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