Synnefo

# filelocker.py - Cross-platform (posix/nt) API for flock-style file locking.

#                 Requires python 1.5.2 or better.

"""Cross-platform (posix/nt) API for flock-style file locking.

Synopsis:

    import filelocker

    with filelocker.lock("lockfile", filelocker.LOCK_EX):

        print "Got it"

Methods:

   lock ( file, flags, tries=10 )

Constants:

   LOCK_EX

   LOCK_SH

   LOCK_NB

Exceptions:

    LockException

Notes:

For the 'nt' platform, this module requires the Python Extensions for Windows.

Be aware that this may not work as expected on Windows 95/98/ME.

History:

I learned the win32 technique for locking files from sample code

provided by John Nielsen <nielsenjf@my-deja.com> in the documentation

that accompanies the win32 modules.

Author: Jonathan Feinberg <jdf@pobox.com>,

        Lowell Alleman <lalleman@mfps.com>

Version: $Id: filelocker.py 5474 2008-05-16 20:53:50Z lowell $

Modified to work as a contextmanager

"""

import os

from contextlib import contextmanager

class LockException(Exception):

    # Error codes:

    LOCK_FAILED = 1

# Import modules for each supported platform

if os.name == 'nt':

    import win32con

    import win32file

    import pywintypes

    LOCK_EX = win32con.LOCKFILE_EXCLUSIVE_LOCK

    LOCK_SH = 0  # the default

    LOCK_NB = win32con.LOCKFILE_FAIL_IMMEDIATELY

    # is there any reason not to reuse the following structure?

    __overlapped = pywintypes.OVERLAPPED()

elif os.name == 'posix':

    import fcntl

    LOCK_EX = fcntl.LOCK_EX

    LOCK_SH = fcntl.LOCK_SH

    LOCK_NB = fcntl.LOCK_NB

else:

    raise RuntimeError("FileLocker only defined for nt and posix platforms")

# --------------------------------------

# Implementation for NT

if os.name == 'nt':

    @contextmanager

    def lock(filename, flags):

        file = open(filename, "w+")

        hfile = win32file._get_osfhandle(file.fileno())

        try:

            win32file.LockFileEx(hfile, flags, 0, -0x10000, __overlapped)

            try:

                yield

            finally:

                file.close()

        except pywintypes.error, exc_value:

            # error: (33, 'LockFileEx',

            #         'The process cannot access the file because another

            #          process has locked a portion of the file.')

            file.close()

            if exc_value[0] == 33:

                raise LockException(LockException.LOCK_FAILED, exc_value[2])

            else:

                # Q:  Are there exceptions/codes we should be dealing with?

                raise

# --------------------------------------

# Implementation for Posix

elif os.name == 'posix':

    @contextmanager

    def lock(filename, flags):

        file = open(filename, "w+")

        try:

            fcntl.flock(file.fileno(), flags)

            try:

                yield

            finally:

                file.close()

        except IOError, exc_value:

            #  IOError: [Errno 11] Resource temporarily unavailable

            file.close()

            if exc_value[0] == 11:

                raise LockException(LockException.LOCK_FAILED, exc_value[1])

            else:

                raise