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# Copyright (C) 2006 Canonical Ltd
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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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# 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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# 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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"""On-disk mutex protecting a resource
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bzr on-disk objects are locked by the existence of a directory with a
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particular name within the control directory. We use this rather than OS
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internal locks (such as flock etc) because they can be seen across all
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transports, including http.
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Objects can be read if there is only physical read access; therefore
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readers can never be required to create a lock, though they will
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check whether a writer is using the lock. Writers can't detect
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whether anyone else is reading from the resource as they write.
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This works because of ordering constraints that make sure readers
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see a consistent view of existing data.
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Waiting for a lock must be done by polling; this can be aborted after
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Locks must always be explicitly released, typically from a try/finally
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block -- they are not released from a finalizer or when Python
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Locks may fail to be released if the process is abruptly terminated
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(machine stop, SIGKILL) or if a remote transport becomes permanently
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disconnected. There is therefore a method to break an existing lock.
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This should rarely be used, and generally only with user approval.
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Locks contain some information on when the lock was taken and by who
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which may guide in deciding whether it can safely be broken. (This is
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similar to the messages displayed by emacs and vim.) Note that if the
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lock holder is still alive they will get no notification that the lock
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has been broken and will continue their work -- so it is important to be
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sure they are actually dead.
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A lock is represented on disk by a directory of a particular name,
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containing an information file. Taking a lock is done by renaming a
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temporary directory into place. We use temporary directories because
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for all known transports and filesystems we believe that exactly one
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attempt to claim the lock will succeed and the others will fail. (Files
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won't do because some filesystems or transports only have
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rename-and-overwrite, making it hard to tell who won.)
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The desired characteristics are:
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* Locks are not reentrant. (That is, a client that tries to take a
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lock it already holds may deadlock or fail.)
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* Stale locks can be guessed at by a heuristic
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* Lost locks can be broken by any client
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* Failed lock operations leave little or no mess
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* Deadlocks are avoided by having a timeout always in use, clients
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desiring indefinite waits can retry or set a silly big timeout.
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Storage formats use the locks, and also need to consider concurrency
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issues underneath the lock. A format may choose not to use a lock
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at all for some operations.
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LockDirs always operate over a Transport. The transport may be readonly, in
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which case the lock can be queried but not acquired.
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Locks are identified by a path name, relative to a base transport.
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Calling code will typically want to make sure there is exactly one LockDir
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object per actual lock on disk. This module does nothing to prevent aliasing
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and deadlocks will likely occur if the locks are aliased.
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In the future we may add a "freshen" method which can be called
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by a lock holder to check that their lock has not been broken, and to
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update the timestamp within it.
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>>> from bzrlib.transport.memory import MemoryTransport
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>>> # typically will be obtained from a BzrDir, Branch, etc
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>>> t = MemoryTransport()
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>>> l = LockDir(t, 'sample-lock')
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>>> # do something here
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from StringIO import StringIO
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from bzrlib.errors import (
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from bzrlib.transport import Transport
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from bzrlib.osutils import rand_chars
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from bzrlib.rio import RioWriter, read_stanza, Stanza
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# XXX: At the moment there is no consideration of thread safety on LockDir
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# objects. This should perhaps be updated - e.g. if two threads try to take a
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# lock at the same time they should *both* get it. But then that's unlikely
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# TODO: Transport could offer a simpler put() method that avoids the
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# rename-into-place for cases like creating the lock template, where there is
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# no chance that the file already exists.
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# TODO: Perhaps store some kind of note like the bzr command line in the lock
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# TODO: Some kind of callback run while polling a lock to show progress
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_DEFAULT_TIMEOUT_SECONDS = 300
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_DEFAULT_POLL_SECONDS = 0.5
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class LockDir(object):
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"""Write-lock guarding access to data."""
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__INFO_NAME = '/info'
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def __init__(self, transport, path):
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"""Create a new LockDir object.
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The LockDir is initially unlocked - this just creates the object.
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:param transport: Transport which will contain the lock
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:param path: Path to the lock within the base directory of the
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assert isinstance(transport, Transport), \
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("not a transport: %r" % transport)
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self.transport = transport
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self._lock_held = False
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self._info_path = path + self.__INFO_NAME
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self.nonce = rand_chars(20)
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return '%s(%s%s)' % (self.__class__.__name__,
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is_held = property(lambda self: self._lock_held)
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def attempt_lock(self):
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"""Take the lock; fail if it's already held.
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If you wish to block until the lock can be obtained, call wait_lock()
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if self.transport.is_readonly():
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raise UnlockableTransport(self.transport)
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tmpname = '%s.pending.%s.tmp' % (self.path, rand_chars(20))
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self.transport.mkdir(tmpname)
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self._prepare_info(sio)
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self.transport.put(tmpname + self.__INFO_NAME, sio)
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# FIXME: this turns into os.rename on posix, but into a fancy rename
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# on Windows that may overwrite existing directory trees.
