django/utils/synch.py

   1 """
   2 Synchronization primitives:
   3
   4     - reader-writer lock (preference to writers)
   5
   6 (Contributed to Django by eugene@lazutkin.com)
   7 """
   8
   9 try:
  10     import threading
  11 except ImportError:
  12     import dummy_threading as threading
  13
  14 class RWLock:
  15     """
  16     Classic implementation of reader-writer lock with preference to writers.
  17
  18     Readers can access a resource simultaneously.
  19     Writers get an exclusive access.
  20
  21     API is self-descriptive:
  22         reader_enters()
  23         reader_leaves()
  24         writer_enters()
  25         writer_leaves()
  26     """
  27     def __init__(self):
  28         self.mutex     = threading.RLock()
  29         self.can_read  = threading.Semaphore(0)
  30         self.can_write = threading.Semaphore(0)
  31         self.active_readers  = 0
  32         self.active_writers  = 0
  33         self.waiting_readers = 0
  34         self.waiting_writers = 0
  35
  36     def reader_enters(self):
  37         self.mutex.acquire()
  38         try:
  39             if self.active_writers == 0 and self.waiting_writers == 0:
  40                 self.active_readers += 1
  41                 self.can_read.release()
  42             else:
  43                 self.waiting_readers += 1
  44         finally:
  45             self.mutex.release()
  46         self.can_read.acquire()
  47
  48     def reader_leaves(self):
  49         self.mutex.acquire()
  50         try:
  51             self.active_readers -= 1
  52             if self.active_readers == 0 and self.waiting_writers != 0:
  53                 self.active_writers  += 1
  54                 self.waiting_writers -= 1
  55                 self.can_write.release()
  56         finally:
  57             self.mutex.release()
  58
  59     def writer_enters(self):
  60         self.mutex.acquire()
  61         try:
  62             if self.active_writers == 0 and self.waiting_writers == 0 and self.active_readers == 0:
  63                 self.active_writers += 1
  64                 self.can_write.release()
  65             else:
  66                 self.waiting_writers += 1
  67         finally:
  68             self.mutex.release()
  69         self.can_write.acquire()
  70
  71     def writer_leaves(self):
  72         self.mutex.acquire()
  73         try:
  74             self.active_writers -= 1
  75             if self.waiting_writers != 0:
  76                 self.active_writers  += 1
  77                 self.waiting_writers -= 1
  78                 self.can_write.release()
  79             elif self.waiting_readers != 0:
  80                 t = self.waiting_readers
  81                 self.waiting_readers = 0
  82                 self.active_readers += t
  83                 while t > 0:
  84                     self.can_read.release()
  85                     t -= 1
  86         finally:
  87             self.mutex.release()