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# Copyright (C) 2006-2011 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 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 breezy.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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>>> token = l.wait_lock()
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>>> # do something here
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Some classes of stale locks can be predicted by checking: the host name is the
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same as the local host name; the user name is the same as the local user; the
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process id no longer exists. The check on user name is not strictly necessary
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but helps protect against colliding host names.
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# TODO: We sometimes have the problem that our attempt to rename '1234' to
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# 'held' fails because the transport server moves into an existing directory,
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# rather than failing the rename. If we made the info file name the same as
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# the locked directory name we would avoid this problem because moving into
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# the held directory would implicitly clash. However this would not mesh with
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# the existing locking code and needs a new format of the containing object.
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# -- robertc, mbp 20070628
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from .decorators import only_raises
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from .errors import (
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from .i18n import gettext
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from .osutils import format_delta, rand_chars, get_host_name
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from .trace import mutter, note
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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: 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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# TODO: Make sure to pass the right file and directory mode bits to all
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# files/dirs created.
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_DEFAULT_TIMEOUT_SECONDS = 30
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_DEFAULT_POLL_SECONDS = 1.0
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class LockDir(lock.Lock):
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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, file_modebits=0o644,
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dir_modebits=0o755, extra_holder_info=None):
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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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:param extra_holder_info: If passed, {str:str} dict of extra or
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updated information to insert into the info file when the lock is
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self.transport = transport
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self._lock_held = False
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self._locked_via_token = False
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self._fake_read_lock = False
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self._held_dir = path + '/held'
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self._held_info_path = self._held_dir + self.__INFO_NAME
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self._file_modebits = file_modebits
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self._dir_modebits = dir_modebits
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self._report_function = note
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self.extra_holder_info = extra_holder_info
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self._warned_about_lock_holder = None
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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 create(self, mode=None):
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"""Create the on-disk lock.
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This is typically only called when the object/directory containing the
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directory is first created. The lock is not held when it's created.
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self._trace("create lock directory")
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self.transport.mkdir(self.path, mode=mode)
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except (TransportError, PathError) as e:
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raise LockFailed(self, e)
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def _attempt_lock(self):
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"""Make the pending directory and attempt to rename into place.
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If the rename succeeds, we read back the info file to check that we
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If we fail to acquire the lock, this method is responsible for
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cleaning up the pending directory if possible. (But it doesn't do
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:returns: The nonce of the lock, if it was successfully acquired.
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:raises LockContention: If the lock is held by someone else. The
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exception contains the info of the current holder of the lock.
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self._trace("lock_write...")
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start_time = time.time()
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tmpname = self._create_pending_dir()
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except (errors.TransportError, PathError) as e:
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self._trace("... failed to create pending dir, %s", e)
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raise LockFailed(self, e)
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self.transport.rename(tmpname, self._held_dir)
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except (errors.TransportError, PathError, DirectoryNotEmpty,
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FileExists, ResourceBusy) as e:
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self._trace("... contention, %s", e)
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other_holder = self.peek()
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self._trace("other holder is %r" % other_holder)
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self._handle_lock_contention(other_holder)
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except BaseException:
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self._remove_pending_dir(tmpname)
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except Exception as e:
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self._trace("... lock failed, %s", e)
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self._remove_pending_dir(tmpname)
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# We must check we really got the lock, because Launchpad's sftp
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# server at one time had a bug were the rename would successfully
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# move the new directory into the existing directory, which was
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# incorrect. It's possible some other servers or filesystems will
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# have a similar bug allowing someone to think they got the lock
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# when it's already held.
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# See <https://bugs.launchpad.net/bzr/+bug/498378> for one case.
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# Strictly the check is unnecessary and a waste of time for most
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# people, but probably worth trapping if something is wrong.
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self._trace("after locking, info=%r", info)
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raise LockFailed(self, "lock was renamed into place, but "
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if info.get('nonce') != self.nonce:
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self._trace("rename succeeded, "
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"but lock is still held by someone else")
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raise LockContention(self)
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self._lock_held = True
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self._trace("... lock succeeded after %dms",
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(time.time() - start_time) * 1000)
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def _handle_lock_contention(self, other_holder):
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"""A lock we want to take is held by someone else.
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This function can: tell the user about it; possibly detect that it's
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safe or appropriate to steal the lock, or just raise an exception.
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If this function returns (without raising an exception) the lock will
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:param other_holder: A LockHeldInfo for the current holder; note that
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it might be None if the lock can be seen to be held but the info
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if (other_holder is not None):
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if other_holder.is_lock_holder_known_dead():
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if self.get_config().get('locks.steal_dead'):
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ui.ui_factory.show_user_warning(
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lock_url=urlutils.join(self.transport.base, self.path),
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other_holder_info=str(other_holder))
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self.force_break(other_holder)
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self._trace("stole lock from dead holder")
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raise LockContention(self)
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def _remove_pending_dir(self, tmpname):
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"""Remove the pending directory
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This is called if we failed to rename into place, so that the pending
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dirs don't clutter up the lockdir.
