Test submodules with empty index or orphaned head

In both of these cases, the submodule data should still be loaded
just (obviously) without the data that comes from either the index
or the HEAD.

This fixes a bug in the orphaned head case.

src/submodule.c

@@ -1176,8 +1176,11 @@ static int load_submodule_config_from_head(
 	git_iterator *i;
 	const git_index_entry *entry;
 
-	if ((error = git_repository_head_tree(&head, repo)) < 0)
-		return error;
+	/* if we can't look up current head, then there's no submodule in it */
+	if (git_repository_head_tree(&head, repo) < 0) {
+		giterr_clear();
+		return 0;
+	}
 
 	if ((error = git_iterator_for_tree(&i, head, 0, NULL, NULL)) < 0) {
 		git_tree_free(head);

tests-clar/submodule/lookup.c

 #include "clar_libgit2.h"
 #include "submodule_helpers.h"
 #include "posix.h"
+#include "git2/sys/repository.h"
 
 static git_repository *g_repo = NULL;
 
@@ -112,3 +113,73 @@ void test_submodule_lookup__foreach(void)
 	cl_git_pass(git_submodule_foreach(g_repo, sm_lookup_cb, &data));
 	cl_assert_equal_i(8, data.count);
 }
+
+void test_submodule_lookup__lookup_even_with_orphaned_head(void)
+{
+	git_reference *orphan;
+	git_submodule *sm;
+
+	/* orphan the head */
+	cl_git_pass(git_reference_symbolic_create(
+		&orphan, g_repo, "HEAD", "refs/heads/garbage", 1));
+	git_reference_free(orphan);
+
+	/* lookup existing */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "sm_unchanged"));
+	cl_assert(sm);
+
+	/* lookup pending change in .gitmodules that is not in HEAD */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "sm_added_and_uncommited"));
+	cl_assert(sm);
+
+	/* lookup pending change in .gitmodules that is neither in HEAD nor index */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "sm_gitmodules_only"));
+	cl_assert(sm);
+
+	/* lookup git repo subdir that is not added as submodule */
+	cl_assert_equal_i(GIT_EEXISTS, git_submodule_lookup(&sm, g_repo, "not-submodule"));
+
+	/* lookup existing directory that is not a submodule */
+	cl_assert_equal_i(GIT_ENOTFOUND, git_submodule_lookup(&sm, g_repo, "just_a_dir"));
+
+	/* lookup existing file that is not a submodule */
+	cl_assert_equal_i(GIT_ENOTFOUND, git_submodule_lookup(&sm, g_repo, "just_a_file"));
+
+	/* lookup non-existent item */
+	cl_assert_equal_i(GIT_ENOTFOUND, git_submodule_lookup(&sm, g_repo, "no_such_file"));
+}
+
+void test_submodule_lookup__lookup_even_with_missing_index(void)
+{
+	git_index *idx;
+	git_submodule *sm;
+
+	/* give the repo an empty index */
+	cl_git_pass(git_index_new(&idx));
+	git_repository_set_index(g_repo, idx);
+	git_index_free(idx);
+
+	/* lookup existing */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "sm_unchanged"));
+	cl_assert(sm);
+
+	/* lookup pending change in .gitmodules that is not in HEAD */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "sm_added_and_uncommited"));
+	cl_assert(sm);
+
+	/* lookup pending change in .gitmodules that is neither in HEAD nor index */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "sm_gitmodules_only"));
+	cl_assert(sm);
+
+	/* lookup git repo subdir that is not added as submodule */
+	cl_assert_equal_i(GIT_EEXISTS, git_submodule_lookup(&sm, g_repo, "not-submodule"));
+
+	/* lookup existing directory that is not a submodule */
+	cl_assert_equal_i(GIT_ENOTFOUND, git_submodule_lookup(&sm, g_repo, "just_a_dir"));
+
+	/* lookup existing file that is not a submodule */
+	cl_assert_equal_i(GIT_ENOTFOUND, git_submodule_lookup(&sm, g_repo, "just_a_file"));
+
+	/* lookup non-existent item */
+	cl_assert_equal_i(GIT_ENOTFOUND, git_submodule_lookup(&sm, g_repo, "no_such_file"));
+}