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374 lines
11 KiB
Python
374 lines
11 KiB
Python
# Copyright 2011 Google Inc.
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain 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,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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"""Locked file interface that should work on Unix and Windows pythons.
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This module first tries to use fcntl locking to ensure serialized access
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to a file, then falls back on a lock file if that is unavialable.
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Usage:
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f = LockedFile('filename', 'r+b', 'rb')
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f.open_and_lock()
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if f.is_locked():
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print 'Acquired filename with r+b mode'
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f.file_handle().write('locked data')
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else:
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print 'Aquired filename with rb mode'
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f.unlock_and_close()
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"""
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__author__ = 'cache@google.com (David T McWherter)'
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import errno
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import logging
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import os
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import time
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from . import util
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logger = logging.getLogger(__name__)
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class CredentialsFileSymbolicLinkError(Exception):
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"""Credentials files must not be symbolic links."""
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class AlreadyLockedException(Exception):
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"""Trying to lock a file that has already been locked by the LockedFile."""
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pass
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def validate_file(filename):
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if os.path.islink(filename):
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raise CredentialsFileSymbolicLinkError(
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'File: %s is a symbolic link.' % filename)
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class _Opener(object):
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"""Base class for different locking primitives."""
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def __init__(self, filename, mode, fallback_mode):
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"""Create an Opener.
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Args:
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filename: string, The pathname of the file.
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mode: string, The preferred mode to access the file with.
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fallback_mode: string, The mode to use if locking fails.
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"""
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self._locked = False
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self._filename = filename
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self._mode = mode
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self._fallback_mode = fallback_mode
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self._fh = None
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def is_locked(self):
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"""Was the file locked."""
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return self._locked
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def file_handle(self):
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"""The file handle to the file. Valid only after opened."""
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return self._fh
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def filename(self):
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"""The filename that is being locked."""
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return self._filename
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def open_and_lock(self, timeout, delay):
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"""Open the file and lock it.
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Args:
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timeout: float, How long to try to lock for.
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delay: float, How long to wait between retries.
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"""
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pass
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def unlock_and_close(self):
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"""Unlock and close the file."""
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pass
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class _PosixOpener(_Opener):
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"""Lock files using Posix advisory lock files."""
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def open_and_lock(self, timeout, delay):
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"""Open the file and lock it.
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Tries to create a .lock file next to the file we're trying to open.
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Args:
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timeout: float, How long to try to lock for.
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delay: float, How long to wait between retries.
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Raises:
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AlreadyLockedException: if the lock is already acquired.
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IOError: if the open fails.
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CredentialsFileSymbolicLinkError if the file is a symbolic link.
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"""
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if self._locked:
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raise AlreadyLockedException('File %s is already locked' %
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self._filename)
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self._locked = False
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validate_file(self._filename)
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try:
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self._fh = open(self._filename, self._mode)
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except IOError, e:
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# If we can't access with _mode, try _fallback_mode and don't lock.
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if e.errno == errno.EACCES:
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self._fh = open(self._filename, self._fallback_mode)
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return
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lock_filename = self._posix_lockfile(self._filename)
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start_time = time.time()
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while True:
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try:
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self._lock_fd = os.open(lock_filename,
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os.O_CREAT|os.O_EXCL|os.O_RDWR)
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self._locked = True
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break
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except OSError, e:
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if e.errno != errno.EEXIST:
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raise
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if (time.time() - start_time) >= timeout:
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logger.warn('Could not acquire lock %s in %s seconds' % (
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lock_filename, timeout))
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# Close the file and open in fallback_mode.
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if self._fh:
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self._fh.close()
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self._fh = open(self._filename, self._fallback_mode)
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return
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time.sleep(delay)
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def unlock_and_close(self):
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"""Unlock a file by removing the .lock file, and close the handle."""
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if self._locked:
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lock_filename = self._posix_lockfile(self._filename)
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os.close(self._lock_fd)
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os.unlink(lock_filename)
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self._locked = False
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self._lock_fd = None
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if self._fh:
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self._fh.close()
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def _posix_lockfile(self, filename):
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"""The name of the lock file to use for posix locking."""
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return '%s.lock' % filename
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try:
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import fcntl
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class _FcntlOpener(_Opener):
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"""Open, lock, and unlock a file using fcntl.lockf."""
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def open_and_lock(self, timeout, delay):
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"""Open the file and lock it.
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Args:
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timeout: float, How long to try to lock for.
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delay: float, How long to wait between retries
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Raises:
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AlreadyLockedException: if the lock is already acquired.
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IOError: if the open fails.
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CredentialsFileSymbolicLinkError if the file is a symbolic link.
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"""
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if self._locked:
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raise AlreadyLockedException('File %s is already locked' %
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self._filename)
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start_time = time.time()
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validate_file(self._filename)
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try:
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self._fh = open(self._filename, self._mode)
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except IOError, e:
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# If we can't access with _mode, try _fallback_mode and don't lock.
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if e.errno == errno.EACCES:
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self._fh = open(self._filename, self._fallback_mode)
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return
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# We opened in _mode, try to lock the file.
