path: root/obnamlib/vfs.py

# Copyright (C) 2008-2015  Lars Wirzenius <liw@liw.fi>
#
# 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.,
# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.


import logging
import os
import stat
import unicodedata
import urlparse

import obnamlib


# Modes (permissions) for new directories and files. We use minimal
# permissions to avoid allowing access to files being restored until
# their original permissions have been set.
#
# Directories are RWX for owner, nothing for anyone else. We need
# the R to be able to do listdir, W to create and remove files, and
# X to access anything in the directory. We _could_ do without R,
# but then everything that needs to later read needs to add the R
# manually, and that's most things, so it's much easier to just add
# R to everything. We can't do without W and X.
#
# Files are RW for owner, nothing for anyone else. Again, we could
# do without R in some cases, but they're few enough that it's easier
# to give R to everything. In any case, if an attacker has gained
# access to our UID, they can already chmod to add R and then they
# can read anyway.

NEW_DIR_MODE = 0700
NEW_FILE_MODE = 0600


class URLSchemeAlreadyRegisteredError(obnamlib.ObnamError):

    msg = 'VFS URL scheme {scheme} already registered'


class UnknownVFSError(obnamlib.ObnamError):

    msg = 'Unknown VFS type: {url}'


class LockFail(obnamlib.ObnamError):

    msg = "Couldn't create lock {lock_name}: {reason}"


class VirtualFileSystem(object):

    '''A virtual filesystem interface.

    The backup program needs to access both local and remote files.
    To make it easier to support all kinds of files both locally and
    remotely, we use a custom virtual filesystem interface so that
    all filesystem access is done the same way. This way, we can
    easily support user data and backup repositories in any combination of
    local and remote filesystems.

    This class defines the interface for such virtual filesystems.
    Sub-classes will actually implement the interface.

    When a VFS is instantiated, it is bound to a base URL. When
    accessing the virtual filesystem, all paths are then given
    relative to the base URL. The Unix syntax for files is used
    for the relative paths: directory components separated by
    slashes, and an initial slash indicating the root of the
    filesystem (in this case, the base URL).

    '''

    def __init__(self, baseurl):
        self.baseurl = baseurl
        self.bytes_read = 0
        self.bytes_written = 0
        logging.debug('VFS: __init__: baseurl=%s', self.baseurl)

    def log_stats(self):
        logging.debug(
            'VFS: baseurl=%s read=%d written=%d',
            self.baseurl, self.bytes_read, self.bytes_written)

    def connect(self):
        '''Connect to filesystem.'''

    def close(self):
        '''Close connection to filesystem.'''
        self.log_stats()

    def reinit(self, new_baseurl, create=False):
        '''Go back to the beginning.

        This behaves like instantiating a new instance, but possibly
        faster for things like SftpFS. If there is a network
        connection already open, it will be reused.

        '''

    def abspath(self, pathname):
        '''Return absolute version of pathname.'''
        return os.path.abspath(os.path.join(self.getcwd(), pathname))

    def getcwd(self):
        '''Return current working directory as absolute pathname.'''

    def chdir(self, pathname):
        '''Change current working directory to pathname.'''

    def listdir(self, pathname):
        '''Return list of basenames of entities at pathname.'''

    def listdir2(self, pathname):
        '''Return list of basenames and stats of entities at pathname.

        The stat entity may be an exception object instead, to indicate
        an error.

        '''

    def lock(self, lockname):
        '''Create a lock file with the given name.'''

    def unlock(self, lockname):
        '''Remove a lock file.'''

    def exists(self, pathname):
        '''Does the file or directory exist?'''

    def mknod(self, pathname, mode):
        '''Create a filesystem node.'''

    def isdir(self, pathname):
        '''Is it a directory?'''

    def mkdir(self, pathname, mode=NEW_DIR_MODE):
        '''Create a directory.

        Parent directories must already exist.

