# This file is part of Checkbox.
#
# Copyright 2012, 2013 Canonical Ltd.
# Written by:
# Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
#
# Checkbox is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License version 3,
# as published by the Free Software Foundation.
#
# Checkbox 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 Checkbox. If not, see <http://www.gnu.org/licenses/>.
"""
:mod:`plainbox.impl.session.storage` -- storage for sessions
============================================================
This module contains storage support code for handling sessions. Using the
:class:`SessionStorageRepository` one can enumerate sessions at a particular
location. Each location is wrapped by a :class:`SessionStorage` instance. That
latter class be used to create (allocate) and remove all of the files
associated with a particular session.
"""
import errno
import logging
import os
import shutil
import stat
import sys
import tempfile
logger = logging.getLogger("plainbox.session.storage")
[docs]class SessionStorageRepository:
"""
Helper class to enumerate filesystem artefacts of current or past Sessions
This class collaborates with :class:`SessionStorage`. The basic
use-case is to open a well-known location and enumerate all the sessions
that are stored there. This allows to create :class:`SessionStorage`
instances to further manage each session (such as remove them by calling
:meth:SessionStorage.remove()`)
"""
_LAST_SESSION_SYMLINK = "last-session"
def __init__(self, location=None):
"""
Initialize new repository at the specified location.
The location does not have to be an existing directory. It will be
created on demand. Typically it should be instantiated with the default
location.
"""
if location is None:
location = self.get_default_location()
self._location = location
@property
[docs] def location(self):
"""
pathname of the repository
"""
return self._location
[docs] def get_last_storage(self):
"""
Find the last session storage object created in this repository.
:returns:
SessionStorage object associated with the last session created in
this repository using legacy mode.
.. note::
This will only return storage objects that were created using
legacy mode. Nonlegacy storage objects will not be returned this
way.
"""
pathname = os.path.join(self.location, self._LAST_SESSION_SYMLINK)
try:
last_storage = os.readlink(pathname)
except OSError:
# The symlink can be gone or not be a real symlink
# in that case just ignore it and return None
return None
else:
# The link may be relative so let's ensure we know the full
# pathname for the subsequent check (which may be performed
# from another directory)
last_storage = os.path.join(self._location, last_storage)
# If the link points to a directory, assume it's okay
if os.path.isdir(last_storage):
return SessionStorage(last_storage)
[docs] def get_storage_list(self):
"""
Enumerate stored sessions in the repository.
If the repository directory is not present then an empty list is
returned.
:returns:
list of :class:`SessionStorage` representing discovered sessions
"""
logger.debug("Enumerating sessions in %s", self._location)
try:
# Try to enumerate the directory
item_list = os.listdir(self._location)
except OSError as exc:
# If the directory does not exist,
# silently return empty collection
if exc.errno == errno.ENOENT:
return []
# Don't silence any other errors
raise
session_list = []
# Check each item by looking for directories
for item in item_list:
pathname = os.path.join(self.location, item)
# Make sure not to follow any symlinks here
stat_result = os.lstat(pathname)
# Consider directories only
if stat.S_ISDIR(stat_result.st_mode):
logger.debug("Found possible session in %r", pathname)
session = SessionStorage(pathname)
session_list.append(session)
# Return the full list
return session_list
def __iter__(self):
"""
Same as :meth:`get_storage_list()`
"""
return iter(self.get_storage_list())
@classmethod
[docs] def get_default_location(cls):
"""
Compute the default location of the session state repository
:returns: ${XDG_CACHE_HOME:-$HOME/.cache}/plainbox/sessions
"""
# Pick XDG_CACHE_HOME from environment
xdg_cache_home = os.environ.get('XDG_CACHE_HOME')
# If not set or empty use the default ~/.cache/
if not xdg_cache_home:
xdg_cache_home = os.path.join(os.path.expanduser('~'), '.cache')
# Use a directory relative to XDG_CACHE_HOME
return os.path.join(xdg_cache_home, 'plainbox', 'sessions')
[docs]class LockedStorageError(IOError):
"""
Exception raised when SessionStorage.save_checkpoint() finds an existing
'next' file from a (presumably) previous call to save_checkpoint() that
got interrupted
"""
[docs]class SessionStorage:
"""
Abstraction for storage area that is used by :class:`SessionState` to
keep some persistent and volatile data.
This class implements functions performing input/output operations
on session checkpoint data. The location property can be used for keeping
any additional files or directories but keep in mind that they will
be removed by :meth:`SessionStorage.remove()`
This class indirectly collaborates with :class:`SessionSuspendHelper` and
:class:`SessionResumeHelper`.