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while True:
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try:
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fcntl.lockf(self._fh.fileno(), fcntl.LOCK_EX)
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self._locked = True
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return
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except IOError, e:
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# If not retrying, then just pass on the error.
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if timeout == 0:
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raise e
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if e.errno != errno.EACCES:
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raise e
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# We could not acquire the lock. Try again.
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if (time.time() - start_time) >= timeout:
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logger.warn('Could not lock %s in %s seconds' % (
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self._filename, timeout))
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if self._fh:
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self._fh.close()
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self._fh = open(self._filename, self._fallback_mode)
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return
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time.sleep(delay)
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def unlock_and_close(self):
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"""Close and unlock the file using the fcntl.lockf primitive."""
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if self._locked:
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fcntl.lockf(self._fh.fileno(), fcntl.LOCK_UN)
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self._locked = False
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if self._fh:
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self._fh.close()
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except ImportError:
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_FcntlOpener = None
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try:
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import pywintypes
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import win32con
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import win32file
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class _Win32Opener(_Opener):
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"""Open, lock, and unlock a file using windows primitives."""
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# Error #33:
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# 'The process cannot access the file because another process'
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FILE_IN_USE_ERROR = 33
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# Error #158:
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# 'The segment is already unlocked.'
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FILE_ALREADY_UNLOCKED_ERROR = 158
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def open_and_lock(self, timeout, delay):
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"""Open the file and lock it.
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Args:
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timeout: float, How long to try to lock for.
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delay: float, How long to wait between retries
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Raises:
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AlreadyLockedException: if the lock is already acquired.
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IOError: if the open fails.
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CredentialsFileSymbolicLinkError if the file is a symbolic link.
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"""
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if self._locked:
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raise AlreadyLockedException('File %s is already locked' %
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self._filename)
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start_time = time.time()
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validate_file(self._filename)
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try:
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self._fh = open(self._filename, self._mode)
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except IOError, e:
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# If we can't access with _mode, try _fallback_mode and don't lock.
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if e.errno == errno.EACCES:
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self._fh = open(self._filename, self._fallback_mode)
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return
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# We opened in _mode, try to lock the file.
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while True:
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try:
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hfile = win32file._get_osfhandle(self._fh.fileno())
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win32file.LockFileEx(
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hfile,
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(win32con.LOCKFILE_FAIL_IMMEDIATELY|
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win32con.LOCKFILE_EXCLUSIVE_LOCK), 0, -0x10000,
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pywintypes.OVERLAPPED())
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self._locked = True
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return
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except pywintypes.error, e:
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if timeout == 0:
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raise e
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# If the error is not that the file is already in use, raise.
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if e[0] != _Win32Opener.FILE_IN_USE_ERROR:
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raise
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# We could not acquire the lock. Try again.
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if (time.time() - start_time) >= timeout:
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logger.warn('Could not lock %s in %s seconds' % (
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self._filename, timeout))
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if self._fh:
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self._fh.close()
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self._fh = open(self._filename, self._fallback_mode)
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return
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time.sleep(delay)
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def unlock_and_close(self):
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"""Close and unlock the file using the win32 primitive."""
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if self._locked:
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try:
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hfile = win32file._get_osfhandle(self._fh.fileno())
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win32file.UnlockFileEx(hfile, 0, -0x10000, pywintypes.OVERLAPPED())
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except pywintypes.error, e:
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if e[0] != _Win32Opener.FILE_ALREADY_UNLOCKED_ERROR:
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raise
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self._locked = False
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if self._fh:
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self._fh.close()
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except ImportError:
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_Win32Opener = None
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class LockedFile(object):
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"""Represent a file that has exclusive access."""
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@util.positional(4)
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def __init__(self, filename, mode, fallback_mode, use_native_locking=True):
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"""Construct a LockedFile.
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Args:
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filename: string, The path of the file to open.
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mode: string, The mode to try to open the file with.
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fallback_mode: string, The mode to use if locking fails.
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use_native_locking: bool, Whether or not fcntl/win32 locking is used.
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"""
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opener = None
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if not opener and use_native_locking:
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if _Win32Opener:
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opener = _Win32Opener(filename, mode, fallback_mode)
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if _FcntlOpener:
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opener = _FcntlOpener(filename, mode, fallback_mode)
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if not opener:
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opener = _PosixOpener(filename, mode, fallback_mode)
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self._opener = opener
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def filename(self):
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"""Return the filename we were constructed with."""
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return self._opener._filename
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def file_handle(self):
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"""Return the file_handle to the opened file."""
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return self._opener.file_handle()
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def is_locked(self):
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"""Return whether we successfully locked the file."""
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return self._opener.is_locked()
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def open_and_lock(self, timeout=0, delay=0.05):
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"""Open the file, trying to lock it.
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Args:
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timeout: float, The number of seconds to try to acquire the lock.
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delay: float, The number of seconds to wait between retry attempts.
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Raises:
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AlreadyLockedException: if the lock is already acquired.
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IOError: if the open fails.
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"""
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self._opener.open_and_lock(timeout, delay)
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def unlock_and_close(self):
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"""Unlock and close a file."""
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self._opener.unlock_and_close()
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