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 class mutex:
  16     def __init__(self):
  17         """Create a new mutex -- initially unlocked."""
  18         self.locked = 0
  19         self.queue = []
  20
  21     def test(self):
  22         """Test the locked bit of the mutex."""
  23         return self.locked
  24
  25     def testandset(self):
  26         """Atomic test-and-set -- grab the lock if it is not set,
  27         return true if it succeeded."""
  28         if not self.locked:
  29             self.locked = 1
  30             return 1
  31         else:
  32             return 0
  33
  34     def lock(self, function, argument):
  35         """Lock a mutex, call the function with supplied argument
  36         when it is acquired.  If the mutex is already locked, place
  37         function and argument in the queue."""
  38         if self.testandset():
  39             function(argument)
  40         else:
  41             self.queue.append((function, argument))
  42
  43     def unlock(self):
  44         """Unlock a mutex.  If the queue is not empty, call the next
  45         function with its argument."""
  46         if self.queue:
  47             function, argument = self.queue[0]
  48             del self.queue[0]
  49             function(argument)
  50         else:
  51             self.locked = 0