        '''

    def makedirs(self, pathname):
        '''Create a directory, and missing parents.'''

    def rmdir(self, pathname):
        '''Remove an empty directory.'''

    def rmtree(self, dirname):
        '''Remove a directory tree, including its contents.'''
        if self.isdir(dirname):
            for pathname, st in self.scan_tree(dirname):
                if stat.S_ISDIR(st.st_mode):
                    self.rmdir(pathname)
                else:
                    self.remove(pathname)

    def remove(self, pathname):
        '''Remove a file.'''

    def rename(self, old, new):
        '''Rename a file.'''

    def lstat(self, pathname):
        '''Like os.lstat.'''

    def get_username(self, uid):
        '''Return name for user, or None if not known.'''

    def get_groupname(self, gid):
        '''Return name for group, or None if not known.'''

    def llistxattr(self, pathname):
        '''Return list of names of extended attributes for file.'''
        return []

    def lgetxattr(self, pathname, attrname):
        '''Return value of an extended attribute.'''

    def lsetxattr(self, pathname, attrname, attrvalue):
        '''Set value of an extended attribute.'''

    def lchown(self, pathname, uid, gid):
        '''Like os.lchown.'''

    def chmod_symlink(self, pathname, mode):
        '''Like os.lchmod, for symlinks only.

        This may fail if the pathname is not a symlink (but it may
        not).  If the target is a symlink, but the platform (e.g.,
        Linux) does not allow setting the permissions of a symlink,
        the method will silently do nothing.

        '''

    def chmod_not_symlink(self, pathname, mode):
        '''Like os.chmod, for non-symlinks only.

        This may fail if pathname is a symlink (but it may not). It
        MUST NOT be called for a symlink; use chmod_symlink instead.

        '''

    def lutimes(self, pathname, atime_sec, atime_nsec, mtime_sec, mtime_nsec):
        '''Like lutimes(2).

        This isn't quite like lutimes, actually. Most importantly, it uses
        nanosecond timestamps rather than microsecond. This is important.

        '''

    def link(self, existing_path, new_path):
        '''Like os.link.'''

    def readlink(self, symlink):
        '''Like os.readlink.'''

    def symlink(self, source, destination):
        '''Like os.symlink.'''

    def open(self, pathname, mode, bufsize=None):
        '''Open a file, like the builtin open() or file() function.

        The return value is a file object like the ones returned
        by the builtin open() function.

        '''

    def cat(self, pathname):
        '''Return the contents of a file.'''

    def write_file(self, pathname, contents):
        '''Write a new file.

        The file must not yet exist. The file is not necessarily written
        atomically, meaning that if the writing fails (connection to
        server drops, for example), the file might exist in a partial
        form. The callers need to deal with this.

        Any directories in pathname will be created if necessary.

        '''

    def overwrite_file(self, pathname, contents):
        '''Like write_file, but overwrites existing file.'''

    def scan_tree(self, dirname, ok=None, dirst=None, log=logging.error,
                  error_handler=None):
        '''Scan a tree for files.

        Return a generator that returns ``(pathname, stat_result)``
        pairs for each file and directory in the tree, in
        depth-first order.

        If ``ok`` is not None, it must be a function that determines
        if a particular file or directory should be returned.
        It gets the pathname and stat result as arguments, and
        should return True or False. If it returns False on a
        directory, ``scan_tree`` will not recurse into the
        directory.

        ``dirst`` is for internal optimization, and should not
        be used by the caller. ``log`` is used by unit tests and
        should not be used by the caller.

        Errors from calling ``listdir`` or ``lstat`` are logged,
        but do not stop the scanning. Such files or directories are
        not returned, however. If `error_handler` is defined, it is
        called once for every problem, giving the name and exception
        as arguments.

