pylons_app/lib/pidlock.py

import os, time
import sys
from warnings import warn
from multiprocessing.util import Finalize
import errno

class LockHeld(Exception):pass


class DaemonLock(object):
    """daemon locking
    USAGE:
    try:
        l = DaemonLock(desc='test lock')
        main()
        l.release()
    except LockHeld:
        sys.exit(1)
    """

    def __init__(self, file=None, callbackfn=None,
                 desc='daemon lock', debug=False):

        self.pidfile = file if file else os.path.join(os.path.dirname(__file__),
                                                      'running.lock')
        self.callbackfn = callbackfn
        self.desc = desc
        self.debug = debug
        self.held = False
        #run the lock automatically !
        self.lock()
        self._finalize = Finalize(self, DaemonLock._on_finalize,
                                    args=(self, debug), exitpriority=10)

    @staticmethod
    def _on_finalize(lock, debug):
        if lock.held:
            if debug:
                print 'leck held finilazing and running lock.release()'
            lock.release()


    def lock(self):
        """locking function, if lock is present it will raise LockHeld exception
        """
        lockname = '%s' % (os.getpid())
        if self.debug:
            print 'running lock'
        self.trylock()
        self.makelock(lockname, self.pidfile)
        return True

    def trylock(self):
        running_pid = False
        if self.debug:
            print 'checking for already running process'
        try:
            pidfile = open(self.pidfile, "r")
            pidfile.seek(0)
            running_pid = int(pidfile.readline())
            
            pidfile.close()
            
            if self.debug:
                print 'lock file present running_pid: %s, checking for execution'\
                % running_pid
            # Now we check the PID from lock file matches to the current
            # process PID
            if running_pid:
                try:
                    os.kill(running_pid, 0)
                except OSError, exc:
                    if exc.errno in (errno.ESRCH, errno.EPERM):
                        print "Lock File is there but the program is not running"
                        print "Removing lock file for the: %s" % running_pid                        
                        self.release()
                    raise
                else:
                    print "You already have an instance of the program running"
                    print "It is running as process %s" % running_pid                    
                    raise LockHeld()
                         
        except IOError, e:
            if e.errno != 2:
                raise

    def release(self):
        """releases the pid by removing the pidfile
        """
        if self.debug:
            print 'trying to release the pidlock'
            
        if self.callbackfn:
            #execute callback function on release
            if self.debug:
                print 'executing callback function %s' % self.callbackfn
            self.callbackfn()
        try:
            if self.debug:
                print 'removing pidfile %s' % self.pidfile
            os.remove(self.pidfile)
            self.held = False
        except OSError, e:
            if self.debug:
                print 'removing pidfile failed %s' % e
            pass

    def makelock(self, lockname, pidfile):
        """
        this function will make an actual lock
        @param lockname: acctual pid of file
        @param pidfile: the file to write the pid in
        """
        if self.debug:
            print 'creating a file %s and pid: %s' % (pidfile, lockname)
        pidfile = open(self.pidfile, "wb")
        pidfile.write(lockname)
        pidfile.close
        self.held = True