credential: treat URL with empty scheme as invalid

Until "credential: refuse to operate when missing host or protocol",
Git's credential handling code interpreted URLs with empty scheme to
mean "give me credentials matching this host for any protocol".

Luckily libcurl does not recognize such URLs (it tries to look for a
protocol named "" and fails). Just in case that changes, let's reject
them within Git as well. This way, credential_from_url is guaranteed to
always produce a "struct credential" with protocol and host set.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

credential.c

@@ -357,7 +357,7 @@ int credential_from_url_gently(struct credential *c, const char *url,
 	 *   (3) proto://<user>:<pass>@<host>/...
 	 */
 	proto_end = strstr(url, "://");
-	if (!proto_end) {
+	if (!proto_end || proto_end == url) {
 		if (!quiet)
 			warning(_("url has no scheme: %s"), url);
 		return -1;
@@ -382,8 +382,7 @@ int credential_from_url_gently(struct credential *c, const char *url,
 		host = at + 1;
 	}
 
-	if (proto_end - url > 0)
-		c->protocol = xmemdupz(url, proto_end - url);
+	c->protocol = xmemdupz(url, proto_end - url);
 	c->host = url_decode_mem(host, slash - host);
 	/* Trim leading and trailing slashes from path */
 	while (*slash == '/')

t/t5550-http-fetch-dumb.sh

@@ -314,6 +314,15 @@ test_expect_success 'remote-http complains cleanly about malformed urls' '
 	test_i18ngrep "url has no scheme" stderr
 '
 
+# NEEDSWORK: Writing commands to git-remote-curl can race against the latter
+# erroring out, producing SIGPIPE. Remove "ok=sigpipe" once transport-helper has
+# learned to handle early remote helper failures more cleanly.
+test_expect_success 'remote-http complains cleanly about empty scheme' '
+	test_must_fail ok=sigpipe git ls-remote \
+		http::${HTTPD_URL#http}/dumb/repo.git 2>stderr &&
+	test_i18ngrep "url has no scheme" stderr
+'
+
 test_expect_success 'redirects can be forbidden/allowed' '
 	test_must_fail git -c http.followRedirects=false \
 		clone $HTTPD_URL/dumb-redir/repo.git dumb-redir &&

t/t7416-submodule-dash-url.sh

@@ -92,6 +92,38 @@ test_expect_success 'fsck rejects relative URL resolving to missing scheme' '
 	grep gitmodulesUrl err
 '
 
+test_expect_success 'fsck rejects empty URL scheme' '
+	git checkout --orphan empty-scheme &&
+	cat >.gitmodules <<-\EOF &&
+	[submodule "foo"]
+		url = http::://one.example.com/foo.git
+	EOF
+	git add .gitmodules &&
+	test_tick &&
+	git commit -m "gitmodules with empty URL scheme" &&
+	test_when_finished "rm -rf dst" &&
+	git init --bare dst &&
+	git -C dst config transfer.fsckObjects true &&
+	test_must_fail git push dst HEAD 2>err &&
+	grep gitmodulesUrl err
+'
+
+test_expect_success 'fsck rejects relative URL resolving to empty scheme' '
+	git checkout --orphan relative-empty-scheme &&
+	cat >.gitmodules <<-\EOF &&
+	[submodule "foo"]
+		url = ../../../:://one.example.com/foo.git
+	EOF
+	git add .gitmodules &&
+	test_tick &&
+	git commit -m "relative gitmodules URL resolving to empty scheme" &&
+	test_when_finished "rm -rf dst" &&
+	git init --bare dst &&
+	git -C dst config transfer.fsckObjects true &&
+	test_must_fail git push dst HEAD 2>err &&
+	grep gitmodulesUrl err
+'
+
 test_expect_success 'fsck permits embedded newline with unrecognized scheme' '
 	git checkout --orphan newscheme &&
 	cat >.gitmodules <<-\EOF &&