usage: allow dying without writing an error message

Sometimes code wants to die in a situation where it already has written
an error message. To use the same error code as `die()` we have to use
`exit(128)`, which is easy to get wrong and leaves magic numbers all
over our codebase.

Teach `die_message_builtin()` to not print any error when passed a
`NULL` pointer as error string. Like this, such users can now call
`die(NULL)` to achieve the same result without any hardcoded error
codes.

Adapt a couple of builtins to use this new pattern to demonstrate that
there is a need for such a helper.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1000,7 +1000,7 @@ static void am_setup(struct am_state *state, enum patch_format patch_format,
 
 	if (!patch_format) {
 		fprintf_ln(stderr, _("Patch format detection failed."));
-		exit(128);
+		die(NULL);
 	}
 
 	if (mkdir(state->dir, 0777) < 0 && errno != EEXIST)
@@ -1178,7 +1178,7 @@ static void NORETURN die_user_resolve(const struct am_state *state)
 		strbuf_release(&sb);
 	}
 
-	exit(128);
+	die(NULL);
 }
 
 /**

builtin/checkout.c

@@ -838,7 +838,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 		init_tree_desc(&trees[0], &tree->object.oid,
 			       tree->buffer, tree->size);
 		if (parse_tree(new_tree) < 0)
-			exit(128);
+			die(NULL);
 		tree = new_tree;
 		init_tree_desc(&trees[1], &tree->object.oid,
 			       tree->buffer, tree->size);
@@ -913,7 +913,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 						     work,
 						     old_tree);
 			if (ret < 0)
-				exit(128);
+				die(NULL);
 			ret = reset_tree(new_tree,
 					 opts, 0,
 					 writeout_error, new_branch_info);

builtin/fetch.c

@@ -992,7 +992,7 @@ static int update_local_ref(struct ref *ref,
 		fast_forward = repo_in_merge_bases(the_repository, current,
 						   updated);
 		if (fast_forward < 0)
-			exit(128);
+			die(NULL);
 		forced_updates_ms += (getnanotime() - t_before) / 1000000;
 	} else {
 		fast_forward = 1;

builtin/submodule--helper.c

@@ -303,7 +303,7 @@ static void runcommand_in_submodule_cb(const struct cache_entry *list_item,
 	char *displaypath;
 
 	if (validate_submodule_path(path) < 0)
-		exit(128);
+		die(NULL);
 
 	displaypath = get_submodule_displaypath(path, info->prefix,
 						info->super_prefix);
@@ -643,7 +643,7 @@ static void status_submodule(const char *path, const struct object_id *ce_oid,
 	};
 
 	if (validate_submodule_path(path) < 0)
-		exit(128);
+		die(NULL);
 
 	if (!submodule_from_path(the_repository, null_oid(the_hash_algo), path))
 		die(_("no submodule mapping found in .gitmodules for path '%s'"),
@@ -1257,7 +1257,7 @@ static void sync_submodule(const char *path, const char *prefix,
 		return;
 
 	if (validate_submodule_path(path) < 0)
-		exit(128);
+		die(NULL);
 
 	sub = submodule_from_path(the_repository, null_oid(the_hash_algo), path);
 
@@ -1402,7 +1402,7 @@ static void deinit_submodule(const char *path, const char *prefix,
 	char *sub_git_dir = xstrfmt("%s/.git", path);
 
 	if (validate_submodule_path(path) < 0)
-		exit(128);
+		die(NULL);
 
 	sub = submodule_from_path(the_repository, null_oid(the_hash_algo), path);
 
@@ -1724,7 +1724,7 @@ static int clone_submodule(const struct module_clone_data *clone_data,
 	char *to_free = NULL;
 
 	if (validate_submodule_path(clone_data_path) < 0)
-		exit(128);
+		die(NULL);
 
 	if (!is_absolute_path(clone_data->path))
 		clone_data_path = to_free = xstrfmt("%s/%s", repo_get_work_tree(the_repository),
@@ -3524,7 +3524,7 @@ static int module_add(int argc, const char **argv, const char *prefix,
 	strip_dir_trailing_slashes(add_data.sm_path);
 
 	if (validate_submodule_path(add_data.sm_path) < 0)
-		exit(128);
+		die(NULL);
 
 	die_on_index_match(add_data.sm_path, force);
 	die_on_repo_without_commits(add_data.sm_path);

usage.c

@@ -67,6 +67,8 @@ static NORETURN void usage_builtin(const char *err, va_list params)
 
 static void die_message_builtin(const char *err, va_list params)
 {
+	if (!err)
+		return;
 	trace2_cmd_error_va(err, params);
 	vreportf(_("fatal: "), err, params);
 }