transport-helper: clarify pushing without refspecs

This has never worked, since it's inception the code simply skips all
the refs, essentially telling fast-export to do nothing.

Let's at least tell the user what's going on.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitremote-helpers.txt

@@ -162,8 +162,8 @@ Miscellaneous capabilities
 	For remote helpers that implement 'import' or 'export', this capability
 	allows the refs to be constrained to a private namespace, instead of
 	writing to refs/heads or refs/remotes directly.
-	It is recommended that all importers providing the 'import' or 'export'
-	capabilities use this.
+	It is recommended that all importers providing the 'import'
+	capability use this. It's mandatory for 'export'.
 +
 A helper advertising the capability
 `refspec refs/heads/*:refs/svn/origin/branches/*`

t/t5801-remote-helpers.sh

@@ -111,13 +111,13 @@ test_expect_success 'pulling without refspecs' '
 	compare_refs local2 HEAD server HEAD
 '
 
-test_expect_failure 'pushing without refspecs' '
+test_expect_success 'pushing without refspecs' '
 	test_when_finished "(cd local2 && git reset --hard origin)" &&
 	(cd local2 &&
 	echo content >>file &&
 	git commit -a -m ten &&
-	GIT_REMOTE_TESTGIT_REFSPEC="" git push) &&
-	compare_refs local2 HEAD server HEAD
+	GIT_REMOTE_TESTGIT_REFSPEC="" test_must_fail git push 2>../error) &&
+	grep "remote-helper doesn.t support push; refspec needed" error
 '
 
 test_expect_success 'pulling without marks' '

transport-helper.c

@@ -785,6 +785,9 @@ static int push_refs_with_export(struct transport *transport,
 	struct string_list revlist_args = STRING_LIST_INIT_NODUP;
 	struct strbuf buf = STRBUF_INIT;
 
+	if (!data->refspecs)
+		die("remote-helper doesn't support push; refspec needed");
+
 	helper = get_helper(transport);
 
 	write_constant(helper->in, "export\n");
@@ -795,8 +798,6 @@ static int push_refs_with_export(struct transport *transport,
 		char *private;
 		unsigned char sha1[20];
 
-		if (!data->refspecs)
-			continue;
 		private = apply_refspecs(data->refspecs, data->refspec_nr, ref->name);
 		if (private && !get_sha1(private, sha1)) {
 			strbuf_addf(&buf, "^%s", private);