base.py

from . import util
from .util import (
    mkhead,
    sm_name,
    sm_section,
    unbare_repo,
    SubmoduleConfigParser,
    find_first_remote_branch
)
from git.objects.util import Traversable
from io import BytesIO                   # need a dict to set bloody .name field
from git.util import (
    Iterable,
    join_path_native,
    to_native_path_linux,
    RemoteProgress,
    rmtree
)

from git.config import (
    SectionConstraint,
    cp
)
from git.exc import (
    InvalidGitRepositoryError,
    NoSuchPathError
)
from git.compat import string_types

import stat
import git

import os
import logging

__all__ = ["Submodule", "UpdateProgress"]


log = logging.getLogger('git.objects.submodule.base')


class UpdateProgress(RemoteProgress):

    """Class providing detailed progress information to the caller who should
    derive from it and implement the ``update(...)`` message"""
    CLONE, FETCH, UPDWKTREE = [1 << x for x in range(RemoteProgress._num_op_codes, RemoteProgress._num_op_codes + 3)]
    _num_op_codes = RemoteProgress._num_op_codes + 3

    __slots__ = tuple()


BEGIN = UpdateProgress.BEGIN
END = UpdateProgress.END
CLONE = UpdateProgress.CLONE
FETCH = UpdateProgress.FETCH
UPDWKTREE = UpdateProgress.UPDWKTREE


# IndexObject comes via util module, its a 'hacky' fix thanks to pythons import
# mechanism which cause plenty of trouble of the only reason for packages and
# modules is refactoring - subpackages shoudn't depend on parent packages
class Submodule(util.IndexObject, Iterable, Traversable):

    """Implements access to a git submodule. They are special in that their sha
    represents a commit in the submodule's repository which is to be checked out
    at the path of this instance.
    The submodule type does not have a string type associated with it, as it exists
    solely as a marker in the tree and index.

    All methods work in bare and non-bare repositories."""

    _id_attribute_ = "name"
    k_modules_file = '.gitmodules'
    k_head_option = 'branch'
    k_head_default = 'master'
    k_default_mode = stat.S_IFDIR | stat.S_IFLNK        # submodules are directories with link-status

    # this is a bogus type for base class compatability
    type = 'submodule'

    __slots__ = ('_parent_commit', '_url', '_branch_path', '_name', '__weakref__')
    _cache_attrs = ('path', '_url', '_branch_path')

    def __init__(self, repo, binsha, mode=None, path=None, name=None, parent_commit=None, url=None, branch_path=None):
        """Initialize this instance with its attributes. We only document the ones
        that differ from ``IndexObject``

        :param repo: Our parent repository
        :param binsha: binary sha referring to a commit in the remote repository, see url parameter
        :param parent_commit: see set_parent_commit()
        :param url: The url to the remote repository which is the submodule
        :param branch_path: full (relative) path to ref to checkout when cloning the remote repository"""
        super(Submodule, self).__init__(repo, binsha, mode, path)
        self.size = 0
        if parent_commit is not None:
            self._parent_commit = parent_commit
        if url is not None:
            self._url = url
        if branch_path is not None:
            assert isinstance(branch_path, string_types)
            self._branch_path = branch_path
        if name is not None:
            self._name = name

    def _set_cache_(self, attr):
        if attr == '_parent_commit':
            # set a default value, which is the root tree of the current head
            self._parent_commit = self.repo.commit()
        elif attr in ('path', '_url', '_branch_path'):
            reader = self.config_reader()
            # default submodule values
            self.path = reader.get_value('path')
            self._url = reader.get_value('url')
            # git-python extension values - optional
            self._branch_path = reader.get_value(self.k_head_option, git.Head.to_full_path(self.k_head_default))
        elif attr == '_name':
            raise AttributeError("Cannot retrieve the name of a submodule if it was not set initially")
        else:
            super(Submodule, self)._set_cache_(attr)
        # END handle attribute name

    def _get_intermediate_items(self, item):
        """:return: all the submodules of our module repository"""
        try:
            return type(self).list_items(item.module())
        except InvalidGitRepositoryError:
            return list()
        # END handle intermeditate items

