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.

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

+ref: refs/heads/master

tests/resources/submodules.git/config

+[core]
+	repositoryformatversion = 0
+	filemode = true
+	bare = true

tests/resources/submodules.git/objects/info/packs

+P pack-b69d04bb39ac274669e2184e45bd90015d02ef5b.pack
+

tests/resources/submodules.git/refs/heads/master

+97896810b3210244a62a82458b8e0819ecfc6850

tests/submodule/lookup.c

@@ -419,3 +419,29 @@ void test_submodule_lookup__cached(void)
 	git_submodule_free(sm);
 	git_submodule_free(sm2);
 }
+
+void test_submodule_lookup__lookup_in_bare_repository_fails(void)
+{
+	git_submodule *sm;
+
+	cl_git_sandbox_cleanup();
+	g_repo = cl_git_sandbox_init("submodules.git");
+
+	cl_git_fail(git_submodule_lookup(&sm, g_repo, "nonexisting"));
+}
+
+static int foreach_cb(git_submodule *sm, const char *name, void *payload)
+{
+	GIT_UNUSED(sm);
+	GIT_UNUSED(name);
+	GIT_UNUSED(payload);
+	return 0;
+}
+
+void test_submodule_lookup__foreach_in_bare_repository_fails(void)
+{
+	cl_git_sandbox_cleanup();
+	g_repo = cl_git_sandbox_init("submodules.git");
+
+	cl_git_fail(git_submodule_foreach(g_repo, foreach_cb, NULL));
+}