Commit | Line | Data |
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920dae64 AT |
1 | """Mutual exclusion -- for use with module sched |
2 | ||
3 | A mutex has two pieces of state -- a 'locked' bit and a queue. | |
4 | When the mutex is not locked, the queue is empty. | |
5 | Otherwise, the queue contains 0 or more (function, argument) pairs | |
6 | representing functions (or methods) waiting to acquire the lock. | |
7 | When the mutex is unlocked while the queue is not empty, | |
8 | the first queue entry is removed and its function(argument) pair called, | |
9 | implying it now has the lock. | |
10 | ||
11 | Of course, no multi-threading is implied -- hence the funny interface | |
12 | for lock, where a function is called once the lock is aquired. | |
13 | """ | |
14 | ||
15 | from collections import deque | |
16 | ||
17 | class mutex: | |
18 | def __init__(self): | |
19 | """Create a new mutex -- initially unlocked.""" | |
20 | self.locked = 0 | |
21 | self.queue = deque() | |
22 | ||
23 | def test(self): | |
24 | """Test the locked bit of the mutex.""" | |
25 | return self.locked | |
26 | ||
27 | def testandset(self): | |
28 | """Atomic test-and-set -- grab the lock if it is not set, | |
29 | return True if it succeeded.""" | |
30 | if not self.locked: | |
31 | self.locked = 1 | |
32 | return True | |
33 | else: | |
34 | return False | |
35 | ||
36 | def lock(self, function, argument): | |
37 | """Lock a mutex, call the function with supplied argument | |
38 | when it is acquired. If the mutex is already locked, place | |
39 | function and argument in the queue.""" | |
40 | if self.testandset(): | |
41 | function(argument) | |
42 | else: | |
43 | self.queue.append((function, argument)) | |
44 | ||
45 | def unlock(self): | |
46 | """Unlock a mutex. If the queue is not empty, call the next | |
47 | function with its argument.""" | |
48 | if self.queue: | |
49 | function, argument = self.queue.popleft() | |
50 | function(argument) | |
51 | else: | |
52 | self.locked = 0 |