Fix custom defaulter from deleting unknown fields

Author: alculquicondor

pkg/webhook/admission/defaulter_custom.go

@@ -71,24 +71,32 @@ func (h *defaulterForType) Handle(ctx context.Context, req Request) Response {
 ctx = NewContextWithRequest(ctx, req)
 
 // Get the object in the request
-obj := h.object.DeepCopyObject()
-if err := h.decoder.Decode(req, obj); err != nil {
+original := h.object.DeepCopyObject()
+if err := h.decoder.Decode(req, original); err != nil {
 return Errored(http.StatusBadRequest, err)
 }
 
 // Default the object
-if err := h.defaulter.Default(ctx, obj); err != nil {
+updated := original.DeepCopyObject()
+if err := h.defaulter.Default(ctx, updated); err != nil {
 var apiStatus apierrors.APIStatus
 if errors.As(err, &apiStatus) {
 return validationResponseFromStatus(false, apiStatus.Status())
 }
 return Denied(err.Error())
 }
 
-// Create the patch
-marshalled, err := json.Marshal(obj)
+// Create the patch.
+// We need to decode and marshall the original because the type registered in the
+// decoder might not match the latest version of the API.
+// Creating a diff from the raw object might cause new fields to be dropped.
+marshalledOriginal, err := json.Marshal(original)
 if err != nil {
 return Errored(http.StatusInternalServerError, err)
 }
-return PatchResponseFromRaw(req.Object.Raw, marshalled)
+marshalledUpdated, err := json.Marshal(updated)
+if err != nil {
+return Errored(http.StatusInternalServerError, err)
+}
+return PatchResponseFromRaw(marshalledOriginal, marshalledUpdated)
 }

pkg/webhook/admission/defaulter_custom_test.go

@@ -20,6 +20,7 @@ import (
 
 . "github.com/onsi/ginkgo/v2"
 . "github.com/onsi/gomega"
+"gomodules.xyz/jsonpatch/v2"
 
 admissionv1 "k8s.io/api/admission/v1"
 "k8s.io/apimachinery/pkg/runtime"
@@ -28,6 +29,27 @@ import (
 
 var _ = Describe("Defaulter Handler", func() {
 
+It("should should not lose unknown fields", func() {
+obj := &TestDefaulter{}
+handler := WithCustomDefaulter(admissionScheme, obj, &TestCustomDefaulter{})
+
+resp := handler.Handle(context.TODO(), Request{
+AdmissionRequest: admissionv1.AdmissionRequest{
+Operation: admissionv1.Create,
+Object: runtime.RawExtension{
+Raw: []byte(`{"newField":"foo"}`),
+},
+},
+})
+Expect(resp.Allowed).Should(BeTrue())
+Expect(resp.Patches).To(Equal([]jsonpatch.JsonPatchOperation{{
+Operation: "add",
+Path: "/replica",
+Value: 2.0,
+}}))
+Expect(resp.Result.Code).Should(Equal(int32(http.StatusOK)))
+})
+
 It("should return ok if received delete verb in defaulter handler", func() {
 obj := &TestDefaulter{}
 handler := WithCustomDefaulter(admissionScheme, obj, &TestCustomDefaulter{})