        '''

        def important_first(items, is_important):
            important = []
            unimportant = []
            for item in items:
                if is_important(item):
                    important.append(item)
                else:
                    unimportant.append(item)
            return important + unimportant

        def is_directory(pair):
            _, metadata = pair
            return (not isinstance(metadata, BaseException) and
                    stat.S_ISDIR(metadata.st_mode))

        def list_files(pathname):
            try:
                pairs = self.listdir2(pathname)
            except OSError, e:
                log('listdir failed: %s: %s' % (e.filename, e.strerror))
                error_handler(pathname, e)
                return []
            else:
                pairs = [(os.path.join(pathname, basename), st)
                         for basename, st in pairs]
                return important_first(pairs, is_directory)

        def lstat(pathname):
            try:
                return self.lstat(pathname)
            except OSError, e:
                log('lstat for dir failed: %s: %s' % (e.filename, e.strerror))
                return e

        def process_dir(dirname, metadata, items):
            new_items = [
                (subname, submeta, False)
                for subname, submeta in list_files(dirname)]
            return new_items + [(dirname, metadata, True)] + items

        error_handler = error_handler or (lambda name, e: None)
        ok = ok or (lambda name, st: True)

        items = [(dirname, lstat(dirname), False)]
        while items:
            filename, metadata, processed_dir = items.pop(0)
            if isinstance(metadata, BaseException):
                error_handler(filename, metadata)
            elif stat.S_ISDIR(metadata.st_mode) and not processed_dir:
                if ok(filename, metadata):
                    items = process_dir(filename, metadata, items)
            elif ok(filename, metadata):
                yield filename, metadata


class VfsFactory(object):

    '''Create new instances of VirtualFileSystem.'''

    def __init__(self):
        self.implementations = {}

    def register(self, scheme, implementation, **kwargs):
        if scheme in self.implementations:
            raise URLSchemeAlreadyRegisteredError(scheme=scheme)
        self.implementations[scheme] = (implementation, kwargs)

    def new(self, url, create=False):
        '''Create a new VFS appropriate for a given URL.'''
        scheme, _, _, _, _, _ = urlparse.urlparse(url)
        if scheme in self.implementations:
            klass, kwargs = self.implementations[scheme]
            return klass(url, create=create, **kwargs)
        raise UnknownVFSError(url=url)


class VfsTests(object):  # pragma: no cover

    '''Re-useable tests for VirtualFileSystem implementations.

    The base class can't be usefully instantiated itself.
    Instead you are supposed to sub-class it and implement the API in
    a suitable way for yourself.

    This class implements a number of tests that the API implementation
    must pass. The implementation's own test class should inherit from
    this class, and unittest.TestCase.

    The test sub-class should define a setUp method that sets the following:

    * self.fs to an instance of the API implementation sub-class
    * self.basepath to the path to the base of the filesystem

    basepath must be operable as a pathname using os.path tools. If
    the VFS implementation operates remotely and wants to operate on a
    URL like 'http://domain/path' as the baseurl, then basepath must be
    just the path portion of the URL.

    The directory indicated by basepath must exist, but must be empty
    at start.

    '''

    non_ascii_name = u'm\u00e4kel\u00e4'.encode('utf-8')

    fs = None
    basepath = None

    # Add some dummy methods to silence pylint.

    def assertTrue(self, *args, **kwargs):
        raise NotImplementedError()

    def assertFalse(self, *args, **kwargs):
        raise NotImplementedError()

    def assertEqual(self, *args, **kwargs):
        raise NotImplementedError()

    def assertRaises(self, *args, **kwargs):
        raise NotImplementedError()

    # Actual tests.

    def test_abspath_returns_input_for_absolute_path(self):
        self.assertEqual(self.fs.abspath('/foo/bar'), '/foo/bar')

    def test_abspath_returns_absolute_path_for_relative_input(self):
        self.assertEqual(self.fs.abspath('foo'),
                         os.path.join(self.basepath, 'foo'))

    def test_abspath_normalizes_path(self):
        self.assertEqual(self.fs.abspath('foo/..'), self.basepath)

    def test_abspath_returns_plain_string(self):
        self.fs.mkdir(self.non_ascii_name)
        self.fs.chdir(self.non_ascii_name)
        self.assertEqual(type(self.fs.abspath('.')), str)