    def __eq__(self, other):
        """Compare with another submodule"""
        # we may only compare by name as this should be the ID they are hashed with
        # Otherwise this type wouldn't be hashable
        # return self.path == other.path and self.url == other.url and super(Submodule, self).__eq__(other)
        return self._name == other._name

    def __ne__(self, other):
        """Compare with another submodule for inequality"""
        return not (self == other)

    def __hash__(self):
        """Hash this instance using its logical id, not the sha"""
        return hash(self._name)

    def __str__(self):
        return self._name

    def __repr__(self):
        return "git.%s(name=%s, path=%s, url=%s, branch_path=%s)"\
               % (type(self).__name__, self._name, self.path, self.url, self.branch_path)

    @classmethod
    def _config_parser(cls, repo, parent_commit, read_only):
        """:return: Config Parser constrained to our submodule in read or write mode
        :raise IOError: If the .gitmodules file cannot be found, either locally or in the repository
            at the given parent commit. Otherwise the exception would be delayed until the first
            access of the config parser"""
        parent_matches_head = repo.head.commit == parent_commit
        if not repo.bare and parent_matches_head:
            fp_module = cls.k_modules_file
            fp_module_path = os.path.join(repo.working_tree_dir, fp_module)
            fp_module = fp_module_path
        else:
            try:
                fp_module = cls._sio_modules(parent_commit)
            except KeyError:
                raise IOError("Could not find %s file in the tree of parent commit %s" %
                              (cls.k_modules_file, parent_commit))
            # END handle exceptions
        # END handle non-bare working tree

        if not read_only and (repo.bare or not parent_matches_head):
            raise ValueError("Cannot write blobs of 'historical' submodule configurations")
        # END handle writes of historical submodules

        return SubmoduleConfigParser(fp_module, read_only=read_only)

    def _clear_cache(self):
        # clear the possibly changed values
        for name in self._cache_attrs:
            try:
                delattr(self, name)
            except AttributeError:
                pass
            # END try attr deletion
        # END for each name to delete

    @classmethod
    def _sio_modules(cls, parent_commit):
        """:return: Configuration file as BytesIO - we only access it through the respective blob's data"""
        sio = BytesIO(parent_commit.tree[cls.k_modules_file].data_stream.read())
        sio.name = cls.k_modules_file
        return sio

    def _config_parser_constrained(self, read_only):
        """:return: Config Parser constrained to our submodule in read or write mode"""
        parser = self._config_parser(self.repo, self._parent_commit, read_only)
        parser.set_submodule(self)
        return SectionConstraint(parser, sm_section(self.name))

    #{ Edit Interface

    @classmethod
    def add(cls, repo, name, path, url=None, branch=None, no_checkout=False):
        """Add a new submodule to the given repository. This will alter the index
        as well as the .gitmodules file, but will not create a new commit.
        If the submodule already exists, no matter if the configuration differs
        from the one provided, the existing submodule will be returned.

        :param repo: Repository instance which should receive the submodule
        :param name: The name/identifier for the submodule
        :param path: repository-relative or absolute path at which the submodule
            should be located
            It will be created as required during the repository initialization.
        :param url: git-clone compatible URL, see git-clone reference for more information
            If None, the repository is assumed to exist, and the url of the first
            remote is taken instead. This is useful if you want to make an existing
            repository a submodule of anotherone.
        :param branch: name of branch at which the submodule should (later) be checked out.
            The given branch must exist in the remote repository, and will be checked
            out locally as a tracking branch.
            It will only be written into the configuration if it not None, which is
            when the checked out branch will be the one the remote HEAD pointed to.
            The result you get in these situation is somewhat fuzzy, and it is recommended
            to specify at least 'master' here.
            Examples are 'master' or 'feature/new'
        :param no_checkout: if True, and if the repository has to be cloned manually,
            no checkout will be performed
        :return: The newly created submodule instance
        :note: works atomically, such that no change will be done if the repository
            update fails for instance"""
        if repo.bare:
            raise InvalidGitRepositoryError("Cannot add submodules to bare repositories")
        # END handle bare repos

