#!/usr/bin/python3
"""
Univention Common Python Library for file locking
"""
# Copyright 2011-2022 Univention GmbH
#
# https://www.univention.de/
#
# All rights reserved.
#
# The source code of this program is made available
# under the terms of the GNU Affero General Public License version 3
# (GNU AGPL V3) as published by the Free Software Foundation.
#
# Binary versions of this program provided by Univention to you as
# well as other copyrighted, protected or trademarked materials like
# Logos, graphics, fonts, specific documentations and configurations,
# cryptographic keys etc. are subject to a license agreement between
# you and Univention and not subject to the GNU AGPL V3.
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# In the case you use this program under the terms of the GNU AGPL V3,
# the program is provided in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public
# License with the Debian GNU/Linux or Univention distribution in file
# /usr/share/common-licenses/AGPL-3; if not, see
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from __future__ import print_function
import fcntl
import os
from typing import IO, Optional, Text # noqa: F401
[docs]def get_lock(name, nonblocking=False):
# type: (str, bool) -> Optional[IO[Text]]
"""
Get a exclusive lock.
:param str name: The name for the lock file.
:param bool nonblocking: Return `None` instead of waiting indefinitely to get the exclusive lock if the lock is already taken.
:returns: a file descriptor for a lock file after the file has been locked exclusively. In non-blocking mode `None` is returned if the lock cannot be gained.
:rtype: file or None
The returned file descriptor has to be kept. Otherwise the lock will
be release automatically on file descriptor's destruction.
>>> fd = get_lock('myapp')
>>> # ...... do some critical stuff ......
>>> release_lock(fd)
>>>
>>> fd = get_lock('myapp', nonblocking=True)
>>> if not fd:
>>> print('cannot get lock')
>>> else:
>>> # ...... do some critical stuff ......
>>> release_lock(fd)
"""
fn = "/var/run/%s.pid" % name
fd = open(fn, 'w')
try:
if nonblocking:
fcntl.lockf(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
else:
fcntl.lockf(fd, fcntl.LOCK_EX)
except IOError as e:
if e.errno == 11:
return None
raise
fd.write('%s\n' % os.getpid())
fd.flush()
return fd
[docs]def release_lock(fd):
# type: (IO[Text]) -> None
"""
Releases the previously gained lock.
:param file fd: The file descriptor of the lock file.
"""
fcntl.lockf(fd, fcntl.LOCK_UN)
fd.close()
