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 class mutex:
  15         #
  16         # Create a new mutex -- initially unlocked
  17         #
  18         def __init__(self):
  19                 self.locked = 0
  20                 self.queue = []
  21         #
  22         # Test the locked bit of the mutex
  23         #
  24         def test(self):
  25                 return self.locked
  26         #
  27         # Atomic test-and-set -- grab the lock if it is not set,
  28         # return true if it succeeded
  29         #
  30         def testandset(self):
  31                 if not self.locked:
  32                         self.locked = 1
  33                         return 1
  34                 else:
  35                         return 0
  36         #
  37         # Lock a mutex, call the function with supplied argument
  38         # when it is acquired.
  39         # If the mutex is already locked, place function and argument
  40         # in the queue.
  41         #
  42         def lock(self, function, argument):
  43                 if self.testandset():
  44                         function(argument)
  45                 else:
  46                         self.queue.append(function, argument)
  47         #
  48         # Unlock a mutex.  If the queue is not empty, call the next
  49         # function with its argument.
  50         #
  51         def unlock(self):
  52                 if self.queue:
  53                         function, argument = self.queue[0]
  54                         del self.queue[0]
  55                         function(argument)
  56                 else:
  57                         self.locked = 0
  58         #