~bzr-pqm/bzr/bzr.dev

# Copyright (C) 2006 Canonical Ltd

# This program is free software; you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation; either version 2 of the License, or

# (at your option) any later version.

# This program is distributed in the hope that it will be useful,

# but WITHOUT ANY WARRANTY; without even the implied warranty of

# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

# GNU General Public License for more details.

# You should have received a copy of the GNU General Public License

# along with this program; if not, write to the Free Software

# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

"""On-disk mutex protecting a resource

bzr on-disk objects are locked by the existence of a directory with a

particular name within the control directory.  We use this rather than OS

internal locks (such as flock etc) because they can be seen across all

transports, including http.

Objects can be read if there is only physical read access; therefore 

readers can never be required to create a lock, though they will

check whether a writer is using the lock.  Writers can't detect

whether anyone else is reading from the resource as they write.

This works because of ordering constraints that make sure readers

see a consistent view of existing data.

Waiting for a lock must be done by polling; this can be aborted after

a timeout.

Locks must always be explicitly released, typically from a try/finally

block -- they are not released from a finalizer or when Python

exits.

Locks may fail to be released if the process is abruptly terminated

(machine stop, SIGKILL) or if a remote transport becomes permanently

disconnected.  There is therefore a method to break an existing lock.

This should rarely be used, and generally only with user approval.

Locks contain some information on when the lock was taken and by who

which may guide in deciding whether it can safely be broken.  (This is

similar to the messages displayed by emacs and vim.) Note that if the

lock holder is still alive they will get no notification that the lock

has been broken and will continue their work -- so it is important to be

sure they are actually dead.

A lock is represented on disk by a directory of a particular name,

containing an information file.  Taking a lock is done by renaming a

temporary directory into place.  We use temporary directories because

for all known transports and filesystems we believe that exactly one

attempt to claim the lock will succeed and the others will fail.  (Files

won't do because some filesystems or transports only have

rename-and-overwrite, making it hard to tell who won.)

The desired characteristics are:

* Locks are not reentrant.  (That is, a client that tries to take a 

  lock it already holds may deadlock or fail.)

* Stale locks can be guessed at by a heuristic

* Lost locks can be broken by any client

* Failed lock operations leave little or no mess

* Deadlocks are avoided by having a timeout always in use, clients

  desiring indefinite waits can retry or set a silly big timeout.

Storage formats use the locks, and also need to consider concurrency

issues underneath the lock.  A format may choose not to use a lock

at all for some operations.

LockDirs always operate over a Transport.  The transport may be readonly, in

which case the lock can be queried but not acquired.

Locks are identified by a path name, relative to a base transport.

Calling code will typically want to make sure there is exactly one LockDir

object per actual lock on disk.  This module does nothing to prevent aliasing

and deadlocks will likely occur if the locks are aliased.

In the future we may add a "freshen" method which can be called

by a lock holder to check that their lock has not been broken, and to 

update the timestamp within it.

Example usage:

>>> from bzrlib.transport.memory import MemoryTransport

>>> # typically will be obtained from a BzrDir, Branch, etc

>>> t = MemoryTransport()

>>> l = LockDir(t, 'sample-lock')

>>> l.create()

>>> l.wait_lock()

>>> # do something here

>>> l.unlock()

"""

import os

import time

from warnings import warn

from StringIO import StringIO

import bzrlib.config

from bzrlib.errors import (

        DirectoryNotEmpty,

        FileExists,

        LockBreakMismatch,

        LockBroken,

        LockContention,

        LockError,

        LockNotHeld,

        NoSuchFile,

        ResourceBusy,

        UnlockableTransport,

        )

from bzrlib.transport import Transport

from bzrlib.osutils import rand_chars

from bzrlib.rio import RioWriter, read_stanza, Stanza

# XXX: At the moment there is no consideration of thread safety on LockDir

# objects.  This should perhaps be updated - e.g. if two threads try to take a

# lock at the same time they should *both* get it.  But then that's unlikely

# to be a good idea.

# TODO: Transport could offer a simpler put() method that avoids the

# rename-into-place for cases like creating the lock template, where there is

# no chance that the file already exists.

# TODO: Perhaps store some kind of note like the bzr command line in the lock

# info?

# TODO: Some kind of callback run while polling a lock to show progress

# indicators.

# TODO: Make sure to pass the right file and directory mode bits to all

# files/dirs created.

_DEFAULT_TIMEOUT_SECONDS = 300

_DEFAULT_POLL_SECONDS = 0.5

class LockDir(object):

    """Write-lock guarding access to data."""

