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Viewing changes to AppServer/google/appengine/_internal/django/utils/synch.py

  • Committer: Chris Bunch
  • Date: 2012-02-17 08:19:21 UTC
  • mfrom: (787.2.3 appscale-raj-merge)
  • Revision ID: cgb@cs.ucsb.edu-20120217081921-pakidyksaenlpzur
merged with main branch, gaining rabbitmq and upgrades for hbase, cassandra, and hypertable, as well as upgrading to gae 1.6.1 for python and go

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AppServer/google/appengine/_internal/django/utils/synch.py
 
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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()