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

from signal import SIGTERM 

class Daemon:
	"""
	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):
		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("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("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()
			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())
		
		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):
		"""
		Start the daemon
		"""
		
		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()

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