submodule.c: submodule_move_head works with broken submodules

Early on in submodule_move_head just after the check if the submodule is
initialized, we need to check if the submodule is populated correctly.

If the submodule is initialized but doesn't look like it is populated,
this is a red flag and can indicate multiple sorts of failures:
(1) The submodule may be recorded at an object name, that is missing.
(2) The submodule '.git' file link may be broken and it is not pointing
    at a repository.

In both cases we want to complain to the user in the non-forced mode,
and in the forced mode ignoring the old state and just moving the
submodule into its new state with a fixed '.git' file link.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -1332,10 +1332,24 @@ int submodule_move_head(const char *path,
 	int ret = 0;
 	struct child_process cp = CHILD_PROCESS_INIT;
 	const struct submodule *sub;
+	int *error_code_ptr, error_code;
 
 	if (!is_submodule_initialized(path))
 		return 0;
 
+	if (flags & SUBMODULE_MOVE_HEAD_FORCE)
+		/*
+		 * Pass non NULL pointer to is_submodule_populated_gently
+		 * to prevent die()-ing. We'll use connect_work_tree_and_git_dir
+		 * to fixup the submodule in the force case later.
+		 */
+		error_code_ptr = &error_code;
+	else
+		error_code_ptr = NULL;
+
+	if (old && !is_submodule_populated_gently(path, error_code_ptr))
+		return 0;
+
 	sub = submodule_from_path(null_sha1, path);
 
 	if (!sub)
@@ -1353,15 +1367,21 @@ int submodule_move_head(const char *path,
 				absorb_git_dir_into_superproject("", path,
 					ABSORB_GITDIR_RECURSE_SUBMODULES);
 		} else {
-			struct strbuf sb = STRBUF_INIT;
-			strbuf_addf(&sb, "%s/modules/%s",
+			char *gitdir = xstrfmt("%s/modules/%s",
 				    get_git_common_dir(), sub->name);
-			connect_work_tree_and_git_dir(path, sb.buf);
-			strbuf_release(&sb);
+			connect_work_tree_and_git_dir(path, gitdir);
+			free(gitdir);
 
 			/* make sure the index is clean as well */
 			submodule_reset_index(path);
 		}
+
+		if (old && (flags & SUBMODULE_MOVE_HEAD_FORCE)) {
+			char *gitdir = xstrfmt("%s/modules/%s",
+				    get_git_common_dir(), sub->name);
+			connect_work_tree_and_git_dir(path, gitdir);
+			free(gitdir);
+		}
 	}
 
 	prepare_submodule_repo_env_no_git_dir(&cp.env_array);

t/lib-submodule-update.sh

@@ -1213,14 +1213,31 @@ test_submodule_forced_switch_recursing () {
 		)
 	'
 	# Updating a submodule from an invalid sha1 updates
-	test_expect_success "$command: modified submodule does not update submodule work tree from invalid commit" '
+	test_expect_success "$command: modified submodule does update submodule work tree from invalid commit" '
 		prolog &&
 		reset_work_tree_to_interested invalid_sub1 &&
 		(
 			cd submodule_update &&
 			git branch -t valid_sub1 origin/valid_sub1 &&
-			test_must_fail $command valid_sub1 &&
-			test_superproject_content origin/invalid_sub1
+			$command valid_sub1 &&
+			test_superproject_content origin/valid_sub1 &&
+			test_submodule_content sub1 origin/valid_sub1
+		)
+	'
+
+	# Old versions of Git were buggy writing the .git link file
+	# (e.g. before f8eaa0ba98b and then moving the superproject repo
+	# whose submodules contained absolute paths)
+	test_expect_success "$command: updating submodules fixes .git links" '
+		prolog &&
+		reset_work_tree_to_interested add_sub1 &&
+		(
+			cd submodule_update &&
+			git branch -t modify_sub1 origin/modify_sub1 &&
+			echo "gitdir: bogus/path" >sub1/.git &&
+			$command modify_sub1 &&
+			test_superproject_content origin/modify_sub1 &&
+			test_submodule_content sub1 origin/modify_sub1
 		)
 	'
 }