submodule-config: ban submodule urls that start with dash

The previous commit taught the submodule code to invoke our
"git clone $url $path" with a "--" separator so that we
aren't confused by urls or paths that start with dashes.

However, that's just one code path. It's not clear if there
are others, and it would be an easy mistake to add one in
the future. Moreover, even with the fix in the previous
commit, it's quite hard to actually do anything useful with
such an entry. Any url starting with a dash must fall into
one of three categories:

 - it's meant as a file url, like "-path". But then any
   clone is not going to have the matching path, since it's
   by definition relative inside the newly created clone. If
   you spell it as "./-path", the submodule code sees the
   "/" and translates this to an absolute path, so it at
   least works (assuming the receiver has the same
   filesystem layout as you). But that trick does not apply
   for a bare "-path".

 - it's meant as an ssh url, like "-host:path". But this
   already doesn't work, as we explicitly disallow ssh
   hostnames that begin with a dash (to avoid option
   injection against ssh).

 - it's a remote-helper scheme, like "-scheme::data". This
   _could_ work if the receiver bends over backwards and
   creates a funny-named helper like "git-remote--scheme".
   But normally there would not be any helper that matches.

Since such a url does not work today and is not likely to do
anything useful in the future, let's simply disallow them
entirely. That protects the existing "git clone" path (in a
belt-and-suspenders way), along with any others that might
exist.

Our tests cover two cases:

  1. A file url with "./" continues to work, showing that
     there's an escape hatch for people with truly silly
     repo names.

  2. A url starting with "-" is rejected.

Note that we expect case (2) to fail, but it would have done
so even without this commit, for the reasons given above.
So instead of just expecting failure, let's also check for
the magic word "ignoring" on stderr. That lets us know that
we failed for the right reason.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule-config.c

@@ -367,6 +367,12 @@ static void warn_multiple_config(const unsigned char *treeish_name,
 			commit_string, name, option);
 }
 
+static void warn_command_line_option(const char *var, const char *value)
+{
+	warning(_("ignoring '%s' which may be interpreted as"
+		  " a command-line option: %s"), var, value);
+}
+
 struct parse_config_parameter {
 	struct submodule_cache *cache;
 	const unsigned char *treeish_name;
@@ -432,6 +438,8 @@ static int parse_config(const char *var, const char *value, void *data)
 	} else if (!strcmp(item.buf, "url")) {
 		if (!value) {
 			ret = config_error_nonbool(var);
+		} else if (looks_like_command_line_option(value)) {
+			warn_command_line_option(var, value);
 		} else if (!me->overwrite && submodule->url) {
 			warn_multiple_config(me->treeish_name, submodule->name,
 					"url");

t/t7416-submodule-dash-url.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+
+test_description='check handling of .gitmodule url with dash'
+. ./test-lib.sh
+
+test_expect_success 'create submodule with protected dash in url' '
+	git init upstream &&
+	git -C upstream commit --allow-empty -m base &&
+	mv upstream ./-upstream &&
+	git submodule add ./-upstream sub &&
+	git add sub .gitmodules &&
+	git commit -m submodule
+'
+
+test_expect_success 'clone can recurse submodule' '
+	test_when_finished "rm -rf dst" &&
+	git clone --recurse-submodules . dst &&
+	echo base >expect &&
+	git -C dst/sub log -1 --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'remove ./ protection from .gitmodules url' '
+	perl -i -pe "s{\./}{}" .gitmodules &&
+	git commit -am "drop protection"
+'
+
+test_expect_success 'clone rejects unprotected dash' '
+	test_when_finished "rm -rf dst" &&
+	test_must_fail git clone --recurse-submodules . dst 2>err &&
+	test_i18ngrep ignoring err
+'
+
+test_done