        path = to_native_path_linux(path)
        if path.endswith('/'):
            path = path[:-1]
        # END handle trailing slash

        if os.path.isabs(path):
            working_tree_linux = to_native_path_linux(repo.working_tree_dir)
            if not path.startswith(working_tree_linux):
                raise ValueError("Submodule checkout path '%s' needs to be within the parents repository at '%s'"
                                 % (working_tree_linux, path))
            path = path[len(working_tree_linux) + 1:]
            if not path:
                raise ValueError("Absolute submodule path '%s' didn't yield a valid relative path" % path)
            # end verify converted relative path makes sense
        # end convert to a relative path

        # assure we never put backslashes into the url, as some operating systems
        # like it ...
        if url is not None:
            url = to_native_path_linux(url)
        # END assure url correctness

        # INSTANTIATE INTERMEDIATE SM
        sm = cls(repo, cls.NULL_BIN_SHA, cls.k_default_mode, path, name, url='invalid-temporary')
        if sm.exists():
            # reretrieve submodule from tree
            try:
                return repo.head.commit.tree[path]
            except KeyError:
                # could only be in index
                index = repo.index
                entry = index.entries[index.entry_key(path, 0)]
                sm.binsha = entry.binsha
                return sm
            # END handle exceptions
        # END handle existing

        # fake-repo - we only need the functionality on the branch instance
        br = git.Head(repo, git.Head.to_full_path(str(branch) or cls.k_head_default))
        has_module = sm.module_exists()
        branch_is_default = branch is None
        if has_module and url is not None:
            if url not in [r.url for r in sm.module().remotes]:
                raise ValueError(
                    "Specified URL '%s' does not match any remote url of the repository at '%s'" % (url, sm.abspath))
            # END check url
        # END verify urls match

        mrepo = None
        if url is None:
            if not has_module:
                raise ValueError("A URL was not given and existing repository did not exsit at %s" % path)
            # END check url
            mrepo = sm.module()
            urls = [r.url for r in mrepo.remotes]
            if not urls:
                raise ValueError("Didn't find any remote url in repository at %s" % sm.abspath)
            # END verify we have url
            url = urls[0]
        else:
            # clone new repo
            kwargs = {'n': no_checkout}
            if not branch_is_default:
                kwargs['b'] = br.name
            # END setup checkout-branch
            mrepo = git.Repo.clone_from(url, os.path.join(repo.working_tree_dir, path), **kwargs)
        # END verify url

        # update configuration and index
        index = sm.repo.index
        writer = sm.config_writer(index=index, write=False)
        writer.set_value('url', url)
        writer.set_value('path', path)

        sm._url = url
        if not branch_is_default:
            # store full path
            writer.set_value(cls.k_head_option, br.path)
            sm._branch_path = br.path
        # END handle path
        writer.release()
        del(writer)

        # we deliberatly assume that our head matches our index !
        pcommit = repo.head.commit
        sm._parent_commit = pcommit
        sm.binsha = mrepo.head.commit.binsha
        index.add([sm], write=True)

        return sm

    def update(self, recursive=False, init=True, to_latest_revision=False, progress=None,
               dry_run=False):
        """Update the repository of this submodule to point to the checkout
        we point at with the binsha of this instance.

        :param recursive: if True, we will operate recursively and update child-
            modules as well.
        :param init: if True, the module repository will be cloned into place if necessary
        :param to_latest_revision: if True, the submodule's sha will be ignored during checkout.
            Instead, the remote will be fetched, and the local tracking branch updated.
            This only works if we have a local tracking branch, which is the case
            if the remote repository had a master branch, or of the 'branch' option
            was specified for this submodule and the branch existed remotely
        :param progress: UpdateProgress instance or None of no progress should be shown
        :param dry_run: if True, the operation will only be simulated, but not performed.
            All performed operations are read-only
        :note: does nothing in bare repositories
        :note: method is definitely not atomic if recurisve is True
        :return: self"""
        if self.repo.bare:
            return self
        # END pass in bare mode

        if progress is None:
            progress = UpdateProgress()
        # END handle progress
        prefix = ''
        if dry_run:
            prefix = "DRY-RUN: "
        # END handle prefix