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self._trace("remove %s", tmpname)
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self.transport.delete(tmpname + self.__INFO_NAME)
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self.transport.rmdir(tmpname)
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except PathError as e:
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note(gettext("error removing pending lock: %s"), e)
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def _create_pending_dir(self):
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tmpname = '%s/%s.tmp' % (self.path, rand_chars(10))
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self.transport.mkdir(tmpname)
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# This may raise a FileExists exception
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# which is okay, it will be caught later and determined
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# to be a LockContention.
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self._trace("lock directory does not exist, creating it")
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self.create(mode=self._dir_modebits)
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# After creating the lock directory, try again
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self.transport.mkdir(tmpname)
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info = LockHeldInfo.for_this_process(self.extra_holder_info)
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self.nonce = info.get('nonce')
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# We use put_file_non_atomic because we just created a new unique
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# directory so we don't have to worry about files existing there.
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# We'll rename the whole directory into place to get atomic
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self.transport.put_bytes_non_atomic(tmpname + self.__INFO_NAME,
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@only_raises(LockNotHeld, LockBroken)
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"""Release a held lock
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if self._fake_read_lock:
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self._fake_read_lock = False
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if not self._lock_held:
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return lock.cant_unlock_not_held(self)
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if self._locked_via_token:
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self._locked_via_token = False
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self._lock_held = False
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old_nonce = self.nonce
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# rename before deleting, because we can't atomically remove the
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start_time = time.time()
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self._trace("unlocking")
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tmpname = '%s/releasing.%s.tmp' % (self.path, rand_chars(20))
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# gotta own it to unlock
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self.transport.rename(self._held_dir, 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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except DirectoryNotEmpty:
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# There might have been junk left over by a rename that moved
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# another locker within the 'held' directory. do a slower
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# deletion where we list the directory and remove everything
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self._trace("doing recursive deletion of non-empty directory "
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self.transport.delete_tree(tmpname)
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self._trace("... unlock succeeded after %dms",
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(time.time() - start_time) * 1000)
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result = lock.LockResult(self.transport.abspath(self.path),
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for hook in self.hooks['lock_released']:
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def break_lock(self):
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"""Break a lock not held by this instance of LockDir.
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This is a UI centric function: it uses the ui.ui_factory to
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prompt for input if a lock is detected and there is any doubt about
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it possibly being still active. force_break is the non-interactive
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:returns: LockResult for the broken lock.
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self._check_not_locked()
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holder_info = self.peek()
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except LockCorrupt as e:
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# The lock info is corrupt.
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if ui.ui_factory.get_boolean(u"Break (corrupt %r)" % (self,)):
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self.force_break_corrupt(e.file_data)
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if holder_info is not None:
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if ui.ui_factory.confirm_action(
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u"Break %(lock_info)s",
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'breezy.lockdir.break',
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dict(lock_info=str(holder_info))):
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result = self.force_break(holder_info)
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ui.ui_factory.show_message(
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"Broke lock %s" % result.lock_url)
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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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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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:param dead_holder_info:
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Must be the result of a previous LockDir.peek() call; this is used
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to check that it's still held by the same process that the user
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decided was dead. If this is not the current holder,
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LockBreakMismatch is raised.
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:returns: LockResult for the broken lock.
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if not isinstance(dead_holder_info, LockHeldInfo):
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raise ValueError("dead_holder_info: %r" % dead_holder_info)
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self._check_not_locked()
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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._held_dir, 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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result = lock.LockResult(self.transport.abspath(self.path),
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current_info.get('nonce'))
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for hook in self.hooks['lock_broken']:
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def force_break_corrupt(self, corrupt_info_lines):
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"""Release a lock that has been corrupted.
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This is very similar to force_break, it except it doesn't assume that
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self.peek() can work.
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:param corrupt_info_lines: the lines of the corrupted info file, used
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to check that the lock hasn't changed between reading the (corrupt)
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info file and calling force_break_corrupt.
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# XXX: this copes with unparseable info files, but what about missing
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# info files? Or missing lock dirs?
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self._check_not_locked()
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tmpname = '%s/broken.%s.tmp' % (self.path, rand_chars(20))
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self.transport.rename(self._held_dir, 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_content = self.transport.get_bytes(broken_info_path)
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broken_lines = osutils.split_lines(broken_content)
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if broken_lines != corrupt_info_lines:
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raise LockBreakMismatch(self, broken_lines, corrupt_info_lines)
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self.transport.delete(broken_info_path)
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self.transport.rmdir(tmpname)
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result = lock.LockResult(self.transport.abspath(self.path))
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for hook in self.hooks['lock_broken']:
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def _check_not_locked(self):
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"""If the lock is held by this instance, raise an error."""
