.*: add verify script for go directive changes

This commit adds a script that checks the changed version in
the go.mod file with a certain maximum version that the go
directive can have.

We set the maximum version of the go directive as 1.20 here
because the oldest go directive that exists on our supported
release branches in k/k is 1.20.

Signed-off-by: Madhav Jivrajani <[email protected]>

Author: MadhavJivrajani

Makefile

@@ -4,6 +4,12 @@ all:
 
 test:
 	go test -race ./...
-	./hack/verify-examples.sh
 	go -C v2 test -race ./...
 	(cd v2 && ./hack/verify-examples.sh)
+
+# We verify for the maximum version of the go directive as 1.20
+# here because the oldest go directive that exists on our supported
+# release branches in k/k is 1.20.
+verify:
+	./hack/verify-examples.sh
+	./hack/verify-go-directive.sh 1.20

hack/verify-go-directive.sh

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+
+# Copyright 2024 The Kubernetes Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+function usage {
+  local script="$(basename $0)"
+
+  echo >&2 "Usage: ${script} <maximum go directive>
+This script should be run at the root of a module.
+
+Compare the go directive in the local working copy's go.mod
+to the specified maximum version it can be. Versions provided
+here are of the form 1.x.y, without the 'go' prefix.
+Examples:
+  ${script} 1.20
+  ${script} 1.21.6
+"
+  exit 1
+}
+
+max="$1"
+# If max is empty, print usage and error
+if [[ -z "${max}" ]]; then
+  usage;
+fi
+
+# Don't specify the version with the go prefix, just 1.x.y will do.
+if [[ ! "${max}" =~ ^[0-9]\.[0-9]+(\.[0-9]+)?$ ]]; then
+    usage
+fi
+
+current=$(awk '/^go / {print $2;}' go.mod)
+if [[ -z "${current}" ]]; then
+  echo >&2 "FAIL: could not get value of go directive from go.mod"
+  exit 1
+fi
+
+if ! printf '%s\n' "${current}" "${max}" | sort --check=silent --version-sort; then
+    echo >&2 "FAIL: current Go directive ${current} is greater than ${max}"
+    exit 1
+fi