root / lib / locking.py @ a95fd5d7
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# Copyright (C) 2006, 2007 Google Inc.
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#
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 2 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful, but
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# WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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# General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program; if not, write to the Free Software
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# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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# 02110-1301, USA.
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"""Module implementing the Ganeti locking code."""
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# pylint: disable-msg=W0613,W0201
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import threading |
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# Wouldn't it be better to define LockingError in the locking module?
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# Well, for now that's how the rest of the code does it...
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from ganeti import errors |
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class SharedLock: |
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"""Implements a shared lock.
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Multiple threads can acquire the lock in a shared way, calling
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acquire_shared(). In order to acquire the lock in an exclusive way threads
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can call acquire_exclusive().
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The lock prevents starvation but does not guarantee that threads will acquire
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the shared lock in the order they queued for it, just that they will
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eventually do so.
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"""
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def __init__(self): |
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"""Construct a new SharedLock"""
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# we have two conditions, c_shr and c_exc, sharing the same lock.
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self.__lock = threading.Lock()
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self.__turn_shr = threading.Condition(self.__lock) |
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self.__turn_exc = threading.Condition(self.__lock) |
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# current lock holders
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self.__shr = set() |
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self.__exc = None |
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# lock waiters
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self.__nwait_exc = 0 |
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self.__nwait_shr = 0 |
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# is this lock in the deleted state?
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self.__deleted = False |
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def __is_sharer(self): |
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"""Is the current thread sharing the lock at this time?"""
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return threading.currentThread() in self.__shr |
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def __is_exclusive(self): |
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"""Is the current thread holding the lock exclusively at this time?"""
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return threading.currentThread() == self.__exc |
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def __is_owned(self, shared=-1): |
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"""Is the current thread somehow owning the lock at this time?
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This is a private version of the function, which presumes you're holding
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the internal lock.
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"""
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if shared < 0: |
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return self.__is_sharer() or self.__is_exclusive() |
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elif shared:
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return self.__is_sharer() |
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else:
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return self.__is_exclusive() |
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def _is_owned(self, shared=-1): |
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"""Is the current thread somehow owning the lock at this time?
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Args:
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shared:
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< 0: check for any type of ownership (default)
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0: check for exclusive ownership
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> 0: check for shared ownership
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"""
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self.__lock.acquire()
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try:
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result = self.__is_owned(shared)
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finally:
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self.__lock.release()
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return result
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def __wait(self,c): |
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"""Wait on the given condition, and raise an exception if the current lock
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is declared deleted in the meantime.
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Args:
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c: condition to wait on
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"""
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c.wait() |
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if self.__deleted: |
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raise errors.LockError('deleted lock') |
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def __exclusive_acquire(self): |
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"""Acquire the lock exclusively.
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This is a private function that presumes you are already holding the
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internal lock. It's defined separately to avoid code duplication between
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acquire() and delete()
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"""
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self.__nwait_exc += 1 |
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try:
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# This is to save ourselves from a nasty race condition that could
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# theoretically make the sharers starve.
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if self.__nwait_shr > 0 or self.__nwait_exc > 1: |
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self.__wait(self.__turn_exc) |
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while len(self.__shr) > 0 or self.__exc is not None: |
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self.__wait(self.__turn_exc) |
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self.__exc = threading.currentThread()
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finally:
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self.__nwait_exc -= 1 |
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def acquire(self, blocking=1, shared=0): |
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"""Acquire a shared lock.
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Args:
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shared: whether to acquire in shared mode. By default an exclusive lock
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will be acquired.
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blocking: whether to block while trying to acquire or to operate in try-lock mode.
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this locking mode is not supported yet.
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"""
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if not blocking: |
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# We don't have non-blocking mode for now
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raise NotImplementedError |
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self.__lock.acquire()
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try:
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if self.__deleted: |
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raise errors.LockError('deleted lock') |
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# We cannot acquire the lock if we already have it
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assert not self.__is_owned(), "double acquire() on a non-recursive lock" |
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if shared:
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self.__nwait_shr += 1 |
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try:
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# If there is an exclusive holder waiting we have to wait. We'll
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# only do this once, though, when we start waiting for the lock. Then
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# we'll just wait while there are no exclusive holders.
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if self.__nwait_exc > 0: |
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# TODO: if !blocking...
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self.__wait(self.__turn_shr) |
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while self.__exc is not None: |
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# TODO: if !blocking...
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self.__wait(self.__turn_shr) |
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self.__shr.add(threading.currentThread())
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finally:
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self.__nwait_shr -= 1 |
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else:
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# TODO: if !blocking...
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# (or modify __exclusive_acquire for non-blocking mode)
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self.__exclusive_acquire()
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finally:
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self.__lock.release()
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return True |
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def release(self): |
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"""Release a Shared Lock.
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You must have acquired the lock, either in shared or in exclusive mode,
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before calling this function.
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"""
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self.__lock.acquire()
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try:
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# Autodetect release type
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if self.__is_exclusive(): |
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self.__exc = None |
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# An exclusive holder has just had the lock, time to put it in shared
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# mode if there are shared holders waiting. Otherwise wake up the next
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# exclusive holder.
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if self.__nwait_shr > 0: |
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self.__turn_shr.notifyAll()
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elif self.__nwait_exc > 0: |
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self.__turn_exc.notify()
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elif self.__is_sharer(): |
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self.__shr.remove(threading.currentThread())
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# If there are shared holders waiting there *must* be an exclusive holder
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# waiting as well; otherwise what were they waiting for?
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assert (self.__nwait_shr == 0 or self.__nwait_exc > 0, |
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"Lock sharers waiting while no exclusive is queueing")
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# If there are no more shared holders and some exclusive holders are
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# waiting let's wake one up.
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if len(self.__shr) == 0 and self.__nwait_exc > 0: |
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self.__turn_exc.notify()
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else:
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assert False, "Cannot release non-owned lock" |
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finally:
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self.__lock.release()
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def delete(self, blocking=1): |
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"""Delete a Shared Lock.
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This operation will declare the lock for removal. First the lock will be
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acquired in exclusive mode if you don't already own it, then the lock
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will be put in a state where any future and pending acquire() fail.
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Args:
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blocking: whether to block while trying to acquire or to operate in
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try-lock mode. this locking mode is not supported yet unless
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you are already holding exclusively the lock.
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"""
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self.__lock.acquire()
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try:
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assert not self.__is_sharer(), "cannot delete() a lock while sharing it" |
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if self.__deleted: |
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raise errors.LockError('deleted lock') |
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if not self.__is_exclusive(): |
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if not blocking: |
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# We don't have non-blocking mode for now
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raise NotImplementedError |
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self.__exclusive_acquire()
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self.__deleted = True |
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self.__exc = None |
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# Wake up everybody, they will fail acquiring the lock and
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# raise an exception instead.
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self.__turn_exc.notifyAll()
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self.__turn_shr.notifyAll()
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finally:
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self.__lock.release()
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