On occasion, one requires the need to lock a file. Now, this is relatively easy if you’re targeting a specific platform because there is often a function in the library to do it for you. But what if you want to target a larger set of platforms? The following is a solution I wrote up today. It’s lockfile creation is an atomic operation and thus doesn’t suffer from any race conditions. It should work in both Windows and Unix environments.
# Copyright (c) 2009, Evan Fosmark # All rights reserved. # # Redistribution and use in source and binary forms, with or without # modification, are permitted provided that the following conditions are met: # # 1. Redistributions of source code must retain the above copyright notice, this # list of conditions and the following disclaimer. # 2. Redistributions in binary form must reproduce the above copyright notice, # this list of conditions and the following disclaimer in the documentation # and/or other materials provided with the distribution. # # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND # ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED # WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE # DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR # ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES # (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND # ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. # # The views and conclusions contained in the software and documentation are those # of the authors and should not be interpreted as representing official policies, # either expressed or implied, of the FreeBSD Project. import os import time import errno class FileLockException(Exception): pass class FileLock(object): """ A file locking mechanism that has context-manager support so you can use it in a with statement. This should be relatively cross compatible as it doesn't rely on msvcrt or fcntl for the locking. """ def __init__(self, file_name, timeout=10, delay=.05): """ Prepare the file locker. Specify the file to lock and optionally the maximum timeout and the delay between each attempt to lock. """ self.is_locked = False self.lockfile = os.path.join(os.getcwd(), "%s.lock" % file_name) self.file_name = file_name self.timeout = timeout self.delay = delay def acquire(self): """ Acquire the lock, if possible. If the lock is in use, it check again every `wait` seconds. It does this until it either gets the lock or exceeds `timeout` number of seconds, in which case it throws an exception. """ start_time = time.time() while True: try: self.fd = os.open(self.lockfile, os.O_CREAT|os.O_EXCL|os.O_RDWR) break; except OSError as e: if e.errno != errno.EEXIST: raise if (time.time() - start_time) >= self.timeout: raise FileLockException("Timeout occured.") time.sleep(self.delay) self.is_locked = True def release(self): """ Get rid of the lock by deleting the lockfile. When working in a `with` statement, this gets automatically called at the end. """ if self.is_locked: os.close(self.fd) os.unlink(self.lockfile) self.is_locked = False def __enter__(self): """ Activated when used in the with statement. Should automatically acquire a lock to be used in the with block. """ if not self.is_locked: self.acquire() return self def __exit__(self, type, value, traceback): """ Activated at the end of the with statement. It automatically releases the lock if it isn't locked. """ if self.is_locked: self.release() def __del__(self): """ Make sure that the FileLock instance doesn't leave a lockfile lying around. """ self.release()
The above class is best used in a context manager fashion through the
with statement like in the example below:
with FileLock("test.txt", timeout=2) as lock: print("Lock acquired.") # Do something with the locked file
The largest downside of this is that the directory the file is located in must be writable. I hope this code helps you. Of course, if you have a better recipe, please share it in the comments. ;)