selftests: mptcp: connect: skip if MPTCP is not supported

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

A new check is then added to make sure MPTCP is supported. If not, the
test stops and is marked as "skipped". Note that this check can also
mark the test as failed if 'SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES' env
var is set to 1: by doing that, we can make sure a test is not being
skipped by mistake.

A new shared file is added here to be able to re-used the same check in
the different selftests we have.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 048d19d444be ("mptcp: add basic kselftest for mptcp")
Cc: [e-mail]
Acked-by: Paolo Abeni <[e-mail]>
Signed-off-by: Matthieu Baerts <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

tools/testing/selftests/net/mptcp/Makefile

···
 
 TEST_GEN_FILES = mptcp_connect pm_nl_ctl mptcp_sockopt mptcp_inq
 
-TEST_FILES := settings
+TEST_FILES := mptcp_lib.sh settings
 
 EXTRA_CLEAN := *.pcap
 

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

···
 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+. "$(dirname "${0}")/mptcp_lib.sh"
+
 time_start=$(date +%s)
 
 optstring="S:R:d:e:l:r:h4cm:f:tC"
···
 		rm -f /tmp/$netns.{nstat,out}
 	done
 }
+
+mptcp_lib_check_mptcp
 
 ip -Version > /dev/null 2>&1
 if [ $? -ne 0 ];then

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

···
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+readonly KSFT_FAIL=1
+readonly KSFT_SKIP=4
+
+# SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES env var can be set when validating all
+# features using the last version of the kernel and the selftests to make sure
+# a test is not being skipped by mistake.
+mptcp_lib_expect_all_features() {
+	[ "${SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES:-}" = "1" ]
+}
+
+# $1: msg
+mptcp_lib_fail_if_expected_feature() {
+	if mptcp_lib_expect_all_features; then
+		echo "ERROR: missing feature: ${*}"
+		exit ${KSFT_FAIL}
+	fi
+
+	return 1
+}
+
+# $1: file
+mptcp_lib_has_file() {
+	local f="${1}"
+
+	if [ -f "${f}" ]; then
+		return 0
+	fi
+
+	mptcp_lib_fail_if_expected_feature "${f} file not found"
+}
+
+mptcp_lib_check_mptcp() {
+	if ! mptcp_lib_has_file "/proc/sys/net/mptcp/enabled"; then
+		echo "SKIP: MPTCP support is not available"
+		exit ${KSFT_SKIP}
+	fi
+}