6d9d4ed012
We need to access the platform-specific file handle locking mechanisms, but they are hidden away in the class implementations. This pushes them into static methods to make it easy to access them while still allowing sub-classing.
281 lines
9.2 KiB
Python
281 lines
9.2 KiB
Python
# -*- coding: utf-8 -*-
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# Copyright 2011 OpenStack Foundation.
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# All Rights Reserved.
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may
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# not use this file except in compliance with the License. You may obtain
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# a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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# License for the specific language governing permissions and limitations
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# under the License.
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import errno
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import logging
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import os
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import threading
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import time
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import six
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from fasteners import _utils
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LOG = logging.getLogger(__name__)
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def _ensure_tree(path):
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"""Create a directory (and any ancestor directories required).
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:param path: Directory to create
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"""
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try:
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os.makedirs(path)
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except OSError as e:
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if e.errno == errno.EEXIST:
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if not os.path.isdir(path):
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raise
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else:
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return False
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elif e.errno == errno.EISDIR:
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return False
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else:
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raise
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else:
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return True
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class _InterProcessLock(object):
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"""An interprocess locking implementation.
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This is a lock implementation which allows multiple locks, working around
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issues like http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=632857 and
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does not require any cleanup. Since the lock is always held on a file
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descriptor rather than outside of the process, the lock gets dropped
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automatically if the process crashes, even if ``__exit__`` is not
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executed.
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There are no guarantees regarding usage by multiple threads in a
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single process here. This lock works only between processes.
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Note these locks are released when the descriptor is closed, so it's not
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safe to close the file descriptor while another thread holds the
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lock. Just opening and closing the lock file can break synchronization,
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so lock files must be accessed only using this abstraction.
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.. warning::
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It is quite useful to read before using (to understand
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the risks involved): http://0pointer.de/blog/projects/locking.html
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"""
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MAX_DELAY = 0.1
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"""
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Default maximum delay we will wait to try to acquire the lock (when
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it's busy/being held by another process).
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"""
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DELAY_INCREMENT = 0.01
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"""
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Default increment we will use (up to max delay) after each attempt before
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next attempt to acquire the lock. For example if 3 attempts have been made
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the calling thread will sleep (0.01 * 3) before the next attempt to
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acquire the lock (and repeat).
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"""
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def __init__(self, path, sleep_func=time.sleep, logger=None):
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self.lockfile = None
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self.path = _utils.canonicalize_path(path)
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self.acquired = False
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self.sleep_func = sleep_func
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self.logger = _utils.pick_first_not_none(logger, LOG)
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def _try_acquire(self, blocking, watch):
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try:
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self.trylock()
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except IOError as e:
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if e.errno in (errno.EACCES, errno.EAGAIN):
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if not blocking or watch.expired():
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return False
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else:
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raise _utils.RetryAgain()
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else:
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raise threading.ThreadError("Unable to acquire lock on"
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" `%(path)s` due to"
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" %(exception)s" %
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{
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'path': self.path,
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'exception': e,
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})
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else:
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return True
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def _do_open(self):
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basedir = os.path.dirname(self.path)
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if basedir:
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made_basedir = _ensure_tree(basedir)
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if made_basedir:
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self.logger.log(_utils.BLATHER,
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'Created lock base path `%s`', basedir)
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# Open in append mode so we don't overwrite any potential contents of
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# the target file. This eliminates the possibility of an attacker
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# creating a symlink to an important file in our lock path.
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if self.lockfile is None or self.lockfile.closed:
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self.lockfile = open(self.path, 'a')
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def acquire(self, blocking=True,
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delay=DELAY_INCREMENT, max_delay=MAX_DELAY,
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timeout=None):
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"""Attempt to acquire the given lock.