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raise AssertionError("can't break own lock: %r" % self)
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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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"""Read one given info file.
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peek() reads the info file of the lock holder, if any.
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return LockHeldInfo.from_info_file_bytes(
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self.transport.get_bytes(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 dict
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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._held_info_path)
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self._trace("peek -> held")
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self._trace("peek -> not held")
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def _prepare_info(self):
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"""Write information about a pending lock to a temporary file.
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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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:return: The lock token.
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:raises LockContention: if the lock is held by someone else.
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if self._fake_read_lock:
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raise LockContention(self)
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result = self._attempt_lock()
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hook_result = lock.LockResult(self.transport.abspath(self.path),
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for hook in self.hooks['lock_acquired']:
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def lock_url_for_display(self):
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"""Give a nicely-printable representation of the URL of this lock."""
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# As local lock urls are correct we display them.
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# We avoid displaying remote lock urls.
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lock_url = self.transport.abspath(self.path)
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if lock_url.startswith('file://'):
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lock_url = lock_url.split('.bzr/')[0]
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def wait_lock(self, timeout=None, poll=None, max_attempts=None):
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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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:param timeout: Approximate maximum amount of time to wait for the
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:param poll: Delay in seconds between retrying the lock.
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:param max_attempts: Maximum number of times to try to lock.
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:return: The lock token.
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timeout = _DEFAULT_TIMEOUT_SECONDS
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poll = _DEFAULT_POLL_SECONDS
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# XXX: the transport interface doesn't let us guard against operations
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# there taking a long time, so the total elapsed time or poll interval
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# may be more than was requested.
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deadline = time.time() + timeout
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lock_url = self.lock_url_for_display()
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return self.attempt_lock()
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except LockContention:
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# possibly report the blockage, then try again
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# TODO: In a few cases, we find out that there's contention by
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# reading the held info and observing that it's not ours. In
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# those cases it's a bit redundant to read it again. However,
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# the normal case (??) is that the rename fails and so we
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# don't know who holds the lock. For simplicity we peek
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new_info = self.peek()
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if new_info is not None and new_info != last_info:
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if last_info is None:
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start = gettext('Unable to obtain')
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start = gettext('Lock owner changed for')
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msg = gettext('{0} lock {1} {2}.').format(start, lock_url,
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if deadline_str is None:
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deadline_str = time.strftime('%H:%M:%S',
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time.localtime(deadline))
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msg += '\n' + gettext(
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'Will continue to try until %s, unless '
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'you press Ctrl-C.') % deadline_str
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msg += '\n' + gettext('See "brz help break-lock" for more.')
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self._report_function(msg)
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if (max_attempts is not None) and (attempt_count >= max_attempts):
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self._trace("exceeded %d attempts")
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raise LockContention(self)
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if time.time() + poll < deadline:
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self._trace("waiting %ss", poll)
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# As timeout is always 0 for remote locks
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# this block is applicable only for local
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self._trace("timeout after waiting %ss", timeout)
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raise LockContention('(local)', lock_url)
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def leave_in_place(self):
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self._locked_via_token = True
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def dont_leave_in_place(self):
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self._locked_via_token = False
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def lock_write(self, token=None):
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"""Wait for and acquire the lock.
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:param token: if this is already locked, then lock_write will fail
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unless the token matches the existing lock.
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:returns: a token if this instance supports tokens, otherwise None.
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:raises TokenLockingNotSupported: when a token is given but this
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instance doesn't support using token locks.
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:raises MismatchedToken: if the specified token doesn't match the token
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of the existing lock.
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A token should be passed in if you know that you have locked the object
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some other way, and need to synchronise this object's state with that
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XXX: docstring duplicated from LockableFiles.lock_write.
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if token is not None:
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self.validate_token(token)
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self._lock_held = True
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self._locked_via_token = True
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return self.wait_lock()
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"""Compatibility-mode shared lock.
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LockDir doesn't support shared read-only locks, so this
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just pretends that the lock is taken but really does nothing.
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# At the moment Branches are commonly locked for read, but
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# we can't rely on that remotely. Once this is cleaned up,
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# reenable this warning to prevent it coming back in
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# warn("LockDir.lock_read falls back to write lock")
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if self._lock_held or self._fake_read_lock:
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raise LockContention(self)
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self._fake_read_lock = True
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def validate_token(self, token):
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if token is not None:
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lock_token = info.get('nonce')
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if token != lock_token:
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raise errors.TokenMismatch(token, lock_token)
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self._trace("revalidated by token %r", token)
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def _trace(self, format, *args):
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if 'lock' not in debug.debug_flags:
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mutter(str(self) + ": " + (format % args))
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def get_config(self):
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"""Get the configuration that governs this lockdir."""