        # to keep things plausible in dry-run mode
        if dry_run:
            mrepo = None
        # END init mrepo

        # ASSURE REPO IS PRESENT AND UPTODATE
        #####################################
        try:
            mrepo = self.module()
            rmts = mrepo.remotes
            len_rmts = len(rmts)
            for i, remote in enumerate(rmts):
                op = FETCH
                if i == 0:
                    op |= BEGIN
                # END handle start

                progress.update(op, i, len_rmts, prefix + "Fetching remote %s of submodule %r" % (remote, self.name))
                #===============================
                if not dry_run:
                    remote.fetch(progress=progress)
                # END handle dry-run
                #===============================
                if i == len_rmts - 1:
                    op |= END
                # END handle end
                progress.update(op, i, len_rmts, prefix + "Done fetching remote of submodule %r" % self.name)
            # END fetch new data
        except InvalidGitRepositoryError:
            if not init:
                return self
            # END early abort if init is not allowed
            import git

            # there is no git-repository yet - but delete empty paths
            module_path = join_path_native(self.repo.working_tree_dir, self.path)
            if not dry_run and os.path.isdir(module_path):
                try:
                    os.rmdir(module_path)
                except OSError:
                    raise OSError("Module directory at %r does already exist and is non-empty" % module_path)
                # END handle OSError
            # END handle directory removal

            # don't check it out at first - nonetheless it will create a local
            # branch according to the remote-HEAD if possible
            progress.update(BEGIN | CLONE, 0, 1, prefix + "Cloning %s to %s in submodule %r" %
                            (self.url, module_path, self.name))
            if not dry_run:
                mrepo = git.Repo.clone_from(self.url, module_path, n=True)
            # END handle dry-run
            progress.update(END | CLONE, 0, 1, prefix + "Done cloning to %s" % module_path)

            if not dry_run:
                # see whether we have a valid branch to checkout
                try:
                    # find  a remote which has our branch - we try to be flexible
                    remote_branch = find_first_remote_branch(mrepo.remotes, self.branch_name)
                    local_branch = mkhead(mrepo, self.branch_path)

                    # have a valid branch, but no checkout - make sure we can figure
                    # that out by marking the commit with a null_sha
                    local_branch.set_object(util.Object(mrepo, self.NULL_BIN_SHA))
                    # END initial checkout + branch creation

                    # make sure HEAD is not detached
                    mrepo.head.set_reference(local_branch, logmsg="submodule: attaching head to %s" % local_branch)
                    mrepo.head.ref.set_tracking_branch(remote_branch)
                except IndexError:
                    log.warn("Failed to checkout tracking branch %s", self.branch_path)
                # END handle tracking branch

                # NOTE: Have to write the repo config file as well, otherwise
                # the default implementation will be offended and not update the repository
                # Maybe this is a good way to assure it doesn't get into our way, but
                # we want to stay backwards compatible too ... . Its so redundant !
                writer = self.repo.config_writer()
                writer.set_value(sm_section(self.name), 'url', self.url)
                writer.release()
            # END handle dry_run
        # END handle initalization

        # DETERMINE SHAS TO CHECKOUT
        ############################
        binsha = self.binsha
        hexsha = self.hexsha
        if mrepo is not None:
            # mrepo is only set if we are not in dry-run mode or if the module existed
            is_detached = mrepo.head.is_detached
        # END handle dry_run

        if mrepo is not None and to_latest_revision:
            msg_base = "Cannot update to latest revision in repository at %r as " % mrepo.working_dir
            if not is_detached:
                rref = mrepo.head.ref.tracking_branch()
                if rref is not None:
                    rcommit = rref.commit
                    binsha = rcommit.binsha
                    hexsha = rcommit.hexsha
                else:
                    log.error("%s a tracking branch was not set for local branch '%s'", msg_base, mrepo.head.ref)
                # END handle remote ref
            else:
                log.error("%s there was no local tracking branch", msg_base)
            # END handle detached head
        # END handle to_latest_revision option

