return None

    @property

    def _empty(self):

        """

        Checks if repository is empty ie. without any changesets

        """

        try:

            self.revisions[0]

        except (KeyError, IndexError):

            return True

        return False

    @LazyProperty

    def revisions(self):

        """

        Returns list of revisions' ids, in ascending order.  Being lazy

        attribute allows external tools to inject shas from cache.

        """

        return self._get_all_revisions()

    @classmethod

    def _run_git_command(cls, cmd, cwd=None):

        """

        Runs given ``cmd`` as git command and returns output bytes in a tuple

        (stdout, stderr) ... or raise RepositoryError.

        :param cmd: git command to be executed

        :param cwd: passed directly to subprocess

        """

        # need to clean fix GIT_DIR !

        gitenv = dict(os.environ)

        gitenv.pop('GIT_DIR', None)

        gitenv['GIT_CONFIG_NOGLOBAL'] = '1'

        assert isinstance(cmd, list), cmd

        cmd = [settings.GIT_EXECUTABLE_PATH, '-c', 'core.quotepath=false'] + cmd

        try:

            p = subprocessio.SubprocessIOChunker(cmd, cwd=cwd, env=gitenv, shell=False)

        except (EnvironmentError, OSError) as err:

            # output from the failing process is in str(EnvironmentError)

            msg = ("Couldn't run git command %s.\n"

                   "Subprocess failed with '%s': %s\n" %

                   (cmd, type(err).__name__, err)

            ).strip()

            log.error(msg)

            raise RepositoryError(msg)

        try:

            stdout = b''.join(p.output)

            stderr = b''.join(p.error)

        finally:

            p.close()

        # TODO: introduce option to make commands fail if they have any stderr output?

        if stderr:

            log.debug('stderr from %s:\n%s', cmd, stderr)

        else:

            log.debug('stderr from %s: None', cmd)

        return stdout, stderr

    def run_git_command(self, cmd):

        """

        Runs given ``cmd`` as git command with cwd set to current repo.

        Returns stdout as unicode str ... or raise RepositoryError.

        """

        cwd = None

        if os.path.isdir(self.path):

            cwd = self.path

        stdout, _stderr = self._run_git_command(cmd, cwd=cwd)

        return safe_str(stdout)

    @classmethod

    def _check_url(cls, url):

        """

        Function will check given url and try to verify if it's a valid

        link. Sometimes it may happened that git will issue basic

        auth request that can cause whole API to hang when used from python

        or other external calls.

        On failures it'll raise urllib2.HTTPError, exception is also thrown

        when the return code is non 200

        """

        # check first if it's not an local url

        if os.path.isdir(url) or url.startswith('file:'):

            return True

        if url.startswith('git://'):

            return True

        if '+' in url[:url.find('://')]:

            url = url[url.find('+') + 1:]

        handlers = []

        url_obj = mercurial.util.url(safe_bytes(url))

        test_uri, authinfo = url_obj.authinfo()

        url_obj.passwd = b'*****'

        cleaned_uri = str(url_obj)

        if authinfo:

            # create a password manager

            passmgr = urllib.request.HTTPPasswordMgrWithDefaultRealm()

            passmgr.add_password(*authinfo)

            handlers.extend((mercurial.url.httpbasicauthhandler(passmgr),

                             mercurial.url.httpdigestauthhandler(passmgr)))

        o = urllib.request.build_opener(*handlers)

        o.addheaders = [('User-Agent', 'git/1.7.8.0')]  # fake some git

        req = urllib.request.Request(

            "%s?%s" % (

                test_uri,

                urllib.parse.urlencode({"service": 'git-upload-pack'})

            ))

        try:

            resp = o.open(req)

            if resp.code != 200:

                raise Exception('Return Code is not 200')

        except Exception as e:

            # means it cannot be cloned

            raise urllib.error.URLError("[%s] org_exc: %s" % (cleaned_uri, e))

        # now detect if it's proper git repo

        gitdata = resp.read()

            raise urllib.error.URLError(

                "url [%s] does not look like an git" % cleaned_uri)

        return True

    def _get_repo(self, create, src_url=None, update_after_clone=False,

                  bare=False):

        if create and os.path.exists(self.path):

            raise RepositoryError("Location already exist")

        if src_url and not create:

            raise RepositoryError("Create should be set to True if src_url is "

                                  "given (clone operation creates repository)")

        try:

            if create and src_url:

                GitRepository._check_url(src_url)

                self.clone(src_url, update_after_clone, bare)

                return Repo(self.path)

            elif create:

                os.makedirs(self.path)

                if bare:

                    return Repo.init_bare(self.path)

                else:

                    return Repo.init(self.path)

            else:

                return Repo(self.path)

        except (NotGitRepository, OSError) as err:

            raise RepositoryError(err)

    def _get_all_revisions(self):

