RF: gitrepo: Rewrite get_submodules() to avoid GitPython

In addition to keeping with our general direction of moving away from
GitPython, this avoids an unresolved issue in GitPython's handling of
submodules [0].

As documented by the new test, there is a change in behavior when
get_submodules() is called in a repository that doesn't have a commit
checked out: it now returns any registered submodules instead of an
empty list.  This is consistent with the behavior of 'git submodule'
and 'datalad subdataset', and there doesn't seem to be an obvious
reason not to support it.

[0]: datalad#3508 (comment)

datalad/support/gitrepo.py

@@ -14,6 +14,7 @@
 from itertools import chain
 import logging
 from collections import OrderedDict
+from collections import namedtuple
 import re
 import shlex
 import time
@@ -555,6 +556,10 @@ def update(self, op_code, cur_count, max_count=None, message=''):
         self._pbar.refresh()
 
 
+# Compatibility kludge.  See GitRepo.get_submodules().
+Submodule = namedtuple("Submodule", ["name", "path", "url"])
+
+
 @add_metaclass(Flyweight)
 class GitRepo(RepoInterface):
     """Representation of a git repository
@@ -2432,15 +2437,47 @@ def get_submodules_(self, paths=None):
             props.update(modinfo.get(path, {}))
             yield props
 
-    def get_submodules(self, sorted_=True):
-        """Return a list of git.Submodule instances for all submodules"""
-        # check whether we have anything in the repo. if not go home early
-        if not self.repo.head.is_valid():
-            return []
-        submodules = self.repo.submodules
+    def get_submodules(self, sorted_=True, paths=None, compat=True):
+        """Return list of submodules.
+
+        Parameters
+        ----------
+        sorted_ : bool, optional
+            Sort submodules by path name.
+        paths : list(pathlib.PurePath), optional
+            Restrict submodules to those under `paths`.
+        compat : bool, optional
+            If true, return a namedtuple that incompletely mimics the
+            attributes of GitPython's Submodule object in hope of backwards
+            compatibility with previous callers. Note that this form should be
+            considered temporary and callers should be updated; this flag will
+            be removed in a future release.
+
+        Returns
+        -------
+        List of submodule namedtuples if `compat` is true or otherwise a list
+        of dictionaries as returned by `get_submodules_`.
+        """
+        xs = self.get_submodules_(paths=paths)
+        if compat:
+            warnings.warn("The attribute-based return value of get_submodules() "
+                          "exists for compatibility purposes and will be removed "
+                          "in an upcoming release",
+                          DeprecationWarning)
+            xs = (Submodule(name=p["gitmodule_name"],
+                            path=text_type(p["path"].relative_to(self.pathobj)),
+                            url=p["gitmodule_url"])
+                  for p in xs)
+
         if sorted_:
-            submodules = sorted(submodules, key=lambda x: x.path)
-        return submodules
+            if compat:
+                def key(x):
+                    return x.path
+            else:
+                def key(x):
+                    return x["path"]
+            xs = sorted(xs, key=key)
+        return list(xs)
 
     def is_submodule_modified(self, name, options=[]):
         """Whether a submodule has new commits

datalad/support/tests/test_gitrepo.py

@@ -1012,6 +1012,16 @@ def test_GitRepo_get_submodules():
     raise SkipTest("TODO")
 
 
+@with_tempfile
+def test_get_submodules_parent_on_unborn_branch(path):
+    repo = GitRepo(path, create=True)
+    subrepo = GitRepo(op.join(path, "sub"), create=True)
+    subrepo.commit(msg="s", options=["--allow-empty"])
+    repo.add_submodule(path="sub")
+    eq_([s.name for s in repo.get_submodules()],
+        ["sub"])
+
+
 def test_kwargs_to_options():
 
     class Some(object):