~bzr-pqm/bzr/bzr.dev

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