In e0a862fdaf (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 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 <redacted>
Signed-off-by: Taylor Blau <redacted>
---

Junio: feel free to queue either, but after thinking on v1 a little more
I think that it is pretty gross and that this version is much cleaner.

Range-diff against v1:
1:  f7a8de14fe ! 1:  bc204f450a builtin/submodule--helper.c: handle missing submodule URLs
    @@ Commit message
         `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). When we then try
    -    and invoke `git submodule-helper` below, we'll get the expected error
    -    message:
    -
    -        error: option `url' requires a value
    -
    -    Note that this means that `--url` and its value must come last when
    -    preparing the argument list, to avoid confusing the next argument as the
    -    value for `--url` if `url` itself is NULL.
    +    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 [off-list ref]
         Signed-off-by: Taylor Blau [off-list ref]
    @@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const st
      			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);
     @@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
    + 	if (suc->update_data->filter_options && suc->update_data->filter_options->choice)
    + 		strvec_pushf(&child->args, "--filter=%s",
    + 			     expand_list_objects_filter_spec(suc->update_data->filter_options));
    ++	strvec_pushl(&child->args, "--url", url, NULL);
    + 	if (suc->update_data->require_init)
      		strvec_push(&child->args, "--require-init");
      	strvec_pushl(&child->args, "--path", sub->path, NULL);
      	strvec_pushl(&child->args, "--name", sub->name, NULL);
    @@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const st
      	if (suc->update_data->references.nr) {
      		struct string_list_item *item;

    -@@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
    - 		strvec_push(&child->args, suc->update_data->single_branch ?
    - 					      "--single-branch" :
    - 					      "--no-single-branch");
    -+	/*
    -+	 * `url` might be NULL, so ensure that this argument is passed
    -+	 * last to avoid confusing the next argument as the value for
    -+	 * `--url`.
    -+	 */
    -+	strvec_pushl(&child->args, "--url", url, NULL);
    -
    - cleanup:
    - 	free(displaypath);

      ## t/t7400-submodule-basic.sh ##
     @@ t/t7400-submodule-basic.sh: test_expect_success 'clone active submodule without submodule url set' '
    @@ t/t7400-submodule-basic.sh: test_expect_success 'clone active submodule without
     +	done &&
     +
     +	test_must_fail git -C multisuper_clone submodule update 2>err &&
    -+	grep "option .url. requires a value" err
    ++	grep "cannot clone submodule .sub[0-3]. without a URL" err
     +'
     +
      test_expect_success 'clone --recurse-submodules with a pathspec works' '

 builtin/submodule--helper.c |  9 ++++++---
 t/t7400-submodule-basic.sh  | 16 ++++++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 6bf8d666ce..efd0761cd0 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c

@@ -2024,14 +2024,17 @@ 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);

@@ -2065,11 +2068,11 @@ static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
 	if (suc->update_data->filter_options && suc->update_data->filter_options->choice)
 		strvec_pushf(&child->args, "--filter=%s",
 			     expand_list_objects_filter_spec(suc->update_data->filter_options));
+	strvec_pushl(&child->args, "--url", url, NULL);
 	if (suc->update_data->require_init)
 		strvec_push(&child->args, "--require-init");
 	strvec_pushl(&child->args, "--path", sub->path, NULL);
 	strvec_pushl(&child->args, "--name", sub->name, NULL);
-	strvec_pushl(&child->args, "--url", url, NULL);
 	if (suc->update_data->references.nr) {
 		struct string_list_item *item;

diff --git a/t/t7400-submodule-basic.sh b/t/t7400-submodule-basic.sh
index eae6a46ef3..d9fbabb2b9 100755
--- a/t/t7400-submodule-basic.sh
+++ b/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 &&
--

2.41.0.rc1.11.gf7a8de14fe.dirty