daemon.py

#!/usr/bin/env python

import sys, os, time, atexit
from signal import SIGTERM 
import fcntl

class Daemon:
    """
    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 as 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 as e:
            sys.stderr.write("fork #2 failed: %d (%s)\n" % (e.errno, e.strerror))
            sys.exit(1) 
    
        if True:
            # 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
        print("try to lock now...")
        self.lockfh = self.lockpidfile()
        if self.lockfh:
            print("locked well")
        else:
            print("not locked")
        atexit.register(self.delpid)
        pid = str(os.getpid())
        # file(self.pidfile,'w+').write("%s\n" % pid)
        # self.lockfh.write(str(os.getpid())) 
        # self.lockfh.flush()


    def delpid(self):
        try:
            os.remove(self.pidfile)
        except OSError:
            # ok if no pidfile
            pass

    def start(self):
        """
        Start the daemon
        """
        
        # Start the daemon
        if self.lockpidfile() is None:
            print("failed to get exclusive lock {}".format(self.pidfile))
            return False
        self.daemonize()
        self.run()
        return True

    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 %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 as 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()
   
    # returns pid number or None
    def lockedpidfile(self):
        try: 
            fp = file(self.pidfile, 'r')
        except IOError:
            return None

        try:
            fcntl.lockf(fp, fcntl.LOCK_EX | fcntl.LOCK_NB)
            return None
        except IOError:
            pidstr = fp.read()
            if pidstr:
                pid = int(pidstr)
                return pid
            else:
                return None
            


    def lockpidfile(self):
        fp = open(self.pidfile, 'w')
        try:
            fcntl.lockf(fp, fcntl.LOCK_EX | fcntl.LOCK_NB)
            fp.write(str(os.getpid()))
            fp.flush()
            return fp
        except IOError:
            return None


    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().
        """