bug: Fakeclient: Do not consider an apply patch to be a strategic merge patch

The fakeclient currently considers an apply patch to be a strategic
merge patch. This is completely wrong, those are different things and
apply patches are not supported in the fakeclientl, because in order to
support them it would need the entire SSA logic which isn't implemented
in upstream yet.

Author: alvaroaleman

pkg/client/fake/client.go

@@ -947,16 +947,18 @@ func dryPatch(action testing.PatchActionImpl, tracker testing.ObjectTracker) (ru
 		if err := json.Unmarshal(modified, obj); err != nil {
 			return nil, err
 		}
-	case types.StrategicMergePatchType, types.ApplyPatchType:
+	case types.StrategicMergePatchType:
 		mergedByte, err := strategicpatch.StrategicMergePatch(old, action.GetPatch(), obj)
 		if err != nil {
 			return nil, err
 		}
 		if err = json.Unmarshal(mergedByte, obj); err != nil {
 			return nil, err
 		}
+	case types.ApplyPatchType:
+		return nil, errors.New("apply patches are not supported in the fake client. Follow https://github.com/kubernetes/kubernetes/issues/115598 for the current status")
 	default:
-		return nil, fmt.Errorf("PatchType is not supported")
+		return nil, fmt.Errorf("%s PatchType is not supported", action.GetPatchType())
 	}
 	return obj, nil
 }

pkg/client/fake/client_test.go

@@ -359,6 +359,26 @@ var _ = Describe("Fake client", func() {
 			Expect(list.Items).To(ConsistOf(*dep2))
 		})
 
+		It("should reject apply patches, they are not supported in the fake client", func() {
+			By("Creating a new configmap")
+			cm := &corev1.ConfigMap{
+				TypeMeta: metav1.TypeMeta{
+					APIVersion: "v1",
+					Kind:       "ConfigMap",
+				},
+				ObjectMeta: metav1.ObjectMeta{
+					Name:      "new-test-cm",
+					Namespace: "ns2",
+				},
+			}
+			err := cl.Create(context.Background(), cm)
+			Expect(err).ToNot(HaveOccurred())
+
+			cm.Data = map[string]string{"foo": "bar"}
+			err = cl.Patch(context.Background(), cm, client.Apply, client.ForceOwnership)
+			Expect(err).To(MatchError(ContainSubstring("apply patches are not supported in the fake client")))
+		})
+
 		It("should be able to Create", func() {
 			By("Creating a new configmap")
 			newcm := &corev1.ConfigMap{