"""
_SESSION_FILE = 'session'
_SESSION_FILE_NEXT = 'session.next'
def __init__(self, location):
"""
Initialize a :class:`SessionStorage` with the given location.
The location is not created. If you want to ensure that it exists
call :meth:`create()` instead.
"""
self._location = location
def __repr__(self):
return "<{} location:{!r}>".format(
self.__class__.__name__, self.location)
@property
[docs] def location(self):
"""
location of the session storage
"""
return self._location
@property
[docs] def session_file(self):
"""
pathname of the session state file
"""
return os.path.join(self._location, self._SESSION_FILE)
@classmethod
[docs] def create(cls, base_dir, legacy_mode=True):
"""
Create a new :class:`SessionStorage` in a random subdirectory
of the specified base directory. The base directory is also
created if necessary.
:param base_dir:
Directory in which a random session directory will be created.
Typically the base directory should be obtained from
:meth:`SessionStorageRepository.get_default_location()`
:param legacy_mode:
If False (defaults to True) then the caller is expected to
handle multiple sessions by itself.
.. note::
Legacy mode is where applications using PlainBox API can only
handle one session. Creating another session replaces whatever was
stored before. In non-legacy mode applications can enumerate
sessions, create arbitrary number of sessions at the same time
and remove sessions once they are no longer necessary.
Legacy mode is implemented with a symbolic link called
'last-session' that keeps track of the last session created using
``legacy_mode=True``. When a new legacy-mode session is created
the target of that symlink is read and recursively removed.
"""
if not os.path.exists(base_dir):
os.makedirs(base_dir)
location = tempfile.mkdtemp(
prefix='pbox-', suffix='.session', dir=base_dir)
logger.debug("Created new storage in %r", location)
self = cls(location)
if legacy_mode:
self._replace_legacy_session(base_dir)
return self
def _replace_legacy_session(self, base_dir):
"""
Remove the previous legacy session and update the 'last-session'
symlink so that it points to this session storage directory.
"""
symlink_pathname = os.path.join(
base_dir, SessionStorageRepository._LAST_SESSION_SYMLINK)
# Try to read and remove the storage referenced to by last-session
# symlink. This can fail if the link file is gone (which is harmless)
# or when it is not an actual symlink (which means that the
# repository is corrupted).
try:
symlink_target = os.readlink(symlink_pathname)
except OSError as exc:
if exc.errno == errno.ENOENT:
pass
elif exc.errno == errno.EINVAL:
logger.warning(
"%r is not a symlink, repository %r must be corrupted",
symlink_pathname, base_dir)
else:
logger.warning(
"Unable to read symlink target from %r: %r",
symlink_pathname, exc)
else:
logger.debug(
"Removing storage associated with last session %r",
symlink_target)
# Remove the old session, note that the symlink may be broken so
# let's ignore any errors here
shutil.rmtree(symlink_target, ignore_errors=True)
# Remove the last-session symlink itself
logger.debug(
"Removing symlink associated with last session: %r",
symlink_pathname)
os.unlink(symlink_pathname)
finally:
# Finally put the last-session synlink that points to this storage
logger.debug("Linking storage %r to last session", self.location)
try:
os.symlink(self.location, symlink_pathname)
except OSError as exc:
logger.error(
"Cannot link %r as %r: %r",
self.location, symlink_pathname, exc)
[docs] def remove(self):
"""
Remove all filesystem entries associated with this instance.
"""
logger.debug("Removing session storage from %r", self._location)
shutil.rmtree(self._location)
[docs] def load_checkpoint(self):
"""
Load checkpoint data from the filesystem
:returns: data from the most recent checkpoint
:rtype: bytes
:raises IOError, OSError:
on various problems related to accessing the filesystem
:raises NotImplementedError:
when openat(2) is not available
"""
if sys.version_info[0:2] >= (3, 3):
return self._load_checkpoint_unix_py33()
else:
return self._load_checkpoint_unix_py32()
[docs] def save_checkpoint(self, data):
"""
Save checkpoint data to the filesystem.
The directory associated with this :class:`SessionStorage` must already
exist. Typically the instance should be obtained by calling
:meth:`SessionStorage.create()` which will ensure that this is already
the case.
:raises TypeError:
if data is not a bytes object.
:raises LockedStorageError:
if leftovers from previous save_checkpoint() have been detected.
Normally those should never be here but in certain cases that is
possible. Callers might want to call :meth:`break_lock()`
to resolve the problem and try again.
:raises IOError, OSError:
on various problems related to accessing the filesystem.
Typically permission errors may be reported here.
:raises NotImplementedError:
when openat(2), renameat(2), unlinkat(2) are not available on this
platform. Should never happen on Linux.
