client-go: NewSelfSignedCACert makes Go 1.15+ compatible cert (#100324)

* NewSelfSignedCACert makes Go 1.15+ compatible cert

As of Go 1.15, X.509 certificates without a SAN no longer
fall back to the CommonName of the certificate.

https://golang.org/doc/go1.15#commonname

Updating NewSelfSignedCACert to produce certificates that
work with this change.

* add missing license

Kubernetes-commit: a41a3ed4ae77a7902577bdb50b35fc5f42ed7c60

util/cert/cert_test.go

@@ -0,0 +1,46 @@
+/*
+Copyright 2021 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.
+*/
+
+package cert_test
+
+import (
+	cryptorand "crypto/rand"
+	"crypto/rsa"
+	"testing"
+
+	"k8s.io/client-go/util/cert"
+)
+
+const COMMON_NAME = "foo.example.com"
+
+// TestSelfSignedCertHasSAN verifies the existing of
+// a SAN on the generated self-signed certificate.
+// a SAN ensures that the certificate is considered
+// valid by default in go 1.15 and above, which
+// turns off fallback to Common Name by default.
+func TestSelfSignedCertHasSAN(t *testing.T) {
+	key, err := rsa.GenerateKey(cryptorand.Reader, 2048)
+	if err != nil {
+		t.Fatalf("rsa key failed to generate: %v", err)
+	}
+	selfSignedCert, err := cert.NewSelfSignedCACert(cert.Config{CommonName: COMMON_NAME}, key)
+	if err != nil {
+		t.Fatalf("self signed certificate failed to generate: %v", err)
+	}
+	if len(selfSignedCert.DNSNames) == 0 {
+		t.Fatalf("self signed certificate has zero DNS names.")
+	}
+}