t/interop: add test of old clients against modern git-daemon

This test just checks that old clients can clone and fetch
from a newer git-daemon. The opposite should also be true,
but it's hard to test ancient versions of git-daemon because
they lack basic options like "--listen".

Note that we have to make a slight tweak to the
lib-git-daemon helper from the regular tests, so that it
starts the daemon with our correct git.a version.

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

Author: peff

t/interop/i5500-git-daemon.sh

@@ -0,0 +1,41 @@
+#!/bin/sh
+
+VERSION_A=.
+VERSION_B=v1.0.0
+
+: ${LIB_GIT_DAEMON_PORT:=5500}
+LIB_GIT_DAEMON_COMMAND='git.a daemon'
+
+test_description='clone and fetch by older client'
+. ./interop-lib.sh
+. "$TEST_DIRECTORY"/lib-git-daemon.sh
+
+start_git_daemon --export-all
+
+repo=$GIT_DAEMON_DOCUMENT_ROOT_PATH/repo
+
+test_expect_success "create repo served by $VERSION_A" '
+	git.a init "$repo" &&
+	git.a -C "$repo" commit --allow-empty -m one
+'
+
+test_expect_success "clone with $VERSION_B" '
+	git.b clone "$GIT_DAEMON_URL/repo" child &&
+	echo one >expect &&
+	git.a -C child log -1 --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success "fetch with $VERSION_B" '
+	git.a -C "$repo" commit --allow-empty -m two &&
+	(
+		cd child &&
+		git.b fetch
+	) &&
+	echo two >expect &&
+	git.a -C child log -1 --format=%s FETCH_HEAD >actual &&
+	test_cmp expect actual
+'
+
+stop_git_daemon
+test_done

t/lib-git-daemon.sh

@@ -46,7 +46,8 @@ start_git_daemon() {
 
 	say >&3 "Starting git daemon ..."
 	mkfifo git_daemon_output
-	git daemon --listen=127.0.0.1 --port="$LIB_GIT_DAEMON_PORT" \
+	${LIB_GIT_DAEMON_COMMAND:-git daemon} \
+		--listen=127.0.0.1 --port="$LIB_GIT_DAEMON_PORT" \
 		--reuseaddr --verbose \
 		--base-path="$GIT_DAEMON_DOCUMENT_ROOT_PATH" \
 		"$@" "$GIT_DAEMON_DOCUMENT_ROOT_PATH" \