1
# Copyright (C) 2006-2010 Canonical Ltd
3
# This program is free software; you can redistribute it and/or modify
4
# it under the terms of the GNU General Public License as published by
5
# the Free Software Foundation; either version 2 of the License, or
6
# (at your option) any later version.
8
# This program is distributed in the hope that it will be useful,
9
# but WITHOUT ANY WARRANTY; without even the implied warranty of
10
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
11
# GNU General Public License for more details.
13
# You should have received a copy of the GNU General Public License
14
# along with this program; if not, write to the Free Software
15
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
17
"""On-disk mutex protecting a resource
19
bzr on-disk objects are locked by the existence of a directory with a
20
particular name within the control directory. We use this rather than OS
21
internal locks (such as flock etc) because they can be seen across all
22
transports, including http.
24
Objects can be read if there is only physical read access; therefore
25
readers can never be required to create a lock, though they will
26
check whether a writer is using the lock. Writers can't detect
27
whether anyone else is reading from the resource as they write.
28
This works because of ordering constraints that make sure readers
29
see a consistent view of existing data.
31
Waiting for a lock must be done by polling; this can be aborted after
34
Locks must always be explicitly released, typically from a try/finally
35
block -- they are not released from a finalizer or when Python
38
Locks may fail to be released if the process is abruptly terminated
39
(machine stop, SIGKILL) or if a remote transport becomes permanently
40
disconnected. There is therefore a method to break an existing lock.
41
This should rarely be used, and generally only with user approval.
42
Locks contain some information on when the lock was taken and by who
43
which may guide in deciding whether it can safely be broken. (This is
44
similar to the messages displayed by emacs and vim.) Note that if the
45
lock holder is still alive they will get no notification that the lock
46
has been broken and will continue their work -- so it is important to be
47
sure they are actually dead.
49
A lock is represented on disk by a directory of a particular name,
50
containing an information file. Taking a lock is done by renaming a
51
temporary directory into place. We use temporary directories because
52
for all known transports and filesystems we believe that exactly one
53
attempt to claim the lock will succeed and the others will fail. (Files
54
won't do because some filesystems or transports only have
55
rename-and-overwrite, making it hard to tell who won.)
57
The desired characteristics are:
59
* Locks are not reentrant. (That is, a client that tries to take a
60
lock it already holds may deadlock or fail.)
61
* Stale locks can be guessed at by a heuristic
62
* Lost locks can be broken by any client
63
* Failed lock operations leave little or no mess
64
* Deadlocks are avoided by having a timeout always in use, clients
65
desiring indefinite waits can retry or set a silly big timeout.
67
Storage formats use the locks, and also need to consider concurrency
68
issues underneath the lock. A format may choose not to use a lock
69
at all for some operations.
71
LockDirs always operate over a Transport. The transport may be readonly, in
72
which case the lock can be queried but not acquired.
74
Locks are identified by a path name, relative to a base transport.
76
Calling code will typically want to make sure there is exactly one LockDir
77
object per actual lock on disk. This module does nothing to prevent aliasing
78
and deadlocks will likely occur if the locks are aliased.
80
In the future we may add a "freshen" method which can be called
81
by a lock holder to check that their lock has not been broken, and to
82
update the timestamp within it.
86
>>> from bzrlib.transport.memory import MemoryTransport
87
>>> # typically will be obtained from a BzrDir, Branch, etc
88
>>> t = MemoryTransport()
89
>>> l = LockDir(t, 'sample-lock')
91
>>> token = l.wait_lock()
