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# 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:
#
# Create a new mutex -- initially unlocked
#
def __init__(self):
self.locked = 0
self.queue = []
#
# Test the locked bit of the mutex
#
def test(self):
return self.locked
#
# Atomic test-and-set -- grab the lock if it is not set,
# return true if it succeeded
#
def testandset(self):
if not self.locked:
self.locked = 1
return 1
else:
return 0
#
# 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.
#
def lock(self, function, argument):
if self.testandset():
function(argument)
else:
self.queue.append((function, argument))
#
# Unlock a mutex. If the queue is not empty, call the next
# function with its argument.
#
def unlock(self):
if self.queue:
function, argument = self.queue[0]
del self.queue[0]
function(argument)
else:
self.locked = 0
#
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