"""
if sys.version_info[0:2] >= (3, 3):
return self._save_checkpoint_unix_py33(data)
else:
return self._save_checkpoint_unix_py32(data)
[docs] def break_lock(self):
"""
Forcibly unlock the storage by removing a file created during
atomic filesystem operations of save_checkpoint().
This method might be useful if save_checkpoint()
raises LockedStorageError. It removes the "next" file that is used
for atomic rename.
"""
_next_session_pathname = os.path.join(
self._location, self._SESSION_FILE_NEXT)
logger.debug(
"Forcibly unlinking 'next' file %r:", _next_session_pathname)
os.unlink(_next_session_pathname)
def _load_checkpoint_unix_py32(self):
_session_pathname = os.path.join(self._location, self._SESSION_FILE)
# Open the location directory
location_fd = os.open(self._location, os.O_DIRECTORY)
logger.debug(
"Opened session directory %r as descriptor %d",
self._location, location_fd)
try:
# Open the current session file in the location directory
session_fd = os.open(_session_pathname, os.O_RDONLY)
logger.debug(
"Opened session state file %r as descriptor %d",
_session_pathname, session_fd)
# Stat the file to know how much to read
session_stat = os.fstat(session_fd)
logger.debug(
"Stat'ed session state file: %s", session_stat)
try:
# Read session data
logger.debug(
"Reading %d bytes of session state", session_stat.st_size)
data = os.read(session_fd, session_stat.st_size)
logger.debug("Read %d bytes of session state", len(data))
if len(data) != session_stat.st_size:
raise IOError("partial read?")
finally:
# Close the session file
logger.debug("Closed descriptor %d", session_fd)
os.close(session_fd)
finally:
# Close the location directory
logger.debug("Closed descriptor %d", location_fd)
os.close(location_fd)
return data
def _load_checkpoint_unix_py33(self):
# Open the location directory
location_fd = os.open(self._location, os.O_DIRECTORY)
try:
# Open the current session file in the location directory
session_fd = os.open(
self._SESSION_FILE, os.O_RDONLY, dir_fd=location_fd)
# Stat the file to know how much to read
session_stat = os.fstat(session_fd)
try:
# Read session data
data = os.read(session_fd, session_stat.st_size)
if len(data) != session_stat.st_size:
raise IOError("partial read?")
finally:
# Close the session file
os.close(session_fd)
finally:
# Close the location directory
os.close(location_fd)
return data
def _save_checkpoint_unix_py32(self, data):
# NOTE: this is like _save_checkpoint_py33 but without all the
# *at() functions (openat, renameat)
#
# Since we cannot those functions there is an implicit race condition
# on all open() calls with another process that renames any of
# the directories that are part of the opened path.
#
# I don't think we can really do anything about this in userspace
# so this, python 3.2 specific version, just does the best effort
# implementation. Some of the comments were redacted but
# but keep in mind that the rename race is always there.
if not isinstance(data, bytes):
raise TypeError("data must be bytes")
logger.debug(
"Saving %d bytes of data (UNIX, python 3.2 or older)", len(data))
# Helper pathnames, needed because we don't have *at functions
_next_session_pathname = os.path.join(
self._location, self._SESSION_FILE_NEXT)
_session_pathname = os.path.join(self._location, self._SESSION_FILE)
# Open the location directory, we need to fsync that later
# XXX: this may fail, maybe we should keep the fd open all the time?
location_fd = os.open(self._location, os.O_DIRECTORY)
logger.debug("Opened %r as descriptor %d", self._location, location_fd)
try:
# Open the "next" file in the location_directory
#
# Use "write" + "create" + "exclusive" flags so that no race
# condition is possible.
#
# This will never return -1, it throws IOError when anything is
# wrong. The caller has to catch this.
#
# As a special exception, this code handles EEXISTS and converts
# that to LockedStorageError that can be especially handled by
# some layer above.
try:
next_session_fd = os.open(
_next_session_pathname,
os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o644)
except IOError as exc:
if exc.errno == errno.EEXISTS:
raise LockedStorageError()
else:
raise
logger.debug(
"Opened next session file %s as descriptor %d",
_next_session_pathname, next_session_fd)
try:
# Write session data to disk
#
# I cannot find conclusive evidence but it seems that
# os.write() handles partial writes internally. In case we do
# get a partial write _or_ we run out of disk space, raise an
# explicit IOError.
num_written = os.write(next_session_fd, data)
logger.debug(
"Wrote %d bytes of data to descriptor %d",
num_written, next_session_fd)
if num_written != len(data):
raise IOError("partial write?")