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# XXX: This really should also use the locationconfig at least, but
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# that seems a bit hard to hook up at the moment. -- mbp 20110329
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# FIXME: The above is still true ;) -- vila 20110811
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return config.GlobalStack()
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class LockHeldInfo(object):
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"""The information recorded about a held lock.
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This information is recorded into the lock when it's taken, and it can be
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read back by any process with access to the lockdir. It can be used, for
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example, to tell the user who holds the lock, or to try to detect whether
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the lock holder is still alive.
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Prior to bzr 2.4 a simple dict was used instead of an object.
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def __init__(self, info_dict):
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self.info_dict = info_dict
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"""Return a debugging representation of this object."""
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return "%s(%r)" % (self.__class__.__name__, self.info_dict)
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"""Return a user-oriented description of this object."""
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d = self.to_readable_dict()
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u'held by %(user)s on %(hostname)s (process #%(pid)s), '
732
u'acquired %(time_ago)s') % d)
734
def to_readable_dict(self):
735
"""Turn the holder info into a dict of human-readable attributes.
737
For example, the start time is presented relative to the current time,
738
rather than as seconds since the epoch.
740
Returns a list of [user, hostname, pid, time_ago] all as readable
743
start_time = self.info_dict.get('start_time')
744
if start_time is None:
745
time_ago = '(unknown)'
747
time_ago = format_delta(
748
time.time() - int(self.info_dict['start_time']))
749
user = self.info_dict.get('user', '<unknown>')
750
hostname = self.info_dict.get('hostname', '<unknown>')
751
pid = self.info_dict.get('pid', '<unknown>')
758
def get(self, field_name):
759
"""Return the contents of a field from the lock info, or None."""
760
return self.info_dict.get(field_name)
763
def for_this_process(cls, extra_holder_info):
764
"""Return a new LockHeldInfo for a lock taken by this process.
767
hostname=get_host_name(),
768
pid=str(os.getpid()),
769
nonce=rand_chars(20).encode('ascii'),
770
start_time=str(int(time.time())),
771
user=get_username_for_lock_info(),
773
if extra_holder_info is not None:
774
info.update(extra_holder_info)
778
s = rio.Stanza(**self.info_dict)
782
def from_info_file_bytes(cls, info_file_bytes):
783
"""Construct from the contents of the held file."""
784
lines = osutils.split_lines(info_file_bytes)
786
stanza = rio.read_stanza(lines)
787
except ValueError as e:
788
mutter('Corrupt lock info file: %r', lines)
789
raise LockCorrupt("could not parse lock info file: " + str(e),
792
# see bug 185013; we fairly often end up with the info file being
793
# empty after an interruption; we could log a message here but
794
# there may not be much we can say
797
ret = stanza.as_dict()
798
ret['nonce'] = ret['nonce'].encode('ascii')
804
def __eq__(self, other):
805
"""Equality check for lock holders."""
806
if type(self) != type(other):
808
return self.info_dict == other.info_dict
810
def __ne__(self, other):
811
return not self == other
813
def is_locked_by_this_process(self):
814
"""True if this process seems to be the current lock holder."""
816
self.get('hostname') == get_host_name()
817
and self.get('pid') == str(os.getpid())
818
and self.get('user') == get_username_for_lock_info())
820
def is_lock_holder_known_dead(self):
821
"""True if the lock holder process is known to be dead.
823
False if it's either known to be still alive, or if we just can't tell.
825
We can be fairly sure the lock holder is dead if it declared the same
826
hostname and there is no process with the given pid alive. If people
827
have multiple machines with the same hostname this may cause trouble.
829
This doesn't check whether the lock holder is in fact the same process
830
calling this method. (In that case it will return true.)
832
if self.get('hostname') != get_host_name():
834
if self.get('hostname') == 'localhost':
837
if self.get('user') != get_username_for_lock_info():
838
# Could well be another local process by a different user, but
839
# just to be safe we won't conclude about this either.
841
pid_str = self.info_dict.get('pid', None)
843
mutter("no pid recorded in %r" % (self, ))
848
mutter("can't parse pid %r from %r"
851
return osutils.is_local_pid_dead(pid)
854
def get_username_for_lock_info():
855
"""Get a username suitable for putting into a lock.
857
It's ok if what's written here is not a proper email address as long
858
as it gives some clue who the user is.
861
return config.GlobalStack().get('email')
862
except errors.NoWhoami:
863
return osutils.getuser_unicode()
added
removed
breezy/lockdir.py
