.fork() # Fork a second child.
except OSError as e:
raise Exception("%s [%d]" % (e.strerror, e.errno))
if (pid == 0): # The second child.
# We probably don't want the file mode creation mask inherited from
# the parent, so we give the child complete control over permissions.
if UMASK is not None:
os.umask(UMASK)
else:
# Parent (the first child) of the second child.
os._exit(0)
else:
# exit() or _exit()?
# _exit is like exit(), but it doesn't call any functions registered
# with atexit (and on_exit) or any registered signal handlers. It also
# closes any open file descriptors. Using exit() may cause all stdio
# streams to be flushed twice and any temporary files may be unexpectedly
# removed. It's therefore recommended that child branches of a fork()
# and the parent branch(es) of a daemon use _exit().
return
# Close all open file descriptors. This prevents the child from keeping
# open any file descriptors inherited from the parent. There is a variety
# of methods to accomplish this task. Three are listed below.
#
# Try the system configuration variable, SC_OPEN_MAX, to obtain the maximum
# number of open file descriptors to close. If it doesn't exist, use
# the default value (configurable).
#
# try:
# maxfd = os.sysconf("SC_OPEN_MAX")
# except (AttributeError, ValueError):
# maxfd = MAXFD
#
# OR
#
# if (os.sysconf_names.has_key("SC_OPEN_MAX")):
# maxfd = os.sysconf("SC_OPEN_MAX")
# else:
# maxfd = MAXFD
#
# OR
#
# Use the getrlimit method to retrieve the maximum file descriptor number
# that can be opened by this process. If there is no limit on the
# resource, use the default value.
#
import resource # Resource usage information.
maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1]
if (maxfd == resource.RLIM_INFINITY):
maxfd = MAXFD
# Iterate through and close all file descriptors.
# for fd in range(0, maxfd):
# try:
# os.close(fd)
# except OSError: # ERROR, fd wasn't open to begin with (ignored)
# pass
# 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(REDIRECT_TO, os.O_RDWR) # standard input (0)
# Duplicate standard input to standard output and standard error.
# os.dup2(0, 1) # standard output (1)
# os.dup2(0, 2) # standard error (2)
si = file('/dev/null', 'r')
so = file(logfile, 'w')
se = so
# Replace those fds with our own
os.dup2(si.fileno(), sys.stdin.fileno())
os.dup2(so.fileno(), sys.stdout.fileno())
os.dup2(se.fileno(), sys.stderr.fileno())
function()
os._exit(0)
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