# `lockfile` — Platform-independent file locking

Warning

This package is deprecated. It is highly preferred that instead of using this code base that instead fasteners or oslo.concurrency is used instead. For any questions or comments or further help needed please email openstack-dev and prefix your email subject with `[oslo][pylockfile]` (for a faster response).

Note

This package is pre-release software. Between versions 0.8 and 0.9 it was changed from a module to a package. It is quite possible that the API and implementation will change again in important ways as people test it and provide feedback and bug fixes. In particular, if the mkdir-based locking scheme is sufficient for both Windows and Unix platforms, the link-based scheme may be deleted so that only a single locking scheme is used, providing cross-platform lockfile cooperation.

Note

The implementation uses the with statement, both in the tests and in the main code, so will only work out-of-the-box with Python 2.5 or later. However, the use of the with statement is minimal, so if you apply the patch in the included 2.4.diff file you can use it with Python 2.4. It’s possible that it will work in Python 2.3 with that patch applied as well, though the doctest code relies on APIs new in 2.4, so will have to be rewritten somewhat to allow testing on 2.3. As they say, patches welcome. `;-)`

The `lockfile` package exports a `LockFile` class which provides a simple API for locking files. Unlike the Windows `msvcrt.locking()` function, the Unix `fcntl.flock()`, `fcntl.lockf()` and the deprecated `posixfile` module, the API is identical across both Unix (including Linux and Mac) and Windows platforms. The lock mechanism relies on the atomic nature of the `link()` (on Unix) and `mkdir()` (On Windows) system calls. It also contains several lock-method-specific modules: `lockfile.linklockfile`, `lockfile.mkdirlockfile`, and `lockfile.sqlitelockfile`, each one exporting a single class. For backwards compatibility with versions before 0.9 the `LinkFileLock`, `MkdirFileLock` and `SQLiteFileLock` objects are exposed as attributes of the top-level lockfile package, though this use was deprecated starting with version 0.9 and will be removed in version 1.0.

Note

The current implementation uses `os.link()` on Unix, but since that function is unavailable on Windows it uses `os.mkdir()` there. At this point it’s not clear that using the `os.mkdir()` method would be insufficient on Unix systems. If it proves to be adequate on Unix then the implementation could be simplified and truly cross-platform locking would be possible.

Note

The current implementation doesn’t provide for shared vs. exclusive locks. It should be possible for multiple reader processes to hold the lock at the same time.

The module defines the following exceptions:

exception `lockfile.``Error`

  This is the base class for all exceptions raised by the `LockFile` class.

exception `lockfile.``LockError`

  This is the base class for all exceptions raised when attempting to lock a file.

exception `lockfile.``UnlockError`

  This is the base class for all exceptions raised when attempting to unlock a file.

exception `lockfile.``LockTimeout`

  This exception is raised if the `LockFile.acquire()` method is called with a timeout which expires before an existing lock is released.

exception `lockfile.``AlreadyLocked`

  This exception is raised if the `LockFile.acquire()` detects a file is already locked when in non-blocking mode.

exception `lockfile.``LockFailed`

  This exception is raised if the `LockFile.acquire()` detects some other condition (such as a non-writable directory) which prevents it from creating its lock file.

exception `lockfile.``NotLocked`

  This exception is raised if the file is not locked when `LockFile.release()` is called.

exception `lockfile.``NotMyLock`

  This exception is raised if the file is locked by another thread or process when `LockFile.release()` is called.

The following classes are provided:

class `linklockfile.``LinkLockFile`(path, threaded=True)

  This class uses the `link(2)()` system call as the basic lock mechanism. path is an object in the file system to be locked. It need not exist, but its directory must exist and be writable at the time the `acquire()` and `release()` methods are called. threaded is optional, but when set to `True` locks will be distinguished between threads in the same process.

class `symlinklockfile.``SymlinkLockFile`(path, threaded=True)

  This class uses the `symlink(2)()` system call as the basic lock mechanism. The parameters have the same meaning and constraints as for the `LinkLockFile` class.

class `mkdirlockfile.``MkdirLockFile`(path, threaded=True)

  This class uses the `mkdir(2)()` system call as the basic lock mechanism. The parameters have the same meaning and constraints as for the `LinkLockFile` class.

class `sqlitelockfile.``SQLiteLockFile`(path, threaded=True)

  This class uses the `sqlite3` module to implement the lock mechanism. The parameters have the same meaning as for the `LinkLockFile` class.

class `lockfile.``LockBase`(path, threaded=True)

  This is the base class for all concrete implementations and is available at the lockfile package level so programmers can implement other locking schemes.