92
>>> # do something here
98
# TODO: We sometimes have the problem that our attempt to rename '1234' to
99
# 'held' fails because the transport server moves into an existing directory,
100
# rather than failing the rename. If we made the info file name the same as
101
# the locked directory name we would avoid this problem because moving into
102
# the held directory would implicitly clash. However this would not mesh with
103
# the existing locking code and needs a new format of the containing object.
104
# -- robertc, mbp 20070628
116
from bzrlib.decorators import only_raises
117
from bzrlib.errors import (
130
from bzrlib.trace import mutter, note
131
from bzrlib.osutils import format_delta, rand_chars, get_host_name
134
from bzrlib.lazy_import import lazy_import
135
lazy_import(globals(), """
136
from bzrlib import rio
139
# XXX: At the moment there is no consideration of thread safety on LockDir
140
# objects. This should perhaps be updated - e.g. if two threads try to take a
141
# lock at the same time they should *both* get it. But then that's unlikely
144
# TODO: Perhaps store some kind of note like the bzr command line in the lock
147
# TODO: Some kind of callback run while polling a lock to show progress
150
# TODO: Make sure to pass the right file and directory mode bits to all
151
# files/dirs created.
154
_DEFAULT_TIMEOUT_SECONDS = 300
155
_DEFAULT_POLL_SECONDS = 1.0
158
class LockDir(lock.Lock):
159
"""Write-lock guarding access to data.
162
__INFO_NAME = '/info'
164
def __init__(self, transport, path, file_modebits=0644, dir_modebits=0755):
165
"""Create a new LockDir object.
167
The LockDir is initially unlocked - this just creates the object.
169
:param transport: Transport which will contain the lock
171
:param path: Path to the lock within the base directory of the
174
self.transport = transport
176
self._lock_held = False
177
self._locked_via_token = False
178
self._fake_read_lock = False
179
self._held_dir = path + '/held'
180
self._held_info_path = self._held_dir + self.__INFO_NAME
181
self._file_modebits = file_modebits
182
self._dir_modebits = dir_modebits
184
self._report_function = note
187
return '%s(%s%s)' % (self.__class__.__name__,
191
is_held = property(lambda self: self._lock_held)
193
def create(self, mode=None):
194
"""Create the on-disk lock.
196
This is typically only called when the object/directory containing the
197
directory is first created. The lock is not held when it's created.
199
self._trace("create lock directory")
201
self.transport.mkdir(self.path, mode=mode)
202
except (TransportError, PathError), e:
203
raise LockFailed(self, e)
206
def _attempt_lock(self):
207
"""Make the pending directory and attempt to rename into place.
209
If the rename succeeds, we read back the info file to check that we
212
If we fail to acquire the lock, this method is responsible for
213
cleaning up the pending directory if possible. (But it doesn't do
216
:returns: The nonce of the lock, if it was successfully acquired.
218
:raises LockContention: If the lock is held by someone else. The exception
219
contains the info of the current holder of the lock.
221
self._trace("lock_write...")
222
start_time = time.time()
224
tmpname = self._create_pending_dir()
225
except (errors.TransportError, PathError), e:
226
self._trace("... failed to create pending dir, %s", e)
227
raise LockFailed(self, e)
229
self.transport.rename(tmpname, self._held_dir)
230
except (errors.TransportError, PathError, DirectoryNotEmpty,
231
FileExists, ResourceBusy), e:
232
self._trace("... contention, %s", e)
233
self._remove_pending_dir(tmpname)
234
raise LockContention(self)
236
self._trace("... lock failed, %s", e)
237
self._remove_pending_dir(tmpname)
239
# We must check we really got the lock, because Launchpad's sftp
240
# server at one time had a bug were the rename would successfully
241
# move the new directory into the existing directory, which was
242
# incorrect. It's possible some other servers or filesystems will
243
# have a similar bug allowing someone to think they got the lock
244
# when it's already held.
246
# See <https://bugs.edge.launchpad.net/bzr/+bug/498378> for one case.
248
# Strictly the check is unnecessary and a waste of time for most
249
# people, but probably worth trapping if something is wrong.
251
self._trace("after locking, info=%r", info)
253
raise LockFailed(self, "lock was renamed into place, but "
255
if info['nonce'] != self.nonce:
256
self._trace("rename succeeded, "
257
"but lock is still held by someone else")
258
raise LockContention(self)
259
self._lock_held = True
260
self._trace("... lock succeeded after %dms",
261
(time.time() - start_time) * 1000)
264
def _remove_pending_dir(self, tmpname):
265
"""Remove the pending directory
267
This is called if we failed to rename into place, so that the pending
268
dirs don't clutter up the lockdir.
