app/django/utils/synch.py
author Lennard de Rijk <ljvderijk@gmail.com>
Sun, 12 Jul 2009 14:03:35 +0200
changeset 2610 95949d4c45d9
parent 54 03e267d67478
permissions -rw-r--r--
Add scope view for GradingSurveyGroup and set access checks. The access checks are now properly set for create, edit and show. To facilitate the scope view the access check have an extra check built in to ensure that a scope is actually present before cheking for an existing Program.

"""
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()