# Copyright (c) 2009, Evan Fosmark
# All rights reserved.
#
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# modification, are permitted provided that the following conditions are met:
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# 1. Redistributions of source code must retain the above copyright notice, this
#    list of conditions and the following disclaimer.
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import os
import time
import errno


class FileLockException(Exception):
    pass


class FileLock(object):
    """ A file locking mechanism that has context-manager support so
        you can use it in a with statement. This should be relatively cross
        compatible as it doesn't rely on msvcrt or fcntl for the locking.
    """

    __slots__ = ('fd', 'is_locked', 'lockfile', 'file_name', 'timeout', 'delay')

    def __init__(self, file_name, timeout = 10, delay = .05):
        """ Prepare the file locker. Specify the file to lock and optionally
            the maximum timeout and the delay between each attempt to lock.
        """
        self.is_locked = False
        self.lockfile = os.path.abspath(
            os.path.expanduser(os.path.expandvars("%s.lock" % file_name)))
        self.file_name = file_name
        self.timeout = timeout
        self.delay = delay


    def acquire(self):
        """ Acquire the lock, if possible. If the lock is in use, it check again
            every `wait` seconds. It does this until it either gets the lock or
            exceeds `timeout` number of seconds, in which case it throws
            an exception.
        """
        start_time = time.time()
        pid = os.getpid()
        while True:
            try:
                self.fd = os.open(self.lockfile,
                                  os.O_CREAT | os.O_EXCL | os.O_RDWR)
                os.write(self.fd, "%d" % pid)
                break
            except OSError as e:
                if e.errno != errno.EEXIST:
                    raise
                if (time.time() - start_time) >= self.timeout:
                    raise FileLockException("Timeout occured.")
                time.sleep(self.delay)
        self.is_locked = True


    def release(self):
        """ Get rid of the lock by deleting the lockfile.
            When working in a `with` statement, this gets automatically
            called at the end.
        """
        if self.is_locked:
            os.close(self.fd)
            os.unlink(self.lockfile)
            self.is_locked = False


    def __enter__(self):
        """ Activated when used in the with statement.
            Should automatically acquire a lock to be used in the with block.
        """
        if not self.is_locked:
            self.acquire()
        return self


    def __exit__(self, type, value, traceback):
        """ Activated at the end of the with statement.
            It automatically releases the lock if it isn't locked.
        """
        if self.is_locked:
            self.release()


    def __del__(self):
        """ Make sure that the FileLock instance doesn't leave a lockfile
            lying around.
        """
        self.release()