270
self._trace("remove %s", tmpname)
272
self.transport.delete(tmpname + self.__INFO_NAME)
273
self.transport.rmdir(tmpname)
275
note("error removing pending lock: %s", e)
277
def _create_pending_dir(self):
278
tmpname = '%s/%s.tmp' % (self.path, rand_chars(10))
280
self.transport.mkdir(tmpname)
282
# This may raise a FileExists exception
283
# which is okay, it will be caught later and determined
284
# to be a LockContention.
285
self._trace("lock directory does not exist, creating it")
286
self.create(mode=self._dir_modebits)
287
# After creating the lock directory, try again
288
self.transport.mkdir(tmpname)
289
self.nonce = rand_chars(20)
290
info_bytes = self._prepare_info()
291
# We use put_file_non_atomic because we just created a new unique
292
# directory so we don't have to worry about files existing there.
293
# We'll rename the whole directory into place to get atomic
295
self.transport.put_bytes_non_atomic(tmpname + self.__INFO_NAME,
299
@only_raises(LockNotHeld, LockBroken)
301
"""Release a held lock
303
if self._fake_read_lock:
304
self._fake_read_lock = False
306
if not self._lock_held:
307
return lock.cant_unlock_not_held(self)
308
if self._locked_via_token:
309
self._locked_via_token = False
310
self._lock_held = False
312
old_nonce = self.nonce
313
# rename before deleting, because we can't atomically remove the
315
start_time = time.time()
316
self._trace("unlocking")
317
tmpname = '%s/releasing.%s.tmp' % (self.path, rand_chars(20))
318
# gotta own it to unlock
320
self.transport.rename(self._held_dir, tmpname)
321
self._lock_held = False
322
self.transport.delete(tmpname + self.__INFO_NAME)
324
self.transport.rmdir(tmpname)
325
except DirectoryNotEmpty, e:
326
# There might have been junk left over by a rename that moved
327
# another locker within the 'held' directory. do a slower
328
# deletion where we list the directory and remove everything
331
# Maybe this should be broader to allow for ftp servers with
332
# non-specific error messages?
333
self._trace("doing recursive deletion of non-empty directory "
335
self.transport.delete_tree(tmpname)
336
self._trace("... unlock succeeded after %dms",
337
(time.time() - start_time) * 1000)
338
result = lock.LockResult(self.transport.abspath(self.path),
340
for hook in self.hooks['lock_released']:
343
def break_lock(self):
344
"""Break a lock not held by this instance of LockDir.
346
This is a UI centric function: it uses the bzrlib.ui.ui_factory to
347
prompt for input if a lock is detected and there is any doubt about
348
it possibly being still active.
350
self._check_not_locked()
351
holder_info = self.peek()
352
if holder_info is not None:
353
lock_info = '\n'.join(self._format_lock_info(holder_info))
354
if bzrlib.ui.ui_factory.get_boolean("Break %s" % lock_info):
355
self.force_break(holder_info)
357
def force_break(self, dead_holder_info):
358
"""Release a lock held by another process.
360
WARNING: This should only be used when the other process is dead; if
361
it still thinks it has the lock there will be two concurrent writers.
362
In general the user's approval should be sought for lock breaks.
364
dead_holder_info must be the result of a previous LockDir.peek() call;
365
this is used to check that it's still held by the same process that
366
the user decided was dead. If this is not the current holder,
367
LockBreakMismatch is raised.
369
After the lock is broken it will not be held by any process.
370
It is possible that another process may sneak in and take the
371
lock before the breaking process acquires it.