        # we must check if this repo is not empty, since later command

        # fails if it is. And it's cheaper to ask than throw the subprocess

        # errors

        try:

            self._repo.head()

        except KeyError:

            return []

        rev_filter = settings.GIT_REV_FILTER

        cmd = ['rev-list', rev_filter, '--reverse', '--date-order']

        try:

            so = self.run_git_command(cmd)

        except RepositoryError:

            # Can be raised for empty repositories

            return []

        return so.splitlines()

    def _get_all_revisions2(self):

        # alternate implementation using dulwich

        includes = [ascii_str(sha) for key, (sha, type_) in self._parsed_refs.items()

                    if type_ != b'T']

        return [c.commit.id for c in self._repo.get_walker(include=includes)]

    def _get_revision(self, revision):

        """

        Given any revision identifier, returns a 40 char string with revision hash.

        """

        if self._empty:

            raise EmptyRepositoryError("There are no changesets yet")

        if revision in (None, '', 'tip', 'HEAD', 'head', -1):

            revision = -1

        if isinstance(revision, int):

            try:

                return self.revisions[revision]

            except IndexError:

                msg = "Revision %r does not exist for %s" % (revision, self.name)

                raise ChangesetDoesNotExistError(msg)

        if isinstance(revision, str):

            if revision.isdigit() and (len(revision) < 12 or len(revision) == revision.count('0')):

                try:

                    return self.revisions[int(revision)]

                except IndexError:

                    msg = "Revision %r does not exist for %s" % (revision, self)

                    raise ChangesetDoesNotExistError(msg)

            # get by branch/tag name

            _ref_revision = self._parsed_refs.get(safe_bytes(revision))

            if _ref_revision:  # and _ref_revision[1] in [b'H', b'RH', b'T']:

                return ascii_str(_ref_revision[0])

            if revision in self.revisions:

                return revision

            # maybe it's a tag ? we don't have them in self.revisions

            if revision in self.tags.values():

                return revision

            if SHA_PATTERN.match(revision):

                msg = "Revision %r does not exist for %s" % (revision, self.name)

                raise ChangesetDoesNotExistError(msg)

        raise ChangesetDoesNotExistError("Given revision %r not recognized" % revision)

    def get_ref_revision(self, ref_type, ref_name):

        :param user: full username, i.e.: "Joe Doe <joe.doe@example.com>"

        :param message: message of the tag's removal commit

        :param date: date of tag's removal commit

        :raises TagDoesNotExistError: if tag with given name does not exists

        """

        if name not in self.tags:

            raise TagDoesNotExistError("Tag %s does not exist" % name)

        if message is None:

            message = "Removed tag %s" % name

        if date is None:

            date = safe_bytes(datetime.datetime.now().strftime('%a, %d %b %Y %H:%M:%S'))

        local = False

        try:

            mercurial.tags.tag(self._repo, safe_bytes(name), mercurial.commands.nullid, safe_bytes(message), local, safe_bytes(user), date)

            self.tags = self._get_tags()

        except mercurial.error.Abort as e:

            raise RepositoryError(e.args[0])

    @LazyProperty

    def bookmarks(self):

        """

        Gets bookmarks for this repository

        """

        return self._get_bookmarks()

    def _get_bookmarks(self):

        if self._empty:

            return {}

        return OrderedDict(sorted(

            ((safe_str(n), ascii_str(h)) for n, h in self._repo._bookmarks.items()),

            reverse=True,

            key=lambda x: x[0],  # sort by name

        ))

    def _get_all_revisions(self):

        return [ascii_str(self._repo[x].hex()) for x in self._repo.filtered(b'visible').changelog.revs()]

    def get_diff(self, rev1, rev2, path='', ignore_whitespace=False,

                  context=3):

        """

        Returns (git like) *diff*, as plain text. Shows changes introduced by

        ``rev2`` since ``rev1``.

        :param rev1: Entry point from which diff is shown. Can be

          ``self.EMPTY_CHANGESET`` - in this case, patch showing all

          the changes since empty state of the repository until ``rev2``

        :param rev2: Until which revision changes should be shown.

        :param ignore_whitespace: If set to ``True``, would not show whitespace

          changes. Defaults to ``False``.

        :param context: How many lines before/after changed lines should be

          shown. Defaults to ``3``. If negative value is passed-in, it will be

          set to ``0`` instead.

