submodule: refuse lookup in bare repositories

While it is technically possible to look up submodules inside of a
bare repository by reading the submodule configuration of a specific
commit, we do not offer this functionality right now. As such, calling
both `git_submodule_lookup` and `git_submodule_foreach` should error out
early when these functions encounter a bare repository. While
`git_submodule_lookup` already does return an error due to not being
able to parse the configuration, `git_submodule_foreach` simply returns
success and never invokes the callback function.

Fix the issue by having both functions check whether the repository is
bare and returning an error in that case.

Author: pks-t

src/submodule.c

@@ -209,6 +209,11 @@ int git_submodule_lookup(
 
 	assert(repo && name);
 
+	if (repo->is_bare) {
+		giterr_set(GITERR_SUBMODULE, "cannot get submodules without a working tree");
+		return -1;
+	}
+
 	if (repo->submodule_cache != NULL) {
 		khiter_t pos = git_strmap_lookup_index(repo->submodule_cache, name);
 		if (git_strmap_valid_index(repo->submodule_cache, pos)) {
@@ -549,6 +554,11 @@ int git_submodule_foreach(
 	int error;
 	size_t i;
 
+	if (repo->is_bare) {
+		giterr_set(GITERR_SUBMODULE, "cannot get submodules without a working tree");
+		return -1;
+	}
+
 	if ((error = git_strmap_alloc(&submodules)) < 0)
 		return error;
 

tests/resources/submodules.git/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/master

tests/resources/submodules.git/config

@@ -0,0 +1,4 @@
+[core]
+	repositoryformatversion = 0
+	filemode = true
+	bare = true

tests/resources/submodules.git/objects/26/a3b32a9b7d97486c5557f5902e8ac94638145e

@@ -0,0 +1,2 @@
+x
%�=
+�0F]�)�0�"�I�*�|��-t{?�2�ilV8���$���m��v��k�k*FDA�=(=|=6�	�DAv=��A}�&'�O�$=

tests/resources/submodules.git/objects/78/308c9251cf4eee8b25a76c7d2790c73d797357

@@ -0,0 +1,3 @@
+x
��[
+�0E��*fʤS�	��K�����4ݿw�ׅ�9p�2MC�F�P@���u.�.�p�!�OY�diYU�'̕8X�bPn���6
+ħԞ��1[q��}0q���c[W�#�1f��R:���SZ�+Y��+{�td�lv����Om��u�_�}�5�i���`	K�

tests/resources/submodules.git/objects/97/896810b3210244a62a82458b8e0819ecfc6850

@@ -0,0 +1,2 @@
+P pack-b69d04bb39ac274669e2184e45bd90015d02ef5b.pack
+