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