    def test_reinit_works(self):
        self.fs.chdir('/')
        self.fs.reinit(self.fs.baseurl)
        self.assertEqual(self.fs.getcwd(), self.basepath)

    def test_reinit_to_nonexistent_filename_raises_OSError(self):
        notexist = os.path.join(self.fs.baseurl, 'thisdoesnotexist')
        self.assertRaises(OSError, self.fs.reinit, notexist)

    def test_reinit_creates_target_if_requested(self):
        self.fs.chdir('/')
        new_baseurl = os.path.join(self.fs.baseurl, 'newdir')
        new_basepath = os.path.join(self.basepath, 'newdir')
        self.fs.reinit(new_baseurl, create=True)
        self.assertEqual(self.fs.getcwd(), new_basepath)

    def test_getcwd_returns_dirname(self):
        self.assertEqual(self.fs.getcwd(), self.basepath)

    def test_getcwd_returns_plain_string(self):
        self.fs.mkdir(self.non_ascii_name)
        self.fs.chdir(self.non_ascii_name)
        self.assertEqual(type(self.fs.getcwd()), str)

    def test_chdir_changes_only_fs_cwd_not_process_cwd(self):
        process_cwd = os.getcwd()
        self.fs.chdir('/')
        self.assertEqual(self.fs.getcwd(), '/')
        self.assertEqual(os.getcwd(), process_cwd)

    def test_chdir_to_nonexistent_raises_exception(self):
        self.assertRaises(OSError, self.fs.chdir, '/foobar')

    def test_chdir_to_relative_works(self):
        pathname = os.path.join(self.basepath, 'foo')
        os.mkdir(pathname)
        self.fs.chdir('foo')
        self.assertEqual(self.fs.getcwd(), pathname)

    def test_chdir_to_dotdot_works(self):
        pathname = os.path.join(self.basepath, 'foo')
        os.mkdir(pathname)
        self.fs.chdir('foo')
        self.fs.chdir('..')
        self.assertEqual(self.fs.getcwd(), self.basepath)

    def test_creates_lock_file(self):
        self.fs.lock('lock')
        self.assertTrue(self.fs.exists('lock'))

    def test_second_lock_fails(self):
        self.fs.lock('lock')
        self.assertRaises(Exception, self.fs.lock, 'lock')

    def test_unlock_removes_lock(self):
        self.fs.lock('lock')
        self.fs.unlock('lock')
        self.assertFalse(self.fs.exists('lock'))

    def test_exists_returns_false_for_nonexistent_file(self):
        self.assertFalse(self.fs.exists('foo'))

    def test_exists_returns_true_for_existing_file(self):
        self.fs.write_file('foo', '')
        self.assertTrue(self.fs.exists('foo'))

    def test_isdir_returns_false_for_nonexistent_file(self):
        self.assertFalse(self.fs.isdir('foo'))

    def test_isdir_returns_false_for_nondir(self):
        self.fs.write_file('foo', '')
        self.assertFalse(self.fs.isdir('foo'))

    def test_isdir_returns_true_for_existing_dir(self):
        self.fs.mkdir('foo')
        self.assertTrue(self.fs.isdir('foo'))

    def test_listdir_returns_plain_strings_only(self):
        self.fs.write_file(u'M\u00E4kel\u00E4'.encode('utf-8'), 'data')
        names = self.fs.listdir('.')
        types = [type(x) for x in names]
        self.assertEqual(types, [str])

    def test_listdir_raises_oserror_if_directory_does_not_exist(self):
        self.assertRaises(OSError, self.fs.listdir, 'foo')

    def test_listdir2_returns_name_stat_pairs(self):
        funny_unicode = u'M\u00E4kel\u00E4'
        funny_utf8 = funny_unicode.encode('utf-8')

        self.fs.write_file(funny_utf8, 'data')
        pairs = self.fs.listdir2('.')
        self.assertEqual(len(pairs), 1)
        self.assertEqual(len(pairs[0]), 2)
        name_utf8, st = pairs[0]

        self.assertEqual(type(name_utf8), str)
        name_unicode = name_utf8.decode('utf-8')

