Fasteners

Cross platform locks for threads and processes.

🔩 Install

pip install fasteners

🔩 Usage

Lock for processes has the same API as the threading.Lock for threads:

import fasteners
import threading

lock = threading.Lock()                                 # for threads
lock = fasteners.InterProcessLock('path/to/lock.file')  # for processes

with lock:
    ... # exclusive access

# or alternatively    

lock.acquire()
... # exclusive access
lock.release()

Reader Writer lock has a similar API, which is the same for threads or processes:

import fasteners

rw_lock = fasteners.ReaderWriterLock()                                 # for threads
rw_lock = fasteners.InterProcessReaderWriterLock('path/to/lock.file')  # for processes

with rw_lock.write_lock():
    ... # write access

with rw_lock.read_lock():
    ... # read access

# or alternatively

rw_lock.acquire_read_lock()
... # read access
rw_lock.release_read_lock()

rw_lock.acquire_write_lock()
... # write access
rw_lock.release_write_lock()

🔩 Overview

Python standard library provides a lock for threads (both a reentrant one, and a non-reentrant one, see below). Fasteners extends this, and provides a lock for processes, as well as Reader Writer locks for both threads and processes.

The specifics of the locks are as follows:

Process locks

The fasteners.InterProcessLock uses fcntl on Unix-like systems and msvc _locking on Windows. As a result, if used cross-platform it guarantees an intersection of their features:

lock	reentrant	mandatory
fcntl	✘	✘
_locking	✔	✔
fasteners.InterProcessLock	✘	✘

The fasteners.InterProcessReaderWriterLock also uses fcntl on Unix-like systems and LockFileEx on Windows. Their features are as follows:

lock	reentrant	mandatory	upgradable	preference
fcntl	✘	✘	✔	reader
LockFileEx	✔	✔	✘	reader
fasteners.InterProcessReaderWriterLock	✘	✘	✘	reader

Thread locks

Fasteners do not provide a simple thread lock, but for the sake of comparison note that the threading module provides both a reentrant and non-reentrant locks:

lock	reentrant	mandatory
threading.Lock	✘	✘
threading.RLock	✔	✘

The fasteners.ReaderWriterLock at the moment is as follows:

lock	reentrant	mandatory	upgradable	preference
fasteners.ReaderWriterLock	✔	✘	✘	reader

🔩 Glossary

To learn more about the various aspects of locks, check the wikipedia pages for locks and readers writer locks as well as the resources listed in the documentation. Here we briefly mention the main notions used above.

Lock - a mechanism that prevents two or more threads or processes from running the same code at the same time.
Readers writer lock - a mechanism that prevents two or more threads from having write (or write and read) access, while allowing multiple readers.
Reentrant lock - a lock that can be acquired (and then released) multiple times, as in:

with lock:
    with lock:
        ... # some code

Mandatory lock (as opposed to advisory lock) - a lock that is enforced by the operating system, rather than by the cooperation between threads or processes
Upgradable readers writer lock - a readers writer lock that can be upgraded from reader to writer (or downgraded from writer to reader) without losing the lock that is already held, as in:

with rw_lock.read_lock():
    ... # read access
    with rw_lock.write_lock():
        ... # write access
    ... # read access

Readers writer lock preference - describes the behaviour when multiple threads or processes are waiting for access. Some of the patterns are:
- Reader preference - If lock is held by readers, then new readers will get immediate access. This can result in writers waiting forever (writer starvation)
- Writer preference - If writer is waiting for a lock, then all the new readers (and writers) will be queued after the writer.
- Phase fair - Lock that alternates between readers and writers.

gthiemonge / fasteners