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137 lines
4.4 KiB
137 lines
4.4 KiB
import logging
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import threading
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log = logging.getLogger(__name__)
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class LockError(Exception):
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pass
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class ReadWriteMutex(object):
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"""A mutex which allows multiple readers, single writer.
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:class:`.ReadWriteMutex` uses a Python ``threading.Condition``
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to provide this functionality across threads within a process.
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The Beaker package also contained a file-lock based version
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of this concept, so that readers/writers could be synchronized
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across processes with a common filesystem. A future Dogpile
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release may include this additional class at some point.
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"""
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def __init__(self):
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# counts how many asynchronous methods are executing
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self.async_ = 0
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# pointer to thread that is the current sync operation
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self.current_sync_operation = None
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# condition object to lock on
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self.condition = threading.Condition(threading.Lock())
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def acquire_read_lock(self, wait=True):
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"""Acquire the 'read' lock."""
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self.condition.acquire()
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try:
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# see if a synchronous operation is waiting to start
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# or is already running, in which case we wait (or just
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# give up and return)
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if wait:
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while self.current_sync_operation is not None:
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self.condition.wait()
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else:
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if self.current_sync_operation is not None:
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return False
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self.async_ += 1
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log.debug("%s acquired read lock", self)
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finally:
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self.condition.release()
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if not wait:
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return True
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def release_read_lock(self):
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"""Release the 'read' lock."""
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self.condition.acquire()
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try:
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self.async_ -= 1
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# check if we are the last asynchronous reader thread
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# out the door.
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if self.async_ == 0:
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# yes. so if a sync operation is waiting, notify_all to wake
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# it up
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if self.current_sync_operation is not None:
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self.condition.notify_all()
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elif self.async_ < 0:
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raise LockError(
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"Synchronizer error - too many "
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"release_read_locks called"
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)
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log.debug("%s released read lock", self)
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finally:
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self.condition.release()
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def acquire_write_lock(self, wait=True):
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"""Acquire the 'write' lock."""
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self.condition.acquire()
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try:
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# here, we are not a synchronous reader, and after returning,
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# assuming waiting or immediate availability, we will be.
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if wait:
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# if another sync is working, wait
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while self.current_sync_operation is not None:
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self.condition.wait()
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else:
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# if another sync is working,
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# we dont want to wait, so forget it
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if self.current_sync_operation is not None:
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return False
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# establish ourselves as the current sync
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# this indicates to other read/write operations
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# that they should wait until this is None again
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self.current_sync_operation = threading.current_thread()
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# now wait again for asyncs to finish
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if self.async_ > 0:
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if wait:
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# wait
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self.condition.wait()
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else:
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# we dont want to wait, so forget it
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self.current_sync_operation = None
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return False
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log.debug("%s acquired write lock", self)
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finally:
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self.condition.release()
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if not wait:
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return True
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def release_write_lock(self):
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"""Release the 'write' lock."""
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self.condition.acquire()
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try:
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if self.current_sync_operation is not threading.current_thread():
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raise LockError(
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"Synchronizer error - current thread doesn't "
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"have the write lock"
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)
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# reset the current sync operation so
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# another can get it
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self.current_sync_operation = None
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# tell everyone to get ready
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self.condition.notify_all()
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log.debug("%s released write lock", self)
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finally:
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# everyone go !!
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self.condition.release()
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