        # update the working tree
        # handles dry_run
        if mrepo is not None and mrepo.head.commit.binsha != binsha:
            progress.update(BEGIN | UPDWKTREE, 0, 1, prefix +
                            "Updating working tree at %s for submodule %r to revision %s"
                            % (self.path, self.name, hexsha))
            if not dry_run:
                if is_detached:
                    # NOTE: for now we force, the user is no supposed to change detached
                    # submodules anyway. Maybe at some point this becomes an option, to
                    # properly handle user modifications - see below for future options
                    # regarding rebase and merge.
                    mrepo.git.checkout(hexsha, force=True)
                else:
                    # TODO: allow to specify a rebase, merge, or reset
                    # TODO: Warn if the hexsha forces the tracking branch off the remote
                    # branch - this should be prevented when setting the branch option
                    mrepo.head.reset(hexsha, index=True, working_tree=True)
                # END handle checkout
            # END handle dry_run
            progress.update(END | UPDWKTREE, 0, 1, prefix + "Done updating working tree for submodule %r" % self.name)
        # END update to new commit only if needed

        # HANDLE RECURSION
        ##################
        if recursive:
            # in dry_run mode, the module might not exist
            if mrepo is not None:
                for submodule in self.iter_items(self.module()):
                    submodule.update(recursive, init, to_latest_revision, progress=progress, dry_run=dry_run)
                # END handle recursive update
            # END handle dry run
        # END for each submodule

        return self

    @unbare_repo
    def move(self, module_path, configuration=True, module=True):
        """Move the submodule to a another module path. This involves physically moving
        the repository at our current path, changing the configuration, as well as
        adjusting our index entry accordingly.

        :param module_path: the path to which to move our module, given as
            repository-relative path. Intermediate directories will be created
            accordingly. If the path already exists, it must be empty.
            Trailling (back)slashes are removed automatically
        :param configuration: if True, the configuration will be adjusted to let
            the submodule point to the given path.
        :param module: if True, the repository managed by this submodule
            will be moved, not the configuration. This will effectively
            leave your repository in an inconsistent state unless the configuration
            and index already point to the target location.
        :return: self
        :raise ValueError: if the module path existed and was not empty, or was a file
        :note: Currently the method is not atomic, and it could leave the repository
            in an inconsistent state if a sub-step fails for some reason
        """
        if module + configuration < 1:
            raise ValueError("You must specify to move at least the module or the configuration of the submodule")
        # END handle input

        module_path = to_native_path_linux(module_path)
        if module_path.endswith('/'):
            module_path = module_path[:-1]
        # END handle trailing slash

        # VERIFY DESTINATION
        if module_path == self.path:
            return self
        # END handle no change

        dest_path = join_path_native(self.repo.working_tree_dir, module_path)
        if os.path.isfile(dest_path):
            raise ValueError("Cannot move repository onto a file: %s" % dest_path)
        # END handle target files

        index = self.repo.index
        tekey = index.entry_key(module_path, 0)
        # if the target item already exists, fail
        if configuration and tekey in index.entries:
            raise ValueError("Index entry for target path did alredy exist")
        # END handle index key already there

        # remove existing destination
        if module:
            if os.path.exists(dest_path):
                if len(os.listdir(dest_path)):
                    raise ValueError("Destination module directory was not empty")
                # END handle non-emptyness

                if os.path.islink(dest_path):
                    os.remove(dest_path)
                else:
                    os.rmdir(dest_path)
                # END handle link
            else:
                # recreate parent directories
                # NOTE: renames() does that now
                pass
            # END handle existance
        # END handle module

        # move the module into place if possible
        cur_path = self.abspath
        renamed_module = False
        if module and os.path.exists(cur_path):
            os.renames(cur_path, dest_path)
            renamed_module = True
        # END move physical module

