fetch: fix segfault in --negotiate-only without --negotiation-tip=*

The recent --negotiate-only option would segfault in the call to
oid_array_for_each() in negotiate_using_fetch() unless one or more
--negotiation-tip=* options were provided.

All of the other tests for the feature combine both, but nothing was
checking this assumption, let's do that and add a test for it. Fixes a
bug in 9c1e657 (fetch: teach independent negotiation (no
packfile), 2021-05-04).

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

builtin/fetch.c

@@ -1990,6 +1990,9 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		fetch_config_from_gitmodules(sfjc, rs);
 	}
 
+	if (negotiate_only && !negotiation_tip.nr)
+		die(_("--negotiate-only needs one or more --negotiate-tip=*"));
+
 	if (deepen_relative) {
 		if (deepen_relative < 0)
 			die(_("Negative depth in --deepen is not supported"));

t/t5702-protocol-v2.sh

@@ -599,6 +599,22 @@ setup_negotiate_only () {
 	test_commit -C client three
 }
 
+test_expect_success 'usage: --negotiate-only without --negotiation-tip' '
+	SERVER="server" &&
+	URI="file://$(pwd)/server" &&
+
+	setup_negotiate_only "$SERVER" "$URI" &&
+
+	cat >err.expect <<-\EOF &&
+	fatal: --negotiate-only needs one or more --negotiate-tip=*
+	EOF
+
+	test_must_fail git -c protocol.version=2 -C client fetch \
+		--negotiate-only \
+		origin 2>err.actual &&
+	test_cmp err.expect err.actual
+'
+
 test_expect_success 'file:// --negotiate-only' '
 	SERVER="server" &&
 	URI="file://$(pwd)/server" &&