+
−"""
+
−Synchronization primitives:
+
−
+
−    - reader-writer lock (preference to writers)
+
−
+
−(Contributed to Django by eugene@lazutkin.com)
+
−"""
+
−
+
−try:
+
−    import threading
+
−except ImportError:
+
−    import dummy_threading as threading
+
−
+
−class RWLock:
+
−    """
+
−    Classic implementation of reader-writer lock with preference to writers.
+
−
+
−    Readers can access a resource simultaneously.
+
−    Writers get an exclusive access.
+
−
+
−    API is self-descriptive:
+
−        reader_enters()
+
−        reader_leaves()
+
−        writer_enters()
+
−        writer_leaves()
+
−    """
+
−    def __init__(self):
+
−        self.mutex     = threading.RLock()
+
−        self.can_read  = threading.Semaphore(0)
+
−        self.can_write = threading.Semaphore(0)
+
−        self.active_readers  = 0
+
−        self.active_writers  = 0
+
−        self.waiting_readers = 0
+
−        self.waiting_writers = 0
+
−
+
−    def reader_enters(self):
+
−        self.mutex.acquire()
+
−        try:
+
−            if self.active_writers == 0 and self.waiting_writers == 0:
+
−                self.active_readers += 1
+
−                self.can_read.release()
+
−            else:
+
−                self.waiting_readers += 1
+
−        finally:
+
−            self.mutex.release()
+
−        self.can_read.acquire()
+
−
+
−    def reader_leaves(self):
+
−        self.mutex.acquire()
+
−        try:
+
−            self.active_readers -= 1
+
−            if self.active_readers == 0 and self.waiting_writers != 0:
+
−                self.active_writers  += 1
+
−                self.waiting_writers -= 1
+
−                self.can_write.release()
+
−        finally:
+
−            self.mutex.release()
+
−
+
−    def writer_enters(self):
+
−        self.mutex.acquire()
+
−        try:
+
−            if self.active_writers == 0 and self.waiting_writers == 0 and self.active_readers == 0:
+
−                self.active_writers += 1
+
−                self.can_write.release()
+
−            else:
+
−                self.waiting_writers += 1
+
−        finally:
+
−            self.mutex.release()
+
−        self.can_write.acquire()
+
−
+
−    def writer_leaves(self):
+
−        self.mutex.acquire()
+
−        try:
+
−            self.active_writers -= 1
+
−            if self.waiting_writers != 0:
+
−                self.active_writers  += 1
+
−                self.waiting_writers -= 1
+
−                self.can_write.release()
+
−            elif self.waiting_readers != 0:
+
−                t = self.waiting_readers
+
−                self.waiting_readers = 0
+
−                self.active_readers += t
+
−                while t > 0:
+
−                    self.can_read.release()
+
−                    t -= 1
+
−        finally:
+
−            self.mutex.release()