    __INFO_NAME = '/info'

    def __init__(self, transport, path, file_modebits=0644, dir_modebits=0755):

        """Create a new LockDir object.

        The LockDir is initially unlocked - this just creates the object.

        :param transport: Transport which will contain the lock

        :param path: Path to the lock within the base directory of the 

            transport.

        """

        assert isinstance(transport, Transport), \

            ("not a transport: %r" % transport)

        self.transport = transport

        self.path = path

        self._lock_held = False

        self._fake_read_lock = False

        self._held_dir = path + '/held'

        self._held_info_path = self._held_dir + self.__INFO_NAME

        self._file_modebits = file_modebits

        self._dir_modebits = dir_modebits

        self.nonce = rand_chars(20)

    def __repr__(self):

        return '%s(%s%s)' % (self.__class__.__name__,

                             self.transport.base,

                             self.path)

    is_held = property(lambda self: self._lock_held)

    def create(self, mode=None):

        """Create the on-disk lock.

        This is typically only called when the object/directory containing the 

        directory is first created.  The lock is not held when it's created.

        """

        if self.transport.is_readonly():

            raise UnlockableTransport(self.transport)

        self.transport.mkdir(self.path, mode=mode)

    def attempt_lock(self):

        """Take the lock; fail if it's already held.

        If you wish to block until the lock can be obtained, call wait_lock()

        instead.

        """

        if self._fake_read_lock:

            raise LockContention(self)

        if self.transport.is_readonly():

            raise UnlockableTransport(self.transport)

        try:

            tmpname = '%s/pending.%s.tmp' % (self.path, rand_chars(20))

            self.transport.mkdir(tmpname)

            sio = StringIO()

            self._prepare_info(sio)

            sio.seek(0)

            self.transport.put(tmpname + self.__INFO_NAME, sio)

            self.transport.rename(tmpname, self._held_dir)

            self._lock_held = True

            self.confirm()

            return

        except (DirectoryNotEmpty, FileExists, ResourceBusy), e:

            pass

        # fall through to here on contention

        raise LockContention(self)

    def unlock(self):

        """Release a held lock

        """

        if self._fake_read_lock:

            self._fake_read_lock = False

            return

        if not self._lock_held:

            raise LockNotHeld(self)

        # rename before deleting, because we can't atomically remove the whole

        # tree

        tmpname = '%s/releasing.%s.tmp' % (self.path, rand_chars(20))

        self.transport.rename(self._held_dir, tmpname)

        self._lock_held = False

        self.transport.delete(tmpname + self.__INFO_NAME)

        self.transport.rmdir(tmpname)

    def force_break(self, dead_holder_info):

        """Release a lock held by another process.

        WARNING: This should only be used when the other process is dead; if

        it still thinks it has the lock there will be two concurrent writers.

        In general the user's approval should be sought for lock breaks.

        dead_holder_info must be the result of a previous LockDir.peek() call;

        this is used to check that it's still held by the same process that

        the user decided was dead.  If this is not the current holder,

        LockBreakMismatch is raised.

        After the lock is broken it will not be held by any process.

        It is possible that another process may sneak in and take the 

        lock before the breaking process acquires it.

