123 lines
3.7 KiB
Python
123 lines
3.7 KiB
Python
# -*- coding: utf-8 -*-
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# vim: tabstop=4 shiftwidth=4 softtabstop=4
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# Copyright (C) 2012 Yahoo! Inc. All Rights Reserved.
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may
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# not use this file except in compliance with the License. You may obtain
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# a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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# License for the specific language governing permissions and limitations
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# under the License.
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import contextlib
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import threading
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class ReaderWriterLock(object):
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"""A simple reader-writer lock.
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Several readers can hold the lock simultaneously, and only one writer.
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Write locks have priority over reads to prevent write starvation.
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Public domain @ http://majid.info/blog/a-reader-writer-lock-for-python/
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"""
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def __init__(self):
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self.rwlock = 0
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self.writers_waiting = 0
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self.monitor = threading.Lock()
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self.readers_ok = threading.Condition(self.monitor)
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self.writers_ok = threading.Condition(self.monitor)
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@contextlib.contextmanager
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def acquire(self, read=True):
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"""Acquire a read or write lock in a context manager."""
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try:
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if read:
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self.acquire_read()
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else:
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self.acquire_write()
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yield self
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finally:
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self.release()
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def acquire_read(self):
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"""Acquire a read lock.
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Several threads can hold this typeof lock.
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It is exclusive with write locks."""
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self.monitor.acquire()
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while self.rwlock < 0 or self.writers_waiting:
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self.readers_ok.wait()
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self.rwlock += 1
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self.monitor.release()
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def acquire_write(self):
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"""Acquire a write lock.
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Only one thread can hold this lock, and only when no read locks
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are also held."""
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self.monitor.acquire()
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while self.rwlock != 0:
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self.writers_waiting += 1
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self.writers_ok.wait()
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self.writers_waiting -= 1
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self.rwlock = -1
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self.monitor.release()
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def release(self):
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"""Release a lock, whether read or write."""
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self.monitor.acquire()
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if self.rwlock < 0:
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self.rwlock = 0
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else:
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self.rwlock -= 1
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wake_writers = self.writers_waiting and self.rwlock == 0
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wake_readers = self.writers_waiting == 0
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self.monitor.release()
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if wake_writers:
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self.writers_ok.acquire()
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self.writers_ok.notify()
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self.writers_ok.release()
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elif wake_readers:
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self.readers_ok.acquire()
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self.readers_ok.notifyAll()
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self.readers_ok.release()
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class LazyPluggable(object):
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"""A pluggable backend loaded lazily based on some value."""
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def __init__(self, pivot, **backends):
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self.__backends = backends
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self.__pivot = pivot
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self.__backend = None
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def __get_backend(self):
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if not self.__backend:
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backend_name = 'sqlalchemy'
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backend = self.__backends[backend_name]
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if isinstance(backend, tuple):
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name = backend[0]
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fromlist = backend[1]
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else:
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name = backend
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fromlist = backend
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self.__backend = __import__(name, None, None, fromlist)
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return self.__backend
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def __getattr__(self, key):
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backend = self.__get_backend()
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return getattr(backend, key)
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