Lib/mutex.py

   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