        """

        # Negative context values make no sense, and will result in

        # errors. Ensure this does not happen.

        if context < 0:

            context = 0

        if hasattr(rev1, 'raw_id'):

            rev1 = getattr(rev1, 'raw_id')

        if hasattr(rev2, 'raw_id'):

            rev2 = getattr(rev2, 'raw_id')

        # Check if given revisions are present at repository (may raise

        # ChangesetDoesNotExistError)

        if rev1 != self.EMPTY_CHANGESET:

            self.get_changeset(rev1)

        self.get_changeset(rev2)

        if path:

            file_filter = mercurial.match.exact(path)

        else:

            file_filter = None

        return b''.join(mercurial.patch.diff(self._repo, rev1, rev2, match=file_filter,

                          opts=mercurial.mdiff.diffopts(git=True,

                                        showfunc=True,

                                        ignorews=ignore_whitespace,

                                        context=context)))

    @classmethod

    def _check_url(cls, url, repoui=None):

        """

        Function will check given url and try to verify if it's a valid

        link. Sometimes it may happened that mercurial will issue basic

        auth request that can cause whole API to hang when used from python

        or other external calls.

        On failures it'll raise urllib2.HTTPError, exception is also thrown

        when the return code is non 200

        """

        # check first if it's not an local url

        if os.path.isdir(url) or url.startswith(b'file:'):

            return True

        if url.startswith(b'ssh:'):

            # in case of invalid uri or authentication issues, sshpeer will

            # throw an exception.

            mercurial.sshpeer.instance(repoui or mercurial.ui.ui(), url, False).lookup(b'tip')

            return True

        url_prefix = None

        if b'+' in url[:url.find(b'://')]:

            url_prefix, url = url.split(b'+', 1)

        handlers = []

        url_obj = mercurial.util.url(url)

        test_uri, authinfo = url_obj.authinfo()

        url_obj.passwd = b'*****'

        cleaned_uri = str(url_obj)

        if authinfo:

            # create a password manager

            passmgr = urllib.request.HTTPPasswordMgrWithDefaultRealm()

            passmgr.add_password(*authinfo)

            handlers.extend((mercurial.url.httpbasicauthhandler(passmgr),

                             mercurial.url.httpdigestauthhandler(passmgr)))

        o = urllib.request.build_opener(*handlers)

        o.addheaders = [('Content-Type', 'application/mercurial-0.1'),

                        ('Accept', 'application/mercurial-0.1')]

        req = urllib.request.Request(

            "%s?%s" % (

                test_uri,

                urllib.parse.urlencode({

                    'cmd': 'between',

                    'pairs': "%s-%s" % ('0' * 40, '0' * 40),

                })

            ))

        try:

            resp = o.open(req)

            if resp.code != 200:

                raise Exception('Return Code is not 200')

        except Exception as e:

            # means it cannot be cloned

            raise urllib.error.URLError("[%s] org_exc: %s" % (cleaned_uri, e))

        if not url_prefix: # skip svn+http://... (and git+... too)

            # now check if it's a proper hg repo

            try:

                mercurial.httppeer.instance(repoui or mercurial.ui.ui(), url, False).lookup(b'tip')

            except Exception as e:

                raise urllib.error.URLError(

                    "url [%s] does not look like an hg repo org_exc: %s"

                    % (cleaned_uri, e))

        return True

    def _get_repo(self, create, src_url=None, update_after_clone=False):

        """

        Function will check for mercurial repository in given path and return

        a localrepo object. If there is no repository in that path it will

        raise an exception unless ``create`` parameter is set to True - in

        that case repository would be created and returned.

        If ``src_url`` is given, would try to clone repository from the

        location at given clone_point. Additionally it'll make update to

        working copy accordingly to ``update_after_clone`` flag

        """

        try:

            if src_url:

                url = safe_bytes(self._get_url(src_url))

                opts = {}

                if not update_after_clone:

                    opts.update({'noupdate': True})

                MercurialRepository._check_url(url, self.baseui)

                mercurial.commands.clone(self.baseui, url, safe_bytes(self.path), **opts)

                # Don't try to create if we've already cloned repo

                create = False

            return mercurial.localrepo.instance(self.baseui, safe_bytes(self.path), create=create)

        except (mercurial.error.Abort, mercurial.error.RepoError) as err:

            if create:

                msg = "Cannot create repository at %s. Original error was %s" \

                    % (self.name, err)

            else:

                msg = "Not valid repository at %s. Original error was %s" \

                    % (self.name, err)

            raise RepositoryError(msg)

    @LazyProperty

    def in_memory_changeset(self):

        return MercurialInMemoryChangeset(self)

    @LazyProperty

    def description(self):