except:
# If anything goes wrong we should unlink the next file.
logger.warning("Unlinking %r", _next_session_pathname)
os.unlink(_next_session_pathname)
else:
# If the write was successful we must flush kernel buffers.
#
# We want to be sure this data is really on disk by now as we
# may crash the machine soon after this method exits.
logger.debug(
"Calling fsync() on descriptor %d", next_session_fd)
os.fsync(next_session_fd)
finally:
# Close the new session file
logger.debug("Closing descriptor %d", next_session_fd)
os.close(next_session_fd)
# Rename FILE_NEXT over FILE.
logger.debug(
"Renaming %r to %r", _next_session_pathname, _session_pathname)
try:
os.rename(_next_session_pathname, _session_pathname)
except:
# Same as above, if we fail we need to unlink the next file
# otherwise any other attempts will not be able to open() it
# with O_EXCL flag.
logger.warning("Unlinking %r", _next_session_pathname)
os.unlink(_next_session_pathname)
# Flush kernel buffers on the directory.
#
# This should ensure the rename operation is really on disk by now.
# As noted above, this is essential for being able to survive
# system crash immediately after exiting this method.
logger.debug("Calling fsync() on descriptor %d", location_fd)
os.fsync(location_fd)
finally:
# Close the location directory
logger.debug("Closing descriptor %d", location_fd)
os.close(location_fd)
def _save_checkpoint_unix_py33(self, data):
if not isinstance(data, bytes):
raise TypeError("data must be bytes")
logger.debug(
"Saving %d bytes of data (UNIX, python 3.2 or older)", len(data))
# Open the location directory, we need to fsync that later
# XXX: this may fail, maybe we should keep the fd open all the time?
location_fd = os.open(self._location, os.O_DIRECTORY)
logger.debug("Opened %r as descriptor %d", self._location, location_fd)
try:
# Open the "next" file in the location_directory
#
# Use openat(2) to ensure we always open a file relative to the
# directory we already opened above. This is essential for fsync(2)
# calls made below.
#
# Use "write" + "create" + "exclusive" flags so that no race
# condition is possible.
#
# This will never return -1, it throws IOError when anything is
# wrong. The caller has to catch this.
#
# As a special exception, this code handles EEXISTS
# (FIleExistsError) and converts that to LockedStorageError
# that can be especially handled by some layer above.
try:
next_session_fd = os.open(
self._SESSION_FILE_NEXT,
os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o644,
dir_fd=location_fd)
except FileExistsError:
raise LockedStorageError()
logger.debug(
"Opened next session file %s as descriptor %d",
self._SESSION_FILE_NEXT, next_session_fd)
try:
# Write session data to disk
#
# I cannot find conclusive evidence but it seems that
# os.write() handles partial writes internally. In case we do
# get a partial write _or_ we run out of disk space, raise an
# explicit IOError.
num_written = os.write(next_session_fd, data)
logger.debug(
"Wrote %d bytes of data to descriptor %d",
num_written, next_session_fd)
if num_written != len(data):
raise IOError("partial write?")
except:
# If anything goes wrong we should unlink the next file. As
# with the open() call above we use unlinkat to prevent race
# conditions.
logger.warning("Unlinking %r", self._SESSION_FILE_NEXT)
os.unlink(self._SESSION_FILE_NEXT, dir_fd=location_fd)
else:
# If the write was successful we must flush kernel buffers.
#
# We want to be sure this data is really on disk by now as we
# may crash the machine soon after this method exits.
logger.debug(
"Calling fsync() on descriptor %d", next_session_fd)
os.fsync(next_session_fd)
finally:
# Close the new session file
logger.debug("Closing descriptor %d", next_session_fd)
os.close(next_session_fd)
# Rename FILE_NEXT over FILE.
#
# Use renameat(2) to ensure that there is no race condition if the
# location (directory) is being moved
logger.debug(
"Renaming %r to %r",
self._SESSION_FILE_NEXT, self._SESSION_FILE)
try:
os.rename(self._SESSION_FILE_NEXT, self._SESSION_FILE,
src_dir_fd=location_fd, dst_dir_fd=location_fd)
except:
# Same as above, if we fail we need to unlink the next file
# otherwise any other attempts will not be able to open() it
# with O_EXCL flag.
logger.warning("Unlinking %r", self._SESSION_FILE_NEXT)
os.unlink(self._SESSION_FILE_NEXT, dir_fd=location_fd)
# Flush kernel buffers on the directory.
#
# This should ensure the rename operation is really on disk by now.
# As noted above, this is essential for being able to survive
# system crash immediately after exiting this method.
logger.debug("Calling fsync() on descriptor %d", location_fd)
os.fsync(location_fd)
finally:
# Close the location directory
logger.debug("Closing descriptor %d", location_fd)
os.close(location_fd)