FieldManager: Reset if we receive nil or a list with one empty item

test/integration/apiserver/apply/apply_test.go

@@ -1578,6 +1578,80 @@ func TestErrorsDontFailPatch(t *testing.T) {
 
 }
 
+// TestClearManagedFieldsWithUpdateEmptyList verifies it's possible to clear the managedFields by sending an empty list.
+func TestClearManagedFieldsWithUpdateEmptyList(t *testing.T) {
+	defer featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, genericfeatures.ServerSideApply, true)()
+
+	_, client, closeFn := setup(t)
+	defer closeFn()
+
+	_, err := client.CoreV1().RESTClient().Patch(types.ApplyPatchType).
+		Namespace("default").
+		Resource("configmaps").
+		Name("test-cm").
+		Param("fieldManager", "apply_test").
+		Body([]byte(`{
+			"apiVersion": "v1",
+			"kind": "ConfigMap",
+			"metadata": {
+				"name": "test-cm",
+				"namespace": "default",
+				"labels": {
+					"test-label": "test"
+				}
+			},
+			"data": {
+				"key": "value"
+			}
+		}`)).
+		Do(context.TODO()).
+		Get()
+	if err != nil {
+		t.Fatalf("Failed to create object using Apply patch: %v", err)
+	}
+
+	_, err = client.CoreV1().RESTClient().Put().
+		Namespace("default").
+		Resource("configmaps").
+		Name("test-cm").
+		Body([]byte(`{
+			"apiVersion": "v1",
+			"kind": "ConfigMap",
+			"metadata": {
+				"name": "test-cm",
+				"namespace": "default",
+				"managedFields": [],
+				"labels": {
+					"test-label": "test"
+				}
+			},
+			"data": {
+				"key": "value"
+			}
+		}`)).Do(context.TODO()).Get()
+	if err != nil {
+		t.Fatalf("Failed to patch object: %v", err)
+	}
+
+	object, err := client.CoreV1().RESTClient().Get().Namespace("default").Resource("configmaps").Name("test-cm").Do(context.TODO()).Get()
+	if err != nil {
+		t.Fatalf("Failed to retrieve object: %v", err)
+	}
+
+	accessor, err := meta.Accessor(object)
+	if err != nil {
+		t.Fatalf("Failed to get meta accessor: %v", err)
+	}
+
+	if managedFields := accessor.GetManagedFields(); len(managedFields) != 0 {
+		t.Fatalf("Failed to clear managedFields, got: %v", managedFields)
+	}
+
+	if labels := accessor.GetLabels(); len(labels) < 1 {
+		t.Fatalf("Expected other fields to stay untouched, got: %v", object)
+	}
+}
+
 var podBytes = []byte(`
 apiVersion: v1
 kind: Pod