373
if not isinstance(dead_holder_info, dict):
374
raise ValueError("dead_holder_info: %r" % dead_holder_info)
375
self._check_not_locked()
376
current_info = self.peek()
377
if current_info is None:
378
# must have been recently released
380
if current_info != dead_holder_info:
381
raise LockBreakMismatch(self, current_info, dead_holder_info)
382
tmpname = '%s/broken.%s.tmp' % (self.path, rand_chars(20))
383
self.transport.rename(self._held_dir, tmpname)
384
# check that we actually broke the right lock, not someone else;
385
# there's a small race window between checking it and doing the
387
broken_info_path = tmpname + self.__INFO_NAME
388
broken_info = self._read_info_file(broken_info_path)
389
if broken_info != dead_holder_info:
390
raise LockBreakMismatch(self, broken_info, dead_holder_info)
391
self.transport.delete(broken_info_path)
392
self.transport.rmdir(tmpname)
393
result = lock.LockResult(self.transport.abspath(self.path),
394
current_info.get('nonce'))
395
for hook in self.hooks['lock_broken']:
398
def _check_not_locked(self):
399
"""If the lock is held by this instance, raise an error."""
401
raise AssertionError("can't break own lock: %r" % self)
404
"""Make sure that the lock is still held by this locker.
406
This should only fail if the lock was broken by user intervention,
407
or if the lock has been affected by a bug.
409
If the lock is not thought to be held, raises LockNotHeld. If
410
the lock is thought to be held but has been broken, raises
413
if not self._lock_held:
414
raise LockNotHeld(self)
417
# no lock there anymore!
418
raise LockBroken(self)
419
if info.get('nonce') != self.nonce:
420
# there is a lock, but not ours
421
raise LockBroken(self)
423
def _read_info_file(self, path):
424
"""Read one given info file.
426
peek() reads the info file of the lock holder, if any.
428
return self._parse_info(self.transport.get_bytes(path))
431
"""Check if the lock is held by anyone.
433
If it is held, this returns the lock info structure as a rio Stanza,
434
which contains some information about the current lock holder.
435
Otherwise returns None.
438
info = self._read_info_file(self._held_info_path)
439
self._trace("peek -> held")
441
except NoSuchFile, e:
442
self._trace("peek -> not held")
444
def _prepare_info(self):
445
"""Write information about a pending lock to a temporary file.
447
# XXX: is creating this here inefficient?
448
config = bzrlib.config.GlobalConfig()
450
user = config.user_email()
451
except errors.NoEmailInUsername:
452
user = config.username()
453
s = rio.Stanza(hostname=get_host_name(),
454
pid=str(os.getpid()),
455
start_time=str(int(time.time())),
461
def _parse_info(self, info_bytes):
462
# TODO: Handle if info_bytes is empty
463
return rio.read_stanza(osutils.split_lines(info_bytes)).as_dict()
465
def attempt_lock(self):
466
"""Take the lock; fail if it's already held.
468
If you wish to block until the lock can be obtained, call wait_lock()
471
:return: The lock token.
472
:raises LockContention: if the lock is held by someone else.
474
if self._fake_read_lock:
475
raise LockContention(self)
476
result = self._attempt_lock()
477
hook_result = lock.LockResult(self.transport.abspath(self.path),
479
for hook in self.hooks['lock_acquired']:
483
def wait_lock(self, timeout=None, poll=None, max_attempts=None):
484
"""Wait a certain period for a lock.
486
If the lock can be acquired within the bounded time, it
487
is taken and this returns. Otherwise, LockContention
488
is raised. Either way, this function should return within
489
approximately `timeout` seconds. (It may be a bit more if
490
a transport operation takes a long time to complete.)
492
:param timeout: Approximate maximum amount of time to wait for the
495
:param poll: Delay in seconds between retrying the lock.
497
:param max_attempts: Maximum number of times to try to lock.
499
:return: The lock token.
