# Copyright 2011 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Locked file interface that should work on Unix and Windows pythons.
This module first tries to use fcntl locking to ensure serialized access
to a file, then falls back on a lock file if that is unavialable.
Usage:
f = LockedFile('filename', 'r+b', 'rb')
f.open_and_lock()
if f.is_locked():
print 'Acquired filename with r+b mode'
f.file_handle().write('locked data')
else:
print 'Aquired filename with rb mode'
f.unlock_and_close()
"""
__author__ = 'cache@google.com (David T McWherter)'
import errno
import logging
import os
import time
from oauth2client import util
logger = logging.getLogger(__name__)
class CredentialsFileSymbolicLinkError(Exception):
"""Credentials files must not be symbolic links."""
class AlreadyLockedException(Exception):
"""Trying to lock a file that has already been locked by the LockedFile."""
pass
def validate_file(filename):
if os.path.islink(filename):
raise CredentialsFileSymbolicLinkError(
'File: %s is a symbolic link.' % filename)
class _Opener(object):
"""Base class for different locking primitives."""
def __init__(self, filename, mode, fallback_mode):
"""Create an Opener.
Args:
filename: string, The pathname of the file.
mode: string, The preferred mode to access the file with.
fallback_mode: string, The mode to use if locking fails.
"""
self._locked = False
self._filename = filename
self._mode = mode
self._fallback_mode = fallback_mode
self._fh = None
def is_locked(self):
"""Was the file locked."""
return self._locked
def file_handle(self):
"""The file handle to the file. Valid only after opened."""
return self._fh
def filename(self):
"""The filename that is being locked."""
return self._filename
def open_and_lock(self, timeout, delay):
"""Open the file and lock it.
Args:
timeout: float, How long to try to lock for.
delay: float, How long to wait between retries.
"""
pass
def unlock_and_close(self):
"""Unlock and close the file."""
pass
class _PosixOpener(_Opener):
"""Lock files using Posix advisory lock files."""
def open_and_lock(self, timeout, delay):
"""Open the file and lock it.
Tries to create a .lock file next to the file we're trying to open.
Args:
timeout: float, How long to try to lock for.
delay: float, How long to wait between retries.
Raises:
AlreadyLockedException: if the lock is already acquired.
IOError: if the open fails.
CredentialsFileSymbolicLinkError if the file is a symbolic link.
"""
if self._locked:
raise AlreadyLockedException('File %s is already locked' %
self._filename)
self._locked = False
validate_file(self._filename)
try:
self._fh = open(self._filename, self._mode)
except IOError, e:
# If we can't access with _mode, try _fallback_mode and don't lock.
if e.errno == errno.EACCES:
self._fh = open(self._filename, self._fallback_mode)
return
lock_filename = self._posix_lockfile(self._filename)
start_time = time.time()
while True:
try:
self._lock_fd = os.open(lock_filename,
os.O_CREAT|os.O_EXCL|os.O_RDWR)
self._locked = True
break
except OSError, e:
if e.errno != errno.EEXIST:
raise
if (time.time() - start_time) >= timeout:
logger.warn('Could not acquire lock %s in %s seconds' % (
lock_filename, timeout))
# Close the file and open in fallback_mode.
if self._fh:
self._fh.close()
self._fh = open(self._filename, self._fallback_mode)
return
time.sleep(delay)
def unlock_and_close(self):
"""Unlock a file by removing the .lock file, and close the handle."""
if self._locked:
lock_filename = self._posix_lockfile(self._filename)
os.close(self._lock_fd)
os.unlink(lock_filename)
self._locked = False
self._lock_fd = None
if self._fh:
self._fh.close()
def _posix_lockfile(self, filename):
"""The name of the lock file to use for posix locking."""
return '%s.lock' % filename
try:
import fcntl
class _FcntlOpener(_Opener):
"""Open, lock, and unlock a file using fcntl.lockf."""
def open_and_lock(self, timeout, delay):
"""Open the file and lock it.
Args:
timeout: float, How long to try to lock for.
delay: float, How long to wait between retries
Raises:
AlreadyLockedException: if the lock is already acquired.
IOError: if the open fails.
CredentialsFileSymbolicLinkError if the file is a symbolic link.
