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 from warnings import warnpy3k
  15 warnpy3k("the mutex module has been removed in Python 3.0", stacklevel=2)
  16 del warnpy3k
  17
  18 from collections import deque
  19
  20 class mutex:
  21     def __init__(self):
  22         """Create a new mutex -- initially unlocked."""
  23         self.locked = 0
  24         self.queue = deque()
  25
  26     def test(self):
  27         """Test the locked bit of the mutex."""
  28         return self.locked
  29
  30     def testandset(self):
  31         """Atomic test-and-set -- grab the lock if it is not set,
  32         return True if it succeeded."""
  33         if not self.locked:
  34             self.locked = 1
  35             return True
  36         else:
  37             return False
  38
  39     def lock(self, function, argument):
  40         """Lock a mutex, call the function with supplied argument
  41         when it is acquired.  If the mutex is already locked, place
  42         function and argument in the queue."""
  43         if self.testandset():
  44             function(argument)
  45         else:
  46             self.queue.append((function, argument))
  47
  48     def unlock(self):
  49         """Unlock a mutex.  If the queue is not empty, call the next
  50         function with its argument."""
  51         if self.queue:
  52             function, argument = self.queue.popleft()
  53             function(argument)
  54         else:
  55             self.locked = 0