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# NB: posix rename will overwrite empty directories, but not
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# non-empty directories.
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self.transport.move(tmpname, self.path)
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self._lock_held = True
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except (DirectoryNotEmpty, FileExists), e:
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# fall through to here on contention
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raise LockContention(self)
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"""Release a held lock
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if not self._lock_held:
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raise LockNotHeld(self)
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# rename before deleting, because we can't atomically remove the whole
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tmpname = '%s.releasing.%s.tmp' % (self.path, rand_chars(20))
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self.transport.rename(self.path, tmpname)
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self._lock_held = False
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self.transport.delete(tmpname + self.__INFO_NAME)
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self.transport.rmdir(tmpname)
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def force_break(self, dead_holder_info):
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"""Release a lock held by another process.
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WARNING: This should only be used when the other process is dead; if
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it still thinks it has the lock there will be two concurrent writers.
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In general the user's approval should be sought for lock breaks.
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dead_holder_info must be the result of a previous LockDir.peek() call;
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this is used to check that it's still held by the same process that
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the user decided was dead. If this is not the current holder,
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LockBreakMismatch is raised.
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After the lock is broken it will not be held by any process.
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It is possible that another process may sneak in and take the
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lock before the breaking process acquires it.
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if not isinstance(dead_holder_info, dict):
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raise ValueError("dead_holder_info: %r" % dead_holder_info)
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raise AssertionError("can't break own lock: %r" % self)
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current_info = self.peek()
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if current_info is None:
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# must have been recently released
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if current_info != dead_holder_info:
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raise LockBreakMismatch(self, current_info, dead_holder_info)
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tmpname = '%s.broken.%s.tmp' % (self.path, rand_chars(20))
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self.transport.rename(self.path, tmpname)
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# check that we actually broke the right lock, not someone else;
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# there's a small race window between checking it and doing the
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broken_info_path = tmpname + self.__INFO_NAME
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broken_info = self._read_info_file(broken_info_path)
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if broken_info != dead_holder_info:
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raise LockBreakMismatch(self, broken_info, dead_holder_info)
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self.transport.delete(broken_info_path)
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self.transport.rmdir(tmpname)
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"""Make sure that the lock is still held by this locker.
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This should only fail if the lock was broken by user intervention,
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or if the lock has been affected by a bug.
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If the lock is not thought to be held, raises LockNotHeld. If
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the lock is thought to be held but has been broken, raises
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if not self._lock_held:
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raise LockNotHeld(self)
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# no lock there anymore!
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raise LockBroken(self)
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if info.get('nonce') != self.nonce:
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# there is a lock, but not ours
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raise LockBroken(self)
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def _read_info_file(self, path):
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return self._parse_info(self.transport.get(path))
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"""Check if the lock is held by anyone.
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If it is held, this returns the lock info structure as a rio Stanza,
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which contains some information about the current lock holder.
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Otherwise returns None.
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info = self._read_info_file(self._info_path)
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assert isinstance(info, dict), \
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"bad parse result %r" % info
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except NoSuchFile, e:
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def _prepare_info(self, outf):
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"""Write information about a pending lock to a temporary file.
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# XXX: is creating this here inefficient?
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config = bzrlib.config.GlobalConfig()
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s = Stanza(hostname=socket.gethostname(),
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pid=str(os.getpid()),
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start_time=str(int(time.time())),
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user=config.user_email(),
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RioWriter(outf).write_stanza(s)
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def _parse_info(self, info_file):
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return read_stanza(info_file.readlines()).as_dict()
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def wait_lock(self, timeout=_DEFAULT_TIMEOUT_SECONDS,
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poll=_DEFAULT_POLL_SECONDS):
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"""Wait a certain period for a lock.
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If the lock can be acquired within the bounded time, it
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is taken and this returns. Otherwise, LockContention
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is raised. Either way, this function should return within
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approximately `timeout` seconds. (It may be a bit more if
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a transport operation takes a long time to complete.)
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# XXX: the transport interface doesn't let us guard
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# against operations there taking a long time.
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deadline = time.time() + timeout
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except LockContention:
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if time.time() + poll < deadline:
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raise LockContention(self)
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def wait(self, timeout=20, poll=0.5):
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"""Wait a certain period for a lock to be released."""
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# XXX: the transport interface doesn't let us guard
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# against operations there taking a long time.
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deadline = time.time() + timeout
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if time.time() + poll < deadline:
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raise LockContention(self)
added
removed
bzrlib/lockdir.py