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:param blocking: whether to wait forever to try to acquire the lock
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:type blocking: bool
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:param delay: when blocking this is the delay time in seconds that
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will be added after each failed acquisition
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:type delay: int/float
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:param max_delay: the maximum delay to have (this limits the
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accumulated delay(s) added after each failed
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acquisition)
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:type max_delay: int/float
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:param timeout: an optional timeout (limits how long blocking
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will occur for)
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:type timeout: int/float
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:returns: whether or not the acquisition succeeded
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:rtype: bool
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"""
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if delay < 0:
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raise ValueError("Delay must be greater than or equal to zero")
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if timeout is not None and timeout < 0:
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raise ValueError("Timeout must be greater than or equal to zero")
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if delay >= max_delay:
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max_delay = delay
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self._do_open()
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watch = _utils.StopWatch(duration=timeout)
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r = _utils.Retry(delay, max_delay,
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sleep_func=self.sleep_func, watch=watch)
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with watch:
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gotten = r(self._try_acquire, blocking, watch)
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if not gotten:
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self.acquired = False
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return False
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else:
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self.acquired = True
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self.logger.log(_utils.BLATHER,
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"Acquired file lock `%s` after waiting %0.3fs [%s"
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" attempts were required]", self.path,
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watch.elapsed(), r.attempts)
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return True
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def _do_close(self):
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if self.lockfile is not None:
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self.lockfile.close()
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self.lockfile = None
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def __enter__(self):
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gotten = self.acquire()
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if not gotten:
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# This shouldn't happen, but just incase...
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raise threading.ThreadError("Unable to acquire a file lock"
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" on `%s` (when used as a"
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" context manager)" % self.path)
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return self
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def release(self):
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"""Release the previously acquired lock."""
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if not self.acquired:
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raise threading.ThreadError("Unable to release an unacquired"
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" lock")
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try:
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self.unlock()
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except IOError:
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self.logger.exception("Could not unlock the acquired lock opened"
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" on `%s`", self.path)
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else:
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self.acquired = False
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try:
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self._do_close()
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except IOError:
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self.logger.exception("Could not close the file handle"
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" opened on `%s`", self.path)
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else:
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self.logger.log(_utils.BLATHER,
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"Unlocked and closed file lock open on"
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" `%s`", self.path)
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def __exit__(self, exc_type, exc_val, exc_tb):
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self.release()
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def exists(self):
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"""Checks if the path that this lock exists at actually exists."""
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return os.path.exists(self.path)
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def trylock(self):
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self._trylock(self.lockfile)
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def unlock(self):
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self._unlock(self.lockfile)
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@staticmethod
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def _trylock():
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raise NotImplementedError()
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@staticmethod
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def _unlock():
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raise NotImplementedError()
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class _WindowsLock(_InterProcessLock):
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"""Interprocess lock implementation that works on windows systems."""
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@staticmethod
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def _trylock(lockfile):
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fileno = lockfile.fileno()
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msvcrt.locking(fileno, msvcrt.LK_NBLCK, 1)
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@staticmethod
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def _unlock(lockfile):
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fileno = lockfile.fileno()
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msvcrt.locking(fileno, msvcrt.LK_UNLCK, 1)
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class _FcntlLock(_InterProcessLock):
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"""Interprocess lock implementation that works on posix systems."""
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@staticmethod
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def _trylock(lockfile):
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fcntl.lockf(lockfile, fcntl.LOCK_EX | fcntl.LOCK_NB)
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@staticmethod
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def _unlock(lockfile):
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fcntl.lockf(lockfile, fcntl.LOCK_UN)
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if os.name == 'nt':
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import msvcrt
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InterProcessLock = _WindowsLock
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else:
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import fcntl
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InterProcessLock = _FcntlLock
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def interprocess_locked(path):
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"""Acquires & releases a interprocess lock around call into
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decorated function."""
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lock = InterProcessLock(path)
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def decorator(f):
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@six.wraps(f)
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def wrapper(*args, **kwargs):
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with lock:
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return f(*args, **kwargs)
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return wrapper
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return decorator
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