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# Copyright Mateusz Kobos, (c) 2011
# released under the MIT licence
import threading
__author__ = "Mateusz Kobos"
class RWLock:
"""
Read-Write locking primitive
Synchronization object used in a solution of so-called second
readers-writers problem. In this problem, many readers can simultaneously
access a share, and a writer has an exclusive access to this share.
Additionally, the following constraints should be met:
1) no reader should be kept waiting if the share is currently opened for
reading unless a writer is also waiting for the share,
2) no writer should be kept waiting for the share longer than absolutely
necessary.
The implementation is based on [1, secs. 4.2.2, 4.2.6, 4.2.7]
with a modification -- adding an additional lock (C{self.__readers_queue})
-- in accordance with [2].
Sources:
[1] A.B. Downey: "The little book of semaphores", Version 2.1.5, 2008
[2] P.J. Courtois, F. Heymans, D.L. Parnas:
"Concurrent Control with 'Readers' and 'Writers'",
Communications of the ACM, 1971 (via [3])
"""
def __init__(self):
"""
A lock giving an even higher priority to the writer in certain
cases (see [2] for a discussion).
"""
self.__read_switch = _LightSwitch()
self.__write_switch = _LightSwitch()
self.__no_readers = threading.Lock()
self.__no_writers = threading.Lock()
self.__readers_queue = threading.Lock()
def reader_acquire(self):
self.__readers_queue.acquire()
self.__no_readers.acquire()
self.__read_switch.acquire(self.__no_writers)
self.__no_readers.release()
self.__readers_queue.release()
def reader_release(self):
self.__read_switch.release(self.__no_writers)
def writer_acquire(self):
self.__write_switch.acquire(self.__no_readers)
self.__no_writers.acquire()
def writer_release(self):
self.__no_writers.release()
self.__write_switch.release(self.__no_readers)
class _LightSwitch:
"""An auxiliary "light switch"-like object. The first thread turns on the
"switch", the last one turns it off (see [1, sec. 4.2.2] for details)."""
def __init__(self):
self.__counter = 0
self.__mutex = threading.Lock()
def acquire(self, lock):
self.__mutex.acquire()
self.__counter += 1
if self.__counter == 1:
lock.acquire()
self.__mutex.release()
def release(self, lock):
self.__mutex.acquire()
self.__counter -= 1
if self.__counter == 0:
lock.release()
self.__mutex.release()