daemon.py

'''
    ***
    Modified generic daemon class
    ***

    Author:     http://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/
                www.boxedice.com

    License:    http://creativecommons.org/licenses/by-sa/3.0/

    Changes:    23rd Jan 2009 (David Mytton <david@boxedice.com>)
                - Replaced hard coded '/dev/null in __init__ with os.devnull
                - Added OS check to conditionally remove code that doesn't work on OS X
                - Added output to console on completion
                - Tidied up formatting
                11th Mar 2009 (David Mytton <david@boxedice.com>)
                - Fixed problem with daemon exiting on Python 2.4 (before SystemExit was part of the Exception base)
                13th Aug 2010 (David Mytton <david@boxedice.com>
                - Fixed unhandled exception if PID file is empty
'''

# Core modules
import atexit
import os
import sys
import time
import signal


class Daemon(object):
    """
    A generic daemon class.

    Usage: subclass the Daemon class and override the run() method
    """
    def __init__(self, pidfile, stdin=os.devnull, stdout=os.devnull, stderr=os.devnull, home_dir='.', umask=022, verbose=1):
        self.stdin = stdin
        self.stdout = stdout
        self.stderr = stderr
        self.pidfile = pidfile
        self.home_dir = home_dir
        self.verbose = verbose
        self.umask = umask
        self.daemon_alive = True

    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("fork #1 failed: %d (%s)\n" % (e.errno, e.strerror))
            sys.exit(1)

        # Decouple from parent environment
        os.chdir(self.home_dir)
        os.setsid()
        os.umask(self.umask)

        # Do 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: %d (%s)\n" % (e.errno, e.strerror))
            sys.exit(1)

        if sys.platform != 'darwin': # This block breaks on OS X
            # Redirect standard file descriptors
            sys.stdout.flush()
            sys.stderr.flush()
            if isinstance(self.stdin, file):
                si = self.stdin
            else:
                si = file(self.stdin, 'r')
            if isinstance(self.stdout, file):
                so = self.stdout
            else:
                so = file(self.stdout, 'a+')
            if self.stderr:
                if isinstance(self.stderr, file):
                    se = self.stderr
                else:
                    se = file(self.stderr, 'a+', 0)
            else:
                se = so
            os.dup2(si.fileno(), sys.stdin.fileno())
            os.dup2(so.fileno(), sys.stdout.fileno())
            os.dup2(se.fileno(), sys.stderr.fileno())

        def sigtermhandler(signum, frame):
            self.daemon_alive = False
        signal.signal(signal.SIGTERM, sigtermhandler)
        signal.signal(signal.SIGINT, sigtermhandler)

        if self.verbose >= 1:
            print "Started"

        # Write pidfile
        atexit.register(self.delpid) # Make sure pid file is removed if we quit
        pid = str(os.getpid())
        file(self.pidfile,'w+').write("%s\n" % pid)

    def delpid(self):
        os.remove(self.pidfile)

    def start(self, *args, **kwargs):
        """
        Start the daemon
        """

        if self.verbose >= 1:
            print "Starting..."

        # 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
        except SystemExit:
            pid = None

        if pid:
            message = "pidfile %s already exists. Is it already running?\n"
            sys.stderr.write(message % self.pidfile)
            sys.exit(1)

        # Start the daemon
        self.daemonize()
        self.run(*args, **kwargs)

    def stop(self):
        """
        Stop the daemon
        """

        if self.verbose >= 1:
            print "Stopping..."

        # Get the pid from the pidfile
        try:
            pf = file(self.pidfile,'r')
            pid = int(pf.read().strip())
            pf.close()
        except IOError:
            pid = None
        except ValueError:
            pid = None

        if not pid:
            message = "pidfile %s does not exist. Not running?\n"
            sys.stderr.write(message % self.pidfile)

            # Just to be sure. A ValueError might occur if the PID file is empty but does actually exist
            if os.path.exists(self.pidfile):
                os.remove(self.pidfile)

            return # Not an error in a restart

        # Try killing the daemon process
        try:
            i = 0
            while 1:
                os.kill(pid, signal.SIGTERM)
                time.sleep(0.1)
                i = i + 1
                if i % 10 == 0:
                    os.kill(pid, signal.SIGHUP)
        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)

        if self.verbose >= 1:
            print "Stopped"

    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().
        """