| 1 | """Mutual exclusion -- for use with module sched
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| 2 |
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| 3 | A mutex has two pieces of state -- a 'locked' bit and a queue.
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| 4 | When the mutex is not locked, the queue is empty.
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| 5 | Otherwise, the queue contains 0 or more (function, argument) pairs
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| 6 | representing functions (or methods) waiting to acquire the lock.
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| 7 | When the mutex is unlocked while the queue is not empty,
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| 8 | the first queue entry is removed and its function(argument) pair called,
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| 9 | implying it now has the lock.
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| 10 |
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| 11 | Of course, no multi-threading is implied -- hence the funny interface
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| 12 | for lock, where a function is called once the lock is aquired.
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| 13 | """
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| 14 |
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| 15 | from collections import deque
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| 16 |
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| 17 | class mutex:
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| 18 | def __init__(self):
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| 19 | """Create a new mutex -- initially unlocked."""
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| 20 | self.locked = 0
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| 21 | self.queue = deque()
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| 22 |
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| 23 | def test(self):
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| 24 | """Test the locked bit of the mutex."""
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| 25 | return self.locked
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| 26 |
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| 27 | def testandset(self):
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| 28 | """Atomic test-and-set -- grab the lock if it is not set,
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| 29 | return True if it succeeded."""
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| 30 | if not self.locked:
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| 31 | self.locked = 1
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| 32 | return True
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| 33 | else:
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| 34 | return False
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