        # See https://en.wikipedia.org/wiki/Unicode_equivalence for
        # background. The NFKD normalisation seems to be the best way
        # to ensure things work across Linux and Mac OS X both (their
        # default normalisation for filenames is different).
        self.assertEqual(
            unicodedata.normalize('NFKD', name_unicode),
            unicodedata.normalize('NFKD', funny_unicode))

        self.assertTrue(hasattr(st, 'st_mode'))
        self.assertFalse(hasattr(st, 'st_mtime'))
        self.assertTrue(hasattr(st, 'st_mtime_sec'))
        self.assertTrue(hasattr(st, 'st_mtime_nsec'))

    def test_listdir2_returns_plain_strings_only(self):
        self.fs.write_file(u'M\u00E4kel\u00E4'.encode('utf-8'), 'data')
        names = [name for name, _ in self.fs.listdir2('.')]
        types = [type(x) for x in names]
        self.assertEqual(types, [str])

    def test_listdir2_raises_oserror_if_directory_does_not_exist(self):
        self.assertRaises(OSError, self.fs.listdir2, 'foo')

    def test_mknod_creates_fifo(self):
        self.fs.mknod('foo', 0600 | stat.S_IFIFO)
        self.assertEqual(self.fs.lstat('foo').st_mode, 0600 | stat.S_IFIFO)

    def test_mkdir_raises_oserror_if_directory_exists(self):
        self.assertRaises(OSError, self.fs.mkdir, '.')

    def test_mkdir_raises_oserror_if_parent_does_not_exist(self):
        self.assertRaises(OSError, self.fs.mkdir, 'foo/bar')

    def test_makedirs_raises_oserror_when_directory_exists(self):
        self.fs.mkdir('foo')
        self.assertRaises(OSError, self.fs.makedirs, 'foo')

    def test_makedirs_creates_directory_when_parent_exists(self):
        self.fs.makedirs('foo')
        self.assertTrue(self.fs.isdir('foo'))

    def test_makedirs_creates_directory_when_parent_does_not_exist(self):
        self.fs.makedirs('foo/bar')
        self.assertTrue(self.fs.isdir('foo/bar'))

    def test_rmdir_removes_directory(self):
        self.fs.mkdir('foo')
        self.fs.rmdir('foo')
        self.assertFalse(self.fs.exists('foo'))

    def test_rmdir_raises_oserror_if_directory_does_not_exist(self):
        self.assertRaises(OSError, self.fs.rmdir, 'foo')

    def test_rmdir_raises_oserror_if_directory_is_not_empty(self):
        self.fs.mkdir('foo')
        self.fs.write_file('foo/bar', '')
        self.assertRaises(OSError, self.fs.rmdir, 'foo')

    def test_rmtree_removes_directory_tree(self):
        self.fs.mkdir('foo')
        self.fs.write_file('foo/bar', '')
        self.fs.rmtree('foo')
        self.assertFalse(self.fs.exists('foo'))

    def test_rmtree_is_silent_when_target_does_not_exist(self):
        self.assertEqual(self.fs.rmtree('foo'), None)

    def test_remove_removes_file(self):
        self.fs.write_file('foo', '')
        self.fs.remove('foo')
        self.assertFalse(self.fs.exists('foo'))

    def test_remove_raises_oserror_if_file_does_not_exist(self):
        self.assertRaises(OSError, self.fs.remove, 'foo')

    def test_rename_renames_file(self):
        self.fs.write_file('foo', 'xxx')
        self.fs.rename('foo', 'bar')
        self.assertFalse(self.fs.exists('foo'))
        self.assertEqual(self.fs.cat('bar'), 'xxx')

    def test_rename_raises_oserror_if_file_does_not_exist(self):
        self.assertRaises(OSError, self.fs.rename, 'foo', 'bar')

