# Copyright (c) 2009, Evan Fosmark
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are met:
#    * Redistributions of source code must retain the above copyright
#      notice, this list of conditions and the following disclaimer.
#    * Redistributions in binary form must reproduce the above copyright
#      notice, this list of conditions and the following disclaimer in the
#      documentation and/or other materials provided with the distribution.
#    * Neither the name of the <organization> nor the
#      names of its contributors may be used to endorse or promote products
#      derived from this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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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.
    """

    def __init__(self, file_name, timeout=10, delay=0.05):
        """Prepare the file locker. Specify the file to lock and optionally
        the maximum timeout and the delay between each attempt to lock.
        """
        if timeout is not None and delay is None:
            raise ValueError(
                "If timeout is not None, then delay must not be None."
            )
        self.is_locked = False
        self.lockfile = os.path.join(os.getcwd(), "%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()
        while True:
            try:
                self.fd = os.open(
                    self.lockfile, os.O_CREAT | os.O_EXCL | os.O_RDWR
                )
                self.is_locked = True  # moved to ensure tag only when locked
                break
            except OSError as e:
                if e.errno != errno.EEXIST:
                    raise
                if self.timeout is None:
                    raise FileLockException(
                        "Could not acquire lock on {}".format(self.file_name)
                    )
                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()