app/django/utils/synch.py
author Pawel Solyga <Pawel.Solyga@gmail.com>
Tue, 20 Jan 2009 22:01:02 +0000
changeset 852 c94bf642be8d
parent 54 03e267d67478
permissions -rw-r--r--
Replace css class 'notice' that is used in User profile template field help text with new 'fieldhelptext' class. Notice is part of CSS for notification bar and we want width limit just for field help text not for notification bar text. Patch by: Pawel Solyga Review by: to-be-reviewed

"""
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()