`lockfile.``locked`(path, timeout=None)

  This function provides a decorator which insures the decorated function is always called with the lock held.

By default, the `LockFile` object refers to the `mkdirlockfile.MkdirLockFile` class on Windows. On all other platforms it refers to the `linklockfile.LinkLockFile` class.

When locking a file the `linklockfile.LinkLockFile` class creates a uniquely named hard link to an empty lock file. That hard link contains the hostname, process id, and if locks between threads are distinguished, the thread identifier. For example, if you want to lock access to a file named “README”, the lock file is named “README.lock”. With per-thread locks enabled the hard link is named HOSTNAME-THREADID-PID. With only per-process locks enabled the hard link is named HOSTNAME–PID.

When using the `mkdirlockfile.MkdirLockFile` class the lock file is a directory. Referring to the example above, README.lock will be a directory and HOSTNAME-THREADID-PID will be an empty file within that directory.

See also

Module `msvcrt`
  Provides the `locking()` function, the standard Windows way of locking (parts of) a file.
Module `posixfile`
  The deprecated (since Python 1.5) way of locking files on Posix systems.
Module `fcntl`
  Provides the current best way to lock files on Unix systems (`lockf()` and `flock()`).

## LockFile Objects

`LockFile` objects support the context manager protocol used by the statement:with statement. The timeout option is not supported when used in this fashion. While support for timeouts could be implemented, there is no support for handling the eventual `Timeout` exceptions raised by the `__enter__()` method, so you would have to protect the with statement with a try statement. The resulting construct would not be any simpler than just using a try statement in the first place.

`LockFile` has the following user-visible methods:

`LockFile.``acquire`(timeout=None)

  Lock the file associated with the `LockFile` object. If the timeout is omitted or `None` the caller will block until the file is unlocked by the object currently holding the lock. If the timeout is zero or a negative number the `AlreadyLocked` exception will be raised if the file is currently locked by another process or thread. If the timeout is positive, the caller will block for that many seconds waiting for the lock to be released. If the lock is not released within that period the `LockTimeout` exception will be raised.

`LockFile.``release`()

  Unlock the file associated with the `LockFile` object. If the file is not currently locked, the `NotLocked` exception is raised. If the file is locked by another thread or process the `NotMyLock` exception is raised.

`lockfile.``is_locked`()

  Return the status of the lock on the current file. If any process or thread (including the current one) is locking the file, `True` is returned, otherwise `False` is returned.

`lockfile.``break_lock`()

  If the file is currently locked, break it.

`lockfile.``i_am_locking`()

  Returns true if the caller holds the lock.

## Examples

This example is the “hello world” for the `lockfile` package:

    from lockfile import LockFile
    lock = LockFile("/some/file/or/other")
    with lock:
        print(lock.path, "is locked.")

To use this with Python 2.4, you can execute:

    from lockfile import LockFile
    lock = LockFile("/some/file/or/other")
    lock.acquire()
    print(lock.path, "is locked.")
    lock.release()

If you don’t want to wait forever, you might try:

    from lockfile import LockFile
    lock = LockFile("/some/file/or/other")
    while not lock.i_am_locking():
        try:
            lock.acquire(timeout=60)    # wait up to 60 seconds
        except LockTimeout:
            lock.break_lock()
            lock.acquire()
    print("I locked", lock.path)
    lock.release()

You can also insure that a lock is always held when appropriately decorated functions are called:

    from lockfile import locked
    @locked("/tmp/mylock")
    def func(a, b):
        return a + b

## Other Libraries

The idea of implementing advisory locking with a standard API is not new with `lockfile`. There are a number of other libraries available:

- locknix - http://pypi.python.org/pypi/locknix - Unix only
- mx.MiscLockFile - from Marc André Lemburg, part of the mx.Base distribution - cross-platform.
- Twisted - http://twistedmatrix.com/trac/browser/trunk/twisted/python/lockfile.py
- zc.lockfile - http://pypi.python.org/pypi/zc.lockfile

## Contacting the Author

If you encounter any problems with `lockfile`, would like help or want to submit a patch, check http://launchpad.net/pylockfile