    def test_rename_works_if_target_exists(self):
        self.fs.write_file('foo', 'foo')
        self.fs.write_file('bar', 'bar')
        self.fs.rename('foo', 'bar')
        self.assertEqual(self.fs.cat('bar'), 'foo')

    def test_lstat_returns_result_with_all_required_fields(self):
        st = self.fs.lstat('.')
        for field in obnamlib.metadata_fields:
            if field.startswith('st_'):
                self.assertTrue(
                    hasattr(st, field), 'stat must return %s' % field)

    def test_lstat_returns_right_filetype_for_directory(self):
        st = self.fs.lstat('.')
        self.assertTrue(stat.S_ISDIR(st.st_mode))

    def test_lstat_raises_oserror_for_nonexistent_entry(self):
        self.assertRaises(OSError, self.fs.lstat, 'notexists')

    def test_chmod_not_symlink_sets_permissions_correctly(self):
        self.fs.mkdir('foo')
        self.fs.chmod_not_symlink('foo', 0777)
        self.assertEqual(self.fs.lstat('foo').st_mode & 0777, 0777)

    def test_chmod_not_symlink_raises_oserror_for_nonexistent_entry(self):
        self.assertRaises(OSError, self.fs.chmod_not_symlink, 'notexists', 0)

    def test_chmod_symlink_raises_oserror_for_nonexistent_entry(self):
        self.assertRaises(OSError, self.fs.chmod_symlink, 'notexists', 0)

    def test_lutimes_sets_times_correctly(self):
        self.fs.mkdir('foo')

        self.fs.lutimes('foo', 1, 2*1000, 3, 4*1000)

        self.assertEqual(self.fs.lstat('foo').st_atime_sec, 1)
        # not all filesystems support sub-second timestamps; those that
        # do not, return 0, so we have to accept either that or the correct
        # value, but no other values
        self.assertTrue(self.fs.lstat('foo').st_atime_nsec in [0, 2*1000])

        self.assertEqual(self.fs.lstat('foo').st_mtime_sec, 3)
        self.assertTrue(self.fs.lstat('foo').st_mtime_nsec in [0, 4*1000])

    def test_lutimes_raises_oserror_for_nonexistent_entry(self):
        self.assertRaises(OSError, self.fs.lutimes, 'notexists', 1, 2, 3, 4)

    def test_link_creates_hard_link(self):
        self.fs.write_file('foo', 'foo')
        self.fs.link('foo', 'bar')
        st1 = self.fs.lstat('foo')
        st2 = self.fs.lstat('bar')
        self.assertEqual(st1, st2)

    def test_symlink_creates_soft_link(self):
        self.fs.symlink('foo', 'bar')
        target = self.fs.readlink('bar')
        self.assertEqual(target, 'foo')

    def test_readlink_returns_plain_string(self):
        self.fs.symlink(self.non_ascii_name, self.non_ascii_name)
        target = self.fs.readlink(self.non_ascii_name)
        self.assertEqual(target, self.non_ascii_name)
        self.assertEqual(type(target), str)

    def test_symlink_raises_oserror_if_name_exists(self):
        self.fs.write_file('foo', 'foo')
        self.assertRaises(OSError, self.fs.symlink, 'bar', 'foo')

    def test_opens_existing_file_ok_for_reading(self):
        self.fs.write_file('foo', '')
        self.assertTrue(self.fs.open('foo', 'r'))

    def test_opens_existing_file_ok_for_writing(self):
        self.fs.write_file('foo', '')
        self.assertTrue(self.fs.open('foo', 'w'))

    def test_open_fails_for_nonexistent_file(self):
        self.assertRaises(IOError, self.fs.open, 'foo', 'r')

    def test_cat_reads_existing_file_ok(self):
        self.fs.write_file('foo', 'bar')
        self.assertEqual(self.fs.cat('foo'), 'bar')

    def test_cat_fails_for_nonexistent_file(self):
        self.assertRaises(IOError, self.fs.cat, 'foo')

