selftests: mptcp: join: skip Fastclose tests if not supported

Selftests are supposed to run on any kernels, including the old ones not
supporting all MPTCP features.

One of them is the support of MP_FASTCLOSE introduced in commit
f284c0c7 ("mptcp: implement fastclose xmit path").

If the MIB counter is not available, the test cannot be verified and the
behaviour will not be the expected one. So we can skip the test if the
counter is missing.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 01542c9b ("selftests: mptcp: add fastclose testcase")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -261,6 +261,19 @@ reset()
 	return 0
 }
 
+# $1: test name ; $2: counter to check
+reset_check_counter()
+{
+	reset "${1}" || return 1
+
+	local counter="${2}"
+
+	if ! nstat -asz "${counter}" | grep -wq "${counter}"; then
+		mark_as_skipped "counter '${counter}' is not available"
+		return 1
+	fi
+}
+
 # $1: test name
 reset_with_cookies()
 {
@@ -3121,14 +3134,14 @@ fullmesh_tests()
 
 fastclose_tests()
 {
-	if reset "fastclose test"; then
+	if reset_check_counter "fastclose test" "MPTcpExtMPFastcloseTx"; then
 		run_tests $ns1 $ns2 10.0.1.1 1024 0 fastclose_client
 		chk_join_nr 0 0 0
 		chk_fclose_nr 1 1
 		chk_rst_nr 1 1 invert
 	fi
 
-	if reset "fastclose server test"; then
+	if reset_check_counter "fastclose server test" "MPTcpExtMPFastcloseRx"; then
 		run_tests $ns1 $ns2 10.0.1.1 1024 0 fastclose_server
 		chk_join_nr 0 0 0
 		chk_fclose_nr 1 1 invert