        # rename the index entry - have to manipulate the index directly as
        # git-mv cannot be used on submodules ... yeah
        try:
            if configuration:
                try:
                    ekey = index.entry_key(self.path, 0)
                    entry = index.entries[ekey]
                    del(index.entries[ekey])
                    nentry = git.IndexEntry(entry[:3] + (module_path,) + entry[4:])
                    index.entries[tekey] = nentry
                except KeyError:
                    raise InvalidGitRepositoryError("Submodule's entry at %r did not exist" % (self.path))
                # END handle submodule doesn't exist

                # update configuration
                writer = self.config_writer(index=index)        # auto-write
                writer.set_value('path', module_path)
                self.path = module_path
                writer.release()
                del(writer)
            # END handle configuration flag
        except Exception:
            if renamed_module:
                os.renames(dest_path, cur_path)
            # END undo module renaming
            raise
        # END handle undo rename

        return self

    @unbare_repo
    def remove(self, module=True, force=False, configuration=True, dry_run=False):
        """Remove this submodule from the repository. This will remove our entry
        from the .gitmodules file and the entry in the .git/config file.

        :param module: If True, the module we point to will be deleted
            as well. If the module is currently on a commit which is not part
            of any branch in the remote, if the currently checked out branch
            working tree, or untracked files,
            is ahead of its tracking branch,  if you have modifications in the
            In case the removal of the repository fails for these reasons, the
            submodule status will not have been altered.
            If this submodule has child-modules on its own, these will be deleted
            prior to touching the own module.
        :param force: Enforces the deletion of the module even though it contains
            modifications. This basically enforces a brute-force file system based
            deletion.
        :param configuration: if True, the submodule is deleted from the configuration,
            otherwise it isn't. Although this should be enabled most of the times,
            this flag enables you to safely delete the repository of your submodule.
        :param dry_run: if True, we will not actually do anything, but throw the errors
            we would usually throw
        :return: self
        :note: doesn't work in bare repositories
        :raise InvalidGitRepositoryError: thrown if the repository cannot be deleted
        :raise OSError: if directories or files could not be removed"""
        if not (module + configuration):
            raise ValueError("Need to specify to delete at least the module, or the configuration")
        # END handle params

        # DELETE MODULE REPOSITORY
        ##########################
        if module and self.module_exists():
            if force:
                # take the fast lane and just delete everything in our module path
                # TODO: If we run into permission problems, we have a highly inconsistent
                # state. Delete the .git folders last, start with the submodules first
                mp = self.abspath
                method = None
                if os.path.islink(mp):
                    method = os.remove
                elif os.path.isdir(mp):
                    method = rmtree
                elif os.path.exists(mp):
                    raise AssertionError("Cannot forcibly delete repository as it was neither a link, nor a directory")
                # END handle brutal deletion
                if not dry_run:
                    assert method
                    method(mp)
                # END apply deletion method
            else:
                # verify we may delete our module
                mod = self.module()
                if mod.is_dirty(untracked_files=True):
                    raise InvalidGitRepositoryError(
                        "Cannot delete module at %s with any modifications, unless force is specified"
                        % mod.working_tree_dir)
                # END check for dirt

                # figure out whether we have new commits compared to the remotes
                # NOTE: If the user pulled all the time, the remote heads might
                # not have been updated, so commits coming from the remote look
                # as if they come from us. But we stay strictly read-only and
                # don't fetch beforhand.
                for remote in mod.remotes:
                    num_branches_with_new_commits = 0
                    rrefs = remote.refs
                    for rref in rrefs:
                        num_branches_with_new_commits += len(mod.git.cherry(rref)) != 0
                    # END for each remote ref
                    # not a single remote branch contained all our commits
                    if num_branches_with_new_commits == len(rrefs):
                        raise InvalidGitRepositoryError(
                            "Cannot delete module at %s as there are new commits" % mod.working_tree_dir)
                    # END handle new commits
                    # have to manually delete references as python's scoping is
                    # not existing, they could keep handles open ( on windows this is a problem )
                    if len(rrefs):
                        del(rref)
                    # END handle remotes
                    del(rrefs)
                    del(remote)
                # END for each remote

                # gently remove all submodule repositories
                for sm in self.children():
                    sm.remove(module=True, force=False, configuration=False, dry_run=dry_run)
                    del(sm)
                # END for each child-submodule

