95 lines
2.6 KiB
Python
95 lines
2.6 KiB
Python
"""
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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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from __future__ import with_statement
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import contextlib
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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(object):
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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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with self.mutex:
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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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self.can_read.acquire()
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def reader_leaves(self):
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with self.mutex:
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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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@contextlib.contextmanager
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def reader(self):
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self.reader_enters()
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try:
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yield
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finally:
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self.reader_leaves()
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def writer_enters(self):
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with self.mutex:
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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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self.can_write.acquire()
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def writer_leaves(self):
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with self.mutex:
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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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@contextlib.contextmanager
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def writer(self):
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self.writer_enters()
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try:
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yield
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finally:
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self.writer_leaves() |