daemon.py

"""
Copyright (C) 2014, Jaguar Land Rover

This program is licensed under the terms and conditions of the
Mozilla Public License, version 2.0.  The full text of the 
Mozilla Public License is at https://www.mozilla.org/MPL/2.0/

Adapted from http://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/

Maintainer: Rudolf Streif (rstreif@jaguarlandrover.com) 
"""

"""
Base class to create Linux daemons.
"""

import os, sys, atexit, time
from signal import *


class Daemon(object):
    """
    A generic daemon class.
    Usage: subclass the Daemon class and override the run() method
    """
    def __init__(self, pidfile, stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):
        self.stdin = stdin
        self.stdout = stdout
        self.stderr = stderr
        self.pidfile = pidfile
       
    def daemonize(self):
        """
        do the UNIX double-fork magic, see Stevens' "Advanced
        Programming in the UNIX Environment" for details (ISBN 0201563177)
        http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC16
         """
        try:
            pid = os.fork()
            if pid > 0:
                # exit first parent
                sys.exit(0)
        except OSError, e:
            sys.stderr.write("Daemon: Fork #1 failed: %d (%s)\n" % (e.errno, e.strerror))
            sys.exit(1)
       
        # decouple from parent environment
        os.chdir("/")
        os.setsid()
        os.umask(0)
       
        # do second fork
        try:
            pid = os.fork()
            if pid > 0:
                # exit from second parent
                sys.exit(0)
        except OSError, e:
            sys.stderr.write("Daemon: Fork #2 failed: %d (%s)\n" % (e.errno, e.strerror))
            sys.exit(1)
       
        # redirect standard file descriptors
        sys.stdout.flush()
        sys.stderr.flush()
        si = file(self.stdin, 'r')
        so = file(self.stdout, 'a+')
        se = file(self.stderr, 'a+', 0)
        os.dup2(si.fileno(), sys.stdin.fileno())
        os.dup2(so.fileno(), sys.stdout.fileno())
        os.dup2(se.fileno(), sys.stderr.fileno())
       
        # write pidfile
        atexit.register(self.delpid)
        pid = str(os.getpid())
        file(self.pidfile,'w+').write("%s\n" % pid)
       
    def delpid(self):
        os.remove(self.pidfile)
 
    def start(self):
         """
         Start the daemon
         """
         # Check for a pidfile to see if the daemon already runs
         try:
             pf = file(self.pidfile,'r')
             pid = int(pf.read().strip())
             pf.close()
         except IOError:
              pid = None
       
         if pid:
             message = "Daemon: Pidfile %s already exist. Daemon already running?\n"
             sys.stderr.write(message % self.pidfile)
             sys.exit(1)
               
         # Start the daemon
         self.daemonize()
         self.run()
 
    def stop(self):
        """
        Stop the daemon
        """
        # Get the pid from the pidfile
        try:
            pf = file(self.pidfile,'r')
            pid = int(pf.read().strip())
            pf.close()
        except IOError:
            pid = None
       
        if not pid:
            message = "Daemon: Pidfile %s does not exist. Daemon not running?\n"
            sys.stderr.write(message % self.pidfile)
            return # not an error in a restart
 
        # Try killing the daemon process       
        try:
            while 1:
                os.kill(pid, SIGTERM)
                time.sleep(0.1)
        except OSError, err:
            err = str(err)
            if err.find("No such process") > 0:
                if os.path.exists(self.pidfile):
                    os.remove(self.pidfile)
            else:
                message = "Daemon: Error shutting down: %s\n"
                sys.stderr.write(message % err)
                sys.exit(1)
 
    def restart(self):
        """
        Restart the daemon
        """
        self.stop()
        self.start()
 
    def run(self):
        """
        You should override this method when you subclass Daemon. It will be called after the process has been
        daemonized by start() or restart().
        """