                # finally delete our own submodule
                if not dry_run:
                    wtd = mod.working_tree_dir
                    del(mod)        # release file-handles (windows)
                    rmtree(wtd)
                # END delete tree if possible
            # END handle force
        # END handle module deletion

        # DELETE CONFIGURATION
        ######################
        if configuration and not dry_run:
            # first the index-entry
            index = self.repo.index
            try:
                del(index.entries[index.entry_key(self.path, 0)])
            except KeyError:
                pass
            # END delete entry
            index.write()

            # now git config - need the config intact, otherwise we can't query
            # inforamtion anymore
            writer = self.repo.config_writer()
            writer.remove_section(sm_section(self.name))
            writer.release()
            writer = self.config_writer()
            writer.remove_section()
            writer.release()
        # END delete configuration

        # void our data not to delay invalid access
        self._clear_cache()

        return self

    def set_parent_commit(self, commit, check=True):
        """Set this instance to use the given commit whose tree is supposed to
        contain the .gitmodules blob.

        :param commit: Commit'ish reference pointing at the root_tree
        :param check: if True, relatively expensive checks will be performed to verify
            validity of the submodule.
        :raise ValueError: if the commit's tree didn't contain the .gitmodules blob.
        :raise ValueError: if the parent commit didn't store this submodule under the
            current path
        :return: self"""
        pcommit = self.repo.commit(commit)
        pctree = pcommit.tree
        if self.k_modules_file not in pctree:
            raise ValueError("Tree of commit %s did not contain the %s file" % (commit, self.k_modules_file))
        # END handle exceptions

        prev_pc = self._parent_commit
        self._parent_commit = pcommit

        if check:
            parser = self._config_parser(self.repo, self._parent_commit, read_only=True)
            if not parser.has_section(sm_section(self.name)):
                self._parent_commit = prev_pc
                raise ValueError("Submodule at path %r did not exist in parent commit %s" % (self.path, commit))
            # END handle submodule did not exist
        # END handle checking mode

        # update our sha, it could have changed
        self.binsha = pctree[self.path].binsha

        self._clear_cache()

        return self

    @unbare_repo
    def config_writer(self, index=None, write=True):
        """:return: a config writer instance allowing you to read and write the data
        belonging to this submodule into the .gitmodules file.

        :param index: if not None, an IndexFile instance which should be written.
            defaults to the index of the Submodule's parent repository.
        :param write: if True, the index will be written each time a configuration
            value changes.
        :note: the parameters allow for a more efficient writing of the index,
            as you can pass in a modified index on your own, prevent automatic writing,
            and write yourself once the whole operation is complete
        :raise ValueError: if trying to get a writer on a parent_commit which does not
            match the current head commit
        :raise IOError: If the .gitmodules file/blob could not be read"""
        writer = self._config_parser_constrained(read_only=False)
        if index is not None:
            writer.config._index = index
        writer.config._auto_write = write
        return writer

    #} END edit interface

    #{ Query Interface

    @unbare_repo
    def module(self):
        """:return: Repo instance initialized from the repository at our submodule path
        :raise InvalidGitRepositoryError: if a repository was not available. This could
            also mean that it was not yet initialized"""
        # late import to workaround circular dependencies
        module_path = self.abspath
        try:
            repo = git.Repo(module_path)
            if repo != self.repo:
                return repo
            # END handle repo uninitialized
        except (InvalidGitRepositoryError, NoSuchPathError):
            raise InvalidGitRepositoryError("No valid repository at %s" % self.path)
        else:
            raise InvalidGitRepositoryError("Repository at %r was not yet checked out" % module_path)
        # END handle exceptions

    def module_exists(self):
        """:return: True if our module exists and is a valid git repository. See module() method"""
        try:
            self.module()
            return True
        except Exception:
            return False
        # END handle exception

