add integration test to validate that ExternalName Services can set internalTrafficPolicy, but the field is dropped on read

Signed-off-by: Andrew Sy Kim <kim.andrewsy@gmail.com>

test/integration/service/service_test.go

@@ -73,3 +73,51 @@ func Test_ExternalNameServiceStopsDefaultingInternalTrafficPolicy(t *testing.T)
 		t.Errorf("service internalTrafficPolicy should be droppped but is set: %v", service.Spec.InternalTrafficPolicy)
 	}
 }
+
+// Test_ExternalNameServiceDropsInternalTrafficPolicy tests that Services accepts the internalTrafficPolicy field on Create,
+// but drops the field on read. This test exists due to historic reasons where the internalTrafficPolicy field was being defaulted
+// in older versions. New versions stop defauting the field and drop on read, but for compatibility reasons we still accept the field.
+func Test_ExternalNameServiceDropsInternalTrafficPolicy(t *testing.T) {
+	controlPlaneConfig := framework.NewIntegrationTestControlPlaneConfig()
+	_, server, closeFn := framework.RunAnAPIServer(controlPlaneConfig)
+	defer closeFn()
+
+	config := restclient.Config{Host: server.URL}
+	client, err := clientset.NewForConfig(&config)
+	if err != nil {
+		t.Fatalf("Error creating clientset: %v", err)
+	}
+
+	ns := framework.CreateTestingNamespace("test-external-name-drops-internal-traffic-policy", server, t)
+	defer framework.DeleteTestingNamespace(ns, server, t)
+
+	internalTrafficPolicy := corev1.ServiceInternalTrafficPolicyCluster
+	service := &corev1.Service{
+		ObjectMeta: metav1.ObjectMeta{
+			Name: "test-123",
+		},
+		Spec: corev1.ServiceSpec{
+			Type:                  corev1.ServiceTypeExternalName,
+			ExternalName:          "foo.bar.com",
+			InternalTrafficPolicy: &internalTrafficPolicy,
+		},
+	}
+
+	service, err = client.CoreV1().Services(ns.Name).Create(context.TODO(), service, metav1.CreateOptions{})
+	if err != nil {
+		t.Fatalf("Error creating test service: %v", err)
+	}
+
+	if service.Spec.InternalTrafficPolicy != nil {
+		t.Errorf("service internalTrafficPolicy should be droppped but is set: %v", service.Spec.InternalTrafficPolicy)
+	}
+
+	service, err = client.CoreV1().Services(ns.Name).Get(context.TODO(), service.Name, metav1.GetOptions{})
+	if err != nil {
+		t.Fatalf("error getting service: %v", err)
+	}
+
+	if service.Spec.InternalTrafficPolicy != nil {
+		t.Errorf("service internalTrafficPolicy should be droppped but is set: %v", service.Spec.InternalTrafficPolicy)
+	}
+}