"""
if self._locked:
raise AlreadyLockedException('File %s is already locked' %
self._filename)
start_time = time.time()
validate_file(self._filename)
try:
self._fh = open(self._filename, self._mode)
except IOError, e:
# If we can't access with _mode, try _fallback_mode and don't lock.
if e.errno == errno.EACCES:
self._fh = open(self._filename, self._fallback_mode)
return
# We opened in _mode, try to lock the file.
while True:
try:
fcntl.lockf(self._fh.fileno(), fcntl.LOCK_EX)
self._locked = True
return
except IOError, e:
# If not retrying, then just pass on the error.
if timeout == 0:
raise e
if e.errno != errno.EACCES:
raise e
# We could not acquire the lock. Try again.
if (time.time() - start_time) >= timeout:
logger.warn('Could not lock %s in %s seconds' % (
self._filename, timeout))
if self._fh:
self._fh.close()
self._fh = open(self._filename, self._fallback_mode)
return
time.sleep(delay)
def unlock_and_close(self):
"""Close and unlock the file using the fcntl.lockf primitive."""
if self._locked:
fcntl.lockf(self._fh.fileno(), fcntl.LOCK_UN)
self._locked = False
if self._fh:
self._fh.close()
except ImportError:
_FcntlOpener = None
try:
import pywintypes
import win32con
import win32file
class _Win32Opener(_Opener):
"""Open, lock, and unlock a file using windows primitives."""
# Error #33:
# 'The process cannot access the file because another process'
FILE_IN_USE_ERROR = 33
# Error #158:
# 'The segment is already unlocked.'
FILE_ALREADY_UNLOCKED_ERROR = 158
def open_and_lock(self, timeout, delay):
"""Open the file and lock it.
Args:
timeout: float, How long to try to lock for.
delay: float, How long to wait between retries
Raises:
AlreadyLockedException: if the lock is already acquired.
IOError: if the open fails.
CredentialsFileSymbolicLinkError if the file is a symbolic link.
"""
if self._locked:
raise AlreadyLockedException('File %s is already locked' %
self._filename)
start_time = time.time()
validate_file(self._filename)
try:
self._fh = open(self._filename, self._mode)
except IOError, e:
# If we can't access with _mode, try _fallback_mode and don't lock.
if e.errno == errno.EACCES:
self._fh = open(self._filename, self._fallback_mode)
return
# We opened in _mode, try to lock the file.
while True:
try:
hfile = win32file._get_osfhandle(self._fh.fileno())
win32file.LockFileEx(
hfile,
(win32con.LOCKFILE_FAIL_IMMEDIATELY|
win32con.LOCKFILE_EXCLUSIVE_LOCK), 0, -0x10000,
pywintypes.OVERLAPPED())
self._locked = True
return
except pywintypes.error, e:
if timeout == 0:
raise e
# If the error is not that the file is already in use, raise.
if e[0] != _Win32Opener.FILE_IN_USE_ERROR:
raise
# We could not acquire the lock. Try again.
if (time.time() - start_time) >= timeout:
logger.warn('Could not lock %s in %s seconds' % (
self._filename, timeout))
if self._fh:
self._fh.close()
self._fh = open(self._filename, self._fallback_mode)
return
time.sleep(delay)
def unlock_and_close(self):
"""Close and unlock the file using the win32 primitive."""
if self._locked:
try:
hfile = win32file._get_osfhandle(self._fh.fileno())
win32file.UnlockFileEx(hfile, 0, -0x10000, pywintypes.OVERLAPPED())
except pywintypes.error, e:
if e[0] != _Win32Opener.FILE_ALREADY_UNLOCKED_ERROR:
raise
self._locked = False
if self._fh:
self._fh.close()
except ImportError:
_Win32Opener = None
class LockedFile(object):
"""Represent a file that has exclusive access."""
@util.positional(4)
def __init__(self, filename, mode, fallback_mode, use_native_locking=True):
"""Construct a LockedFile.
Args:
filename: string, The path of the file to open.
mode: string, The mode to try to open the file with.
fallback_mode: string, The mode to use if locking fails.
use_native_locking: bool, Whether or not fcntl/win32 locking is used.
"""
opener = None
if not opener and use_native_locking:
if _Win32Opener:
opener = _Win32Opener(filename, mode, fallback_mode)
if _FcntlOpener:
opener = _FcntlOpener(filename, mode, fallback_mode)
if not opener:
opener = _PosixOpener(filename, mode, fallback_mode)
self._opener = opener
def filename(self):
"""Return the filename we were constructed with."""
return self._opener._filename
def file_handle(self):
"""Return the file_handle to the opened file."""
return self._opener.file_handle()
def is_locked(self):
"""Return whether we successfully locked the file."""
return self._opener.is_locked()
def open_and_lock(self, timeout=0, delay=0.05):
"""Open the file, trying to lock it.
Args:
timeout: float, The number of seconds to try to acquire the lock.
delay: float, The number of seconds to wait between retry attempts.
Raises:
AlreadyLockedException: if the lock is already acquired.
IOError: if the open fails.
"""
self._opener.open_and_lock(timeout, delay)
def unlock_and_close(self):
"""Unlock and close a file."""
self._opener.unlock_and_close()