    def test_has_read_nothing_initially(self):
        self.assertEqual(self.fs.bytes_read, 0)

    def test_cat_updates_bytes_read(self):
        self.fs.write_file('foo', 'bar')
        self.fs.cat('foo')
        self.assertEqual(self.fs.bytes_read, 3)

    def test_write_fails_if_file_exists_already(self):
        self.fs.write_file('foo', 'bar')
        self.assertRaises(OSError, self.fs.write_file, 'foo', 'foobar')

    def test_write_creates_missing_directories(self):
        self.fs.write_file('foo/bar', 'yo')
        self.assertEqual(self.fs.cat('foo/bar'), 'yo')

    def test_write_leaves_existing_file_intact(self):
        self.fs.write_file('foo', 'bar')
        try:
            self.fs.write_file('foo', 'foobar')
        except OSError:
            pass
        self.assertEqual(self.fs.cat('foo'), 'bar')

    def test_overwrite_creates_new_file_ok(self):
        self.fs.overwrite_file('foo', 'bar')
        self.assertEqual(self.fs.cat('foo'), 'bar')

    def test_overwrite_replaces_existing_file(self):
        self.fs.write_file('foo', 'bar')
        self.fs.overwrite_file('foo', 'foobar')
        self.assertEqual(self.fs.cat('foo'), 'foobar')

    def test_has_written_nothing_initially(self):
        self.assertEqual(self.fs.bytes_written, 0)

    def test_write_updates_written(self):
        self.fs.write_file('foo', 'foo')
        self.assertEqual(self.fs.bytes_written, 3)

    def test_overwrite_updates_written(self):
        self.fs.overwrite_file('foo', 'foo')
        self.assertEqual(self.fs.bytes_written, 3)

    def set_up_scan_tree(self):
        self.dirs = ['foo', 'foo/bar', 'foo/bar/subway', 'foobar']
        self.dirs = [os.path.join(self.basepath, x) for x in self.dirs]
        for dirname in self.dirs:
            self.fs.mkdir(dirname)
        self.dirs.insert(0, self.basepath)
        self.fs.symlink('foo', 'symfoo')
        self.pathnames = self.dirs + [os.path.join(self.basepath, 'symfoo')]

    def test_scan_tree_returns_nothing_if_listdir_fails(self):
        self.set_up_scan_tree()

        def raiser(dirname):
            raise OSError(123, 'oops', dirname)

        def logerror(msg):
            pass

        self.fs.listdir2 = raiser
        result = list(self.fs.scan_tree(self.basepath, log=logerror))
        self.assertEqual(len(result), 1)
        pathname, _ = result[0]
        self.assertEqual(pathname, self.basepath)

    def test_scan_tree_returns_the_right_stuff(self):
        self.set_up_scan_tree()
        result = list(self.fs.scan_tree(self.basepath))
        pathnames = [pathname for pathname, _ in result]
        self.assertEqual(sorted(pathnames), sorted(self.pathnames),
                         'pathnames: %r   --- self.pathnames: %r' %
                         (pathnames, self.pathnames))

    def test_scan_tree_filters_away_unwanted(self):
        def ok(pathname, st):
            return stat.S_ISDIR(st.st_mode)
        self.set_up_scan_tree()
        result = list(self.fs.scan_tree(self.basepath, ok=ok))
        pathnames = [pathname for pathname, _ in result]
        self.assertEqual(sorted(pathnames), sorted(self.dirs))

    def test_scan_tree_filters_away_unwanted_subdirs(self):
        def ok(pathname, st):
            return not pathname.endswith('bar')
        self.set_up_scan_tree()
        result = list(self.fs.scan_tree(self.basepath, ok=ok))
        pathnames = [pathname for pathname, _ in result]
        self.assertEqual(
            sorted(pathnames),
            sorted([self.basepath] +
                   [os.path.join(self.basepath, x)
                    for x in ['foo', 'symfoo']]))