builtin/submodule--helper.c: handle missing submodule URLs

In e0a862f (submodule helper: convert relative URL to absolute URL if
needed, 2018-10-16), `prepare_to_clone_next_submodule()` lost the
ability to handle URL-less submodules, due to a change from:

    if (repo_get_config_string_const(the_repostiory, sb.buf, &url))
        url = sub->url;

to

    if (repo_get_config_string_const(the_repostiory, sb.buf, &url)) {
        if (starts_with_dot_slash(sub->url) ||
            starts_with_dot_dot_slash(sub->url)) {
                /* ... */
            }
    }

, which will segfault when `sub->url` is NULL, since both
`starts_with_dot_slash()` does not guard its arguments as non-NULL.

Guard the checks to both of the above functions by first checking
whether `sub->url` is non-NULL. There is no need to check whether `sub`
itself is NULL, since we already perform this check earlier in
`prepare_to_clone_next_submodule()`.

By adding a NULL-ness check on `sub->url`, we'll fall into the 'else'
branch, setting `url` to `sub->url` (which is NULL). Before attempting
to invoke `git submodule--helper clone`, check whether `url` is NULL,
and die() if it is.

Reported-by: Tribo Dar <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

builtin/submodule--helper.c

@@ -2016,14 +2016,17 @@ static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "submodule.%s.url", sub->name);
 	if (repo_config_get_string_tmp(the_repository, sb.buf, &url)) {
-		if (starts_with_dot_slash(sub->url) ||
-		    starts_with_dot_dot_slash(sub->url)) {
+		if (sub->url && (starts_with_dot_slash(sub->url) ||
+				 starts_with_dot_dot_slash(sub->url))) {
 			url = resolve_relative_url(sub->url, NULL, 0);
 			need_free_url = 1;
 		} else
 			url = sub->url;
 	}
 
+	if (!url)
+		die(_("cannot clone submodule '%s' without a URL"), sub->name);
+
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/.git", ce->name);
 	needs_cloning = !file_exists(sb.buf);

t/t7400-submodule-basic.sh

@@ -1351,6 +1351,22 @@ test_expect_success 'clone active submodule without submodule url set' '
 	)
 '
 
+test_expect_success 'update submodules without url set in .gitconfig' '
+	test_when_finished "rm -rf multisuper_clone" &&
+	git clone file://"$pwd"/multisuper multisuper_clone &&
+
+	git -C multisuper_clone submodule init &&
+	for s in sub0 sub1 sub2 sub3
+	do
+		key=submodule.$s.url &&
+		git -C multisuper_clone config --local --unset $key &&
+		git -C multisuper_clone config --file .gitmodules --unset $key || return 1
+	done &&
+
+	test_must_fail git -C multisuper_clone submodule update 2>err &&
+	grep "cannot clone submodule .sub[0-3]. without a URL" err
+'
+
 test_expect_success 'clone --recurse-submodules with a pathspec works' '
 	test_when_finished "rm -rf multisuper_clone" &&
 	cat >expected <<-\EOF &&