app/django/utils/synch.py
author Pawel Solyga <Pawel.Solyga@gmail.com>
Thu, 11 Sep 2008 12:01:20 +0000
changeset 127 70206a64da4a
parent 54 03e267d67478
permissions -rw-r--r--
Created custom_widgets.py and added TinyMCE django widget that can be later used for particular form fields. Patch by: Pawel Solyga Review by: to-be-reviewed
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"""
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Synchronization primitives:
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    - reader-writer lock (preference to writers)
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(Contributed to Django by eugene@lazutkin.com)
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"""
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try:
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    import threading
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except ImportError:
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    import dummy_threading as threading
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class RWLock:
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    """
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    Classic implementation of reader-writer lock with preference to writers.
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    Readers can access a resource simultaneously.
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    Writers get an exclusive access.
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    API is self-descriptive:
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        reader_enters()
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        reader_leaves()
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        writer_enters()
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        writer_leaves()
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    """
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    def __init__(self):
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        self.mutex     = threading.RLock()
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        self.can_read  = threading.Semaphore(0)
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        self.can_write = threading.Semaphore(0)
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        self.active_readers  = 0
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        self.active_writers  = 0
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        self.waiting_readers = 0
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        self.waiting_writers = 0
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    def reader_enters(self):
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        self.mutex.acquire()
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        try:
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            if self.active_writers == 0 and self.waiting_writers == 0:
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                self.active_readers += 1
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                self.can_read.release()
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            else:
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                self.waiting_readers += 1
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        finally:
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            self.mutex.release()
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        self.can_read.acquire()
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    def reader_leaves(self):
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        self.mutex.acquire()
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        try:
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            self.active_readers -= 1
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            if self.active_readers == 0 and self.waiting_writers != 0:
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                self.active_writers  += 1
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                self.waiting_writers -= 1
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                self.can_write.release()
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        finally:
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            self.mutex.release()
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    def writer_enters(self):
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        self.mutex.acquire()
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        try:
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            if self.active_writers == 0 and self.waiting_writers == 0 and self.active_readers == 0:
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                self.active_writers += 1
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                self.can_write.release()
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            else:
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                self.waiting_writers += 1
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        finally:
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            self.mutex.release()
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        self.can_write.acquire()
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    def writer_leaves(self):
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        self.mutex.acquire()
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        try:
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            self.active_writers -= 1
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            if self.waiting_writers != 0:
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                self.active_writers  += 1
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                self.waiting_writers -= 1
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                self.can_write.release()
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            elif self.waiting_readers != 0:
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                t = self.waiting_readers
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                self.waiting_readers = 0
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                self.active_readers += t
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                while t > 0:
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                    self.can_read.release()
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                    t -= 1
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        finally:
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            self.mutex.release()