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227 lines
10 KiB
Python
227 lines
10 KiB
Python
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# -*- coding: iso-8859-15 -*-
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"""locks.py - Read-Write lock thread lock implementation
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See the class documentation for more info.
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Copyright (C) 2007, Heiko Wundram.
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Released under the BSD-license.
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found at: http://code.activestate.com/recipes/502283-read-write-lock-class-rlock-like/
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"""
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# Imports
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# -------
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from threading import Condition, Lock, currentThread
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from time import time
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# Read write lock
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# ---------------
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class ReadWriteLock(object):
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"""Read-Write lock class. A read-write lock differs from a standard
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threading.RLock() by allowing multiple threads to simultaneously hold a
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read lock, while allowing only a single thread to hold a write lock at the
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same point of time.
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When a read lock is requested while a write lock is held, the reader
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is blocked; when a write lock is requested while another write lock is
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held or there are read locks, the writer is blocked.
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Writers are always preferred by this implementation: if there are blocked
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threads waiting for a write lock, current readers may request more read
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locks (which they eventually should free, as they starve the waiting
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writers otherwise), but a new thread requesting a read lock will not
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be granted one, and block. This might mean starvation for readers if
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two writer threads interweave their calls to acquireWrite() without
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leaving a window only for readers.
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In case a current reader requests a write lock, this can and will be
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satisfied without giving up the read locks first, but, only one thread
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may perform this kind of lock upgrade, as a deadlock would otherwise
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occur. After the write lock has been granted, the thread will hold a
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full write lock, and not be downgraded after the upgrading call to
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acquireWrite() has been match by a corresponding release().
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"""
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def __init__(self):
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"""Initialize this read-write lock."""
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# Condition variable, used to signal waiters of a change in object
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# state.
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self.__condition = Condition(Lock())
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# Initialize with no writers.
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self.__writer = None
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self.__upgradewritercount = 0
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self.__pendingwriters = []
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# Initialize with no readers.
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self.__readers = {}
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def acquireRead(self, timeout=None):
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"""Acquire a read lock for the current thread, waiting at most
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timeout seconds or doing a non-blocking check in case timeout is <= 0.
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In case timeout is None, the call to acquireRead blocks until the
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lock request can be serviced.
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In case the timeout expires before the lock could be serviced, a
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RuntimeError is thrown."""
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if timeout is not None:
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endtime = time() + timeout
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me = currentThread()
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self.__condition.acquire()
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try:
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if self.__writer is me:
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# If we are the writer, grant a new read lock, always.
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self.__writercount += 1
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return
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while True:
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if self.__writer is None:
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# Only test anything if there is no current writer.
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if self.__upgradewritercount or self.__pendingwriters:
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if me in self.__readers:
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# Only grant a read lock if we already have one
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# in case writers are waiting for their turn.
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# This means that writers can't easily get starved
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# (but see below, readers can).
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self.__readers[me] += 1
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return
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# No, we aren't a reader (yet), wait for our turn.
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else:
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# Grant a new read lock, always, in case there are
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# no pending writers (and no writer).
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self.__readers[me] = self.__readers.get(me, 0) + 1
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return
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if timeout is not None:
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remaining = endtime - time()
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if remaining <= 0:
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# Timeout has expired, signal caller of this.
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raise RuntimeError("Acquiring read lock timed out")
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self.__condition.wait(remaining)
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else:
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self.__condition.wait()
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finally:
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self.__condition.release()
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def acquireWrite(self, timeout=None):
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"""Acquire a write lock for the current thread, waiting at most
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timeout seconds or doing a non-blocking check in case timeout is <= 0.
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In case the write lock cannot be serviced due to the deadlock
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condition mentioned above, a ValueError is raised.
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In case timeout is None, the call to acquireWrite blocks until the
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lock request can be serviced.
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In case the timeout expires before the lock could be serviced, a
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RuntimeError is thrown."""
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if timeout is not None:
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endtime = time() + timeout
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me, upgradewriter = currentThread(), False
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self.__condition.acquire()
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try:
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if self.__writer is me:
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# If we are the writer, grant a new write lock, always.
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self.__writercount += 1
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return
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elif me in self.__readers:
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# If we are a reader, no need to add us to pendingwriters,
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# we get the upgradewriter slot.
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if self.__upgradewritercount:
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# If we are a reader and want to upgrade, and someone
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# else also wants to upgrade, there is no way we can do
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# this except if one of us releases all his read locks.
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# Signal this to user.
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raise ValueError(
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"Inevitable dead lock, denying write lock"
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)
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upgradewriter = True
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self.__upgradewritercount = self.__readers.pop(me)
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else:
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# We aren't a reader, so add us to the pending writers queue
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# for synchronization with the readers.
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self.__pendingwriters.append(me)
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while True:
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if not self.__readers and self.__writer is None:
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# Only test anything if there are no readers and writers.
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if self.__upgradewritercount:
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if upgradewriter:
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# There is a writer to upgrade, and it's us. Take
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# the write lock.
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self.__writer = me
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self.__writercount = self.__upgradewritercount + 1
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self.__upgradewritercount = 0
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return
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# There is a writer to upgrade, but it's not us.
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# Always leave the upgrade writer the advance slot,
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# because he presumes he'll get a write lock directly
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# from a previously held read lock.
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elif self.__pendingwriters[0] is me:
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# If there are no readers and writers, it's always
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# fine for us to take the writer slot, removing us
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# from the pending writers queue.
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# This might mean starvation for readers, though.
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self.__writer = me
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self.__writercount = 1
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self.__pendingwriters = self.__pendingwriters[1:]
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return
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if timeout is not None:
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remaining = endtime - time()
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if remaining <= 0:
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# Timeout has expired, signal caller of this.
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if upgradewriter:
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# Put us back on the reader queue. No need to
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# signal anyone of this change, because no other
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# writer could've taken our spot before we got
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# here (because of remaining readers), as the test
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# for proper conditions is at the start of the
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# loop, not at the end.
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self.__readers[me] = self.__upgradewritercount
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self.__upgradewritercount = 0
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else:
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# We were a simple pending writer, just remove us
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# from the FIFO list.
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self.__pendingwriters.remove(me)
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raise RuntimeError("Acquiring write lock timed out")
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self.__condition.wait(remaining)
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else:
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self.__condition.wait()
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finally:
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self.__condition.release()
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def release(self):
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"""Release the currently held lock.
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In case the current thread holds no lock, a ValueError is thrown."""
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me = currentThread()
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self.__condition.acquire()
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try:
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if self.__writer is me:
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# We are the writer, take one nesting depth away.
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self.__writercount -= 1
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if not self.__writercount:
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# No more write locks; take our writer position away and
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# notify waiters of the new circumstances.
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self.__writer = None
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self.__condition.notifyAll()
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elif me in self.__readers:
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# We are a reader currently, take one nesting depth away.
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self.__readers[me] -= 1
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if not self.__readers[me]:
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# No more read locks, take our reader position away.
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del self.__readers[me]
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if not self.__readers:
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# No more readers, notify waiters of the new
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# circumstances.
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self.__condition.notifyAll()
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else:
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raise ValueError("Trying to release unheld lock")
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finally:
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self.__condition.release()
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