        """

        if not isinstance(dead_holder_info, dict):

            raise ValueError("dead_holder_info: %r" % dead_holder_info)

        if self._lock_held:

            raise AssertionError("can't break own lock: %r" % self)

        current_info = self.peek()

        if current_info is None:

            # must have been recently released

            return

        if current_info != dead_holder_info:

            raise LockBreakMismatch(self, current_info, dead_holder_info)

        tmpname = '%s/broken.%s.tmp' % (self.path, rand_chars(20))

        self.transport.rename(self._held_dir, tmpname)

        # check that we actually broke the right lock, not someone else;

        # there's a small race window between checking it and doing the 

        # rename.

        broken_info_path = tmpname + self.__INFO_NAME

        broken_info = self._read_info_file(broken_info_path)

        if broken_info != dead_holder_info:

            raise LockBreakMismatch(self, broken_info, dead_holder_info)

        self.transport.delete(broken_info_path)

        self.transport.rmdir(tmpname)

    def confirm(self):

        """Make sure that the lock is still held by this locker.

        This should only fail if the lock was broken by user intervention,

        or if the lock has been affected by a bug.

        If the lock is not thought to be held, raises LockNotHeld.  If

        the lock is thought to be held but has been broken, raises 

        LockBroken.

        """

        if not self._lock_held:

            raise LockNotHeld(self)

        info = self.peek()

        if info is None:

            # no lock there anymore!

            raise LockBroken(self)

        if info.get('nonce') != self.nonce:

            # there is a lock, but not ours

            raise LockBroken(self)

    def _read_info_file(self, path):

        """Read one given info file.

        peek() reads the info file of the lock holder, if any.

        """

        return self._parse_info(self.transport.get(path))

    def peek(self):

        """Check if the lock is held by anyone.

        If it is held, this returns the lock info structure as a rio Stanza,

        which contains some information about the current lock holder.

        Otherwise returns None.

        """

        try:

            info = self._read_info_file(self._held_info_path)

            assert isinstance(info, dict), \

                    "bad parse result %r" % info

            return info

        except NoSuchFile, e:

            return None

    def _prepare_info(self, outf):

        """Write information about a pending lock to a temporary file.

        """

        import socket

        # XXX: is creating this here inefficient?

        config = bzrlib.config.GlobalConfig()

        s = Stanza(hostname=socket.gethostname(),

                   pid=str(os.getpid()),

                   start_time=str(int(time.time())),

                   nonce=self.nonce,

                   user=config.user_email(),

                   )

        RioWriter(outf).write_stanza(s)

    def _parse_info(self, info_file):

        return read_stanza(info_file.readlines()).as_dict()

    def wait_lock(self, timeout=_DEFAULT_TIMEOUT_SECONDS,

                  poll=_DEFAULT_POLL_SECONDS):

        """Wait a certain period for a lock.

        If the lock can be acquired within the bounded time, it

        is taken and this returns.  Otherwise, LockContention

        is raised.  Either way, this function should return within

        approximately `timeout` seconds.  (It may be a bit more if

        a transport operation takes a long time to complete.)

        """

        # XXX: the transport interface doesn't let us guard 

        # against operations there taking a long time.

        deadline = time.time() + timeout

        while True:

            try:

                self.attempt_lock()

                return

            except LockContention:

                pass

            if time.time() + poll < deadline:

                time.sleep(poll)

            else:

                raise LockContention(self)

    def lock_write(self):

        """Wait for and acquire the lock."""

        self.attempt_lock()

    def lock_read(self):

        """Compatability-mode shared lock.

        LockDir doesn't support shared read-only locks, so this 

        just pretends that the lock is taken but really does nothing.

        """

        # At the moment Branches are commonly locked for read, but 

        # we can't rely on that remotely.  Once this is cleaned up,

        # reenable this warning to prevent it coming back in 

        # -- mbp 20060303

        ## warn("LockDir.lock_read falls back to write lock")

        if self._lock_held or self._fake_read_lock:

            raise LockContention(self)

        self._fake_read_lock = True

    def wait(self, timeout=20, poll=0.5):

        """Wait a certain period for a lock to be released."""

        # XXX: the transport interface doesn't let us guard 

        # against operations there taking a long time.

        deadline = time.time() + timeout

        while True:

            if self.peek():

                return

            if time.time() + poll < deadline:

                time.sleep(poll)

            else:

                raise LockContention(self)