502
timeout = _DEFAULT_TIMEOUT_SECONDS
504
poll = _DEFAULT_POLL_SECONDS
505
# XXX: the transport interface doesn't let us guard against operations
506
# there taking a long time, so the total elapsed time or poll interval
507
# may be more than was requested.
508
deadline = time.time() + timeout
515
return self.attempt_lock()
516
except LockContention:
517
# possibly report the blockage, then try again
519
# TODO: In a few cases, we find out that there's contention by
520
# reading the held info and observing that it's not ours. In
521
# those cases it's a bit redundant to read it again. However,
522
# the normal case (??) is that the rename fails and so we
523
# don't know who holds the lock. For simplicity we peek
525
new_info = self.peek()
526
if new_info is not None and new_info != last_info:
527
if last_info is None:
528
start = 'Unable to obtain'
530
start = 'Lock owner changed for'
532
formatted_info = self._format_lock_info(new_info)
533
if deadline_str is None:
534
deadline_str = time.strftime('%H:%M:%S',
535
time.localtime(deadline))
536
lock_url = self.transport.abspath(self.path)
537
# See <https://bugs.edge.launchpad.net/bzr/+bug/250451>
538
# the URL here is sometimes not one that is useful to the
539
# user, perhaps being wrapped in a lp-%d or chroot decorator,
540
# especially if this error is issued from the server.
541
self._report_function('%s %s\n'
543
'%s\n' # locked ... ago
544
'Will continue to try until %s, unless '
545
'you press Ctrl-C.\n'
546
'See "bzr help break-lock" for more.',
554
if (max_attempts is not None) and (attempt_count >= max_attempts):
555
self._trace("exceeded %d attempts")
556
raise LockContention(self)
557
if time.time() + poll < deadline:
558
self._trace("waiting %ss", poll)
561
self._trace("timeout after waiting %ss", timeout)
562
raise LockContention(self)
564
def leave_in_place(self):
565
self._locked_via_token = True
567
def dont_leave_in_place(self):
568
self._locked_via_token = False
570
def lock_write(self, token=None):
571
"""Wait for and acquire the lock.
573
:param token: if this is already locked, then lock_write will fail
574
unless the token matches the existing lock.
575
:returns: a token if this instance supports tokens, otherwise None.
576
:raises TokenLockingNotSupported: when a token is given but this
577
instance doesn't support using token locks.
578
:raises MismatchedToken: if the specified token doesn't match the token
579
of the existing lock.
581
A token should be passed in if you know that you have locked the object
582
some other way, and need to synchronise this object's state with that
585
XXX: docstring duplicated from LockableFiles.lock_write.
587
if token is not None:
588
self.validate_token(token)
590
self._lock_held = True
591
self._locked_via_token = True
594
return self.wait_lock()
597
"""Compatibility-mode shared lock.
599
LockDir doesn't support shared read-only locks, so this
600
just pretends that the lock is taken but really does nothing.
602
# At the moment Branches are commonly locked for read, but
603
# we can't rely on that remotely. Once this is cleaned up,
604
# reenable this warning to prevent it coming back in
606
## warn("LockDir.lock_read falls back to write lock")
607
if self._lock_held or self._fake_read_lock:
608
raise LockContention(self)
609
self._fake_read_lock = True
611
def _format_lock_info(self, info):
612
"""Turn the contents of peek() into something for the user"""
613
lock_url = self.transport.abspath(self.path)
614
delta = time.time() - int(info['start_time'])
616
'lock %s' % (lock_url,),
617
'held by %(user)s on host %(hostname)s [process #%(pid)s]' % info,
618
'locked %s' % (format_delta(delta),),
621
def validate_token(self, token):
622
if token is not None:
628
lock_token = info.get('nonce')
629
if token != lock_token:
630
raise errors.TokenMismatch(token, lock_token)
632
self._trace("revalidated by token %r", token)
634
def _trace(self, format, *args):
635
if 'lock' not in debug.debug_flags:
637
mutter(str(self) + ": " + (format % args))
added
removed
bzrlib/lockdir.py
