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2475.4.1
by Martin Pool
Start adding CountedLock class to partially replace LockableFiles |
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# Copyright (C) 2007 Canonical Ltd
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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,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU 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., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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"""Counted lock class"""
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from bzrlib.errors import ( |
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LockError, |
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ReadOnlyError, |
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)
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# TODO: Pass through lock tokens on lock_write and read, and return them...
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#
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# TODO: Allow upgrading read locks to write? Conceptually difficult.
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class CountedLock(object): |
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"""Decorator around a lock that makes it reentrant.
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This can be used with any object that provides a basic Lock interface,
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including LockDirs and OS file locks.
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"""
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def __init__(self, real_lock): |
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self._real_lock = real_lock |
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self._lock_mode = None |
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self._lock_count = 0 |
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def break_lock(self): |
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self._real_lock.break_lock() |
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self._lock_mode = None |
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self._lock_count = 0 |
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def is_locked(self): |
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return self._lock_mode is not None |
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def lock_read(self): |
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"""Acquire the lock in read mode.
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If the lock is already held in either read or write mode this
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increments the count and succeeds. If the lock is not already held,
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it is taken in read mode.
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"""
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if self._lock_mode: |
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assert self._lock_mode in ('r', 'w'), \ |
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"invalid lock mode %r" % self._lock_mode |
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self._lock_count += 1 |
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else: |
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assert self._lock_count == 0 |
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self._real_lock.lock_read() |
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self._lock_count = 1 |
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self._lock_mode = 'r' |
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def lock_write(self): |
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"""Acquire the lock in write mode.
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If the lock was originally acquired in read mode this will fail.
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"""
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if self._lock_count == 0: |
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assert self._lock_mode is None |
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self._real_lock.lock_write() |
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self._lock_mode = 'w' |
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elif self._lock_mode != 'w': |
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raise ReadOnlyError(self) |
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self._lock_count += 1 |
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def unlock(self): |
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if self._lock_count == 0: |
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raise LockError("%s not locked" % (self,)) |
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elif self._lock_count == 1: |
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self._real_lock.unlock() |
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self._lock_mode = None |
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self._lock_count -= 1 |