mirror of https://github.com/python/cpython
52 lines
1.5 KiB
Python
52 lines
1.5 KiB
Python
"""Mutual exclusion -- for use with module sched
|
|
|
|
A mutex has two pieces of state -- a 'locked' bit and a queue.
|
|
When the mutex is not locked, the queue is empty.
|
|
Otherwise, the queue contains 0 or more (function, argument) pairs
|
|
representing functions (or methods) waiting to acquire the lock.
|
|
When the mutex is unlocked while the queue is not empty,
|
|
the first queue entry is removed and its function(argument) pair called,
|
|
implying it now has the lock.
|
|
|
|
Of course, no multi-threading is implied -- hence the funny interface
|
|
for lock, where a function is called once the lock is aquired.
|
|
"""
|
|
|
|
class mutex:
|
|
def __init__(self):
|
|
"""Create a new mutex -- initially unlocked."""
|
|
self.locked = 0
|
|
self.queue = []
|
|
|
|
def test(self):
|
|
"""Test the locked bit of the mutex."""
|
|
return self.locked
|
|
|
|
def testandset(self):
|
|
"""Atomic test-and-set -- grab the lock if it is not set,
|
|
return true if it succeeded."""
|
|
if not self.locked:
|
|
self.locked = 1
|
|
return 1
|
|
else:
|
|
return 0
|
|
|
|
def lock(self, function, argument):
|
|
"""Lock a mutex, call the function with supplied argument
|
|
when it is acquired. If the mutex is already locked, place
|
|
function and argument in the queue."""
|
|
if self.testandset():
|
|
function(argument)
|
|
else:
|
|
self.queue.append((function, argument))
|
|
|
|
def unlock(self):
|
|
"""Unlock a mutex. If the queue is not empty, call the next
|
|
function with its argument."""
|
|
if self.queue:
|
|
function, argument = self.queue[0]
|
|
del self.queue[0]
|
|
function(argument)
|
|
else:
|
|
self.locked = 0
|