    def exists(self):
        """
        :return: True if the submodule exists, False otherwise. Please note that
            a submodule may exist (in the .gitmodules file) even though its module
            doesn't exist on disk"""
        # keep attributes for later, and restore them if we have no valid data
        # this way we do not actually alter the state of the object
        loc = locals()
        for attr in self._cache_attrs:
            try:
                if hasattr(self, attr):
                    loc[attr] = getattr(self, attr)
                # END if we have the attribute cache
            except cp.NoSectionError:
                # on PY3, this can happen apparently ... don't know why this doesn't happen on PY2
                pass
        # END for each attr
        self._clear_cache()

        try:
            try:
                self.path
                return True
            except Exception:
                return False
            # END handle exceptions
        finally:
            for attr in self._cache_attrs:
                if attr in loc:
                    setattr(self, attr, loc[attr])
                # END if we have a cache
            # END reapply each attribute
        # END handle object state consistency

    @property
    def branch(self):
        """:return: The branch instance that we are to checkout
        :raise InvalidGitRepositoryError: if our module is not yet checked out"""
        return mkhead(self.module(), self._branch_path)

    @property
    def branch_path(self):
        """
        :return: full (relative) path as string to the branch we would checkout
            from the remote and track"""
        return self._branch_path

    @property
    def branch_name(self):
        """:return: the name of the branch, which is the shortest possible branch name"""
        # use an instance method, for this we create a temporary Head instance
        # which uses a repository that is available at least ( it makes no difference )
        return git.Head(self.repo, self._branch_path).name

    @property
    def url(self):
        """:return: The url to the repository which our module-repository refers to"""
        return self._url

    @property
    def parent_commit(self):
        """:return: Commit instance with the tree containing the .gitmodules file
        :note: will always point to the current head's commit if it was not set explicitly"""
        return self._parent_commit

    @property
    def name(self):
        """:return: The name of this submodule. It is used to identify it within the
            .gitmodules file.
        :note: by default, the name is the path at which to find the submodule, but
            in git-python it should be a unique identifier similar to the identifiers
            used for remotes, which allows to change the path of the submodule
            easily
        """
        return self._name

    def config_reader(self):
        """
        :return: ConfigReader instance which allows you to qurey the configuration values
            of this submodule, as provided by the .gitmodules file
        :note: The config reader will actually read the data directly from the repository
            and thus does not need nor care about your working tree.
        :note: Should be cached by the caller and only kept as long as needed
        :raise IOError: If the .gitmodules file/blob could not be read"""
        return self._config_parser_constrained(read_only=True)

    def children(self):
        """
        :return: IterableList(Submodule, ...) an iterable list of submodules instances
            which are children of this submodule or 0 if the submodule is not checked out"""
        return self._get_intermediate_items(self)

    #} END query interface

    #{ Iterable Interface

    @classmethod
    def iter_items(cls, repo, parent_commit='HEAD'):
        """:return: iterator yielding Submodule instances available in the given repository"""
        pc = repo.commit(parent_commit)         # parent commit instance
        try:
            parser = cls._config_parser(repo, pc, read_only=True)
        except IOError:
            raise StopIteration
        # END handle empty iterator

        rt = pc.tree                                # root tree

        for sms in parser.sections():
            n = sm_name(sms)
            p = parser.get_value(sms, 'path')
            u = parser.get_value(sms, 'url')
            b = cls.k_head_default
            if parser.has_option(sms, cls.k_head_option):
                b = str(parser.get_value(sms, cls.k_head_option))
            # END handle optional information

            # get the binsha
            index = repo.index
            try:
                sm = rt[p]
            except KeyError:
                # try the index, maybe it was just added
                try:
                    entry = index.entries[index.entry_key(p, 0)]
                    sm = Submodule(repo, entry.binsha, entry.mode, entry.path)
                except KeyError:
                    raise InvalidGitRepositoryError(
                        "Gitmodule path %r did not exist in revision of parent commit %s" % (p, parent_commit))
                # END handle keyerror
            # END handle critical error

            # fill in remaining info - saves time as it doesn't have to be parsed again
            sm._name = n
            sm._parent_commit = pc
            sm._branch_path = git.Head.to_full_path(b)
            sm._url = u

            yield sm
        # END for each section

    #} END iterable interface