daemon.py

#! /usr/bin/env python

import sys
import os
import time
import atexit

from signal import SIGTERM


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's "Advanced
        Programming in the UNIX Environment" for detaill (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(
                "fork #1 failed:{1} ({2}) \n".format(e.errno, e.strerror))
            sys.exit(1)

        # decouble from parent enviroment
        os.chdir("/")
        os.setsid()
        os.umask(0)

        # do the second fork
        try:
            pid = os.fork()
            if pid > 0:
                # exit from second parent
                sys.exit(0)
        except OSError, e:
            sys.stderr.write(
                "fork #2 failed:{0} (1)\n".format(e.errno, e.strerror))

        # 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("{0}\n".format(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 = "pidfile {0} already exist. Daemon already running?\n"
            sys.stderr.write(message.format(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 = "pidfile {0} does not exist. Deamon not running?\n"
            sys.stderr.write(message.format(self.pidfile))
            return

        # try to kill the Daemon
        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:
                print str(err)
                sys.exit(1)

    def restart(self):
        """
        Restart the daemon
        """
        self.stop()
        self.start()

    def run(self):
        """
        You should override the method when you subclass Daemon. It will
        be called after the daemonized by start() or restart()
        """


class DaemonDemo(Daemon):

    def run(self):
        while True:
            sys.stdout.write("help me!\n")
            sys.stdout.flush()
            time.sleep(1)


if __name__ == '__main__':
    pid = "/tmp/daemon-example.pid"
    stdout = "/tmp/daemon-example.stdout"
    daemon = DaemonDemo(pidfile=pid, stdout=stdout)
    if len(sys.argv) == 2:
        if 'start' == sys.argv[1]:
            daemon.start()
        elif 'stop' == sys.argv[1]:
            daemon.stop()
        elif 'restart' == sys.argv[1]:
            daemon.restart()
        else:
            print 'Unknown command'
            sys.exit(2)

        sys.exit(0)
    else:
        print 'usage:%s start|stop|restart '.format(sys.argv[1])
        sys.exit(2)