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

staging/src/k8s.io/apiserver/pkg/endpoints/handlers/fieldmanager/fieldmanager.go

@@ -18,6 +18,7 @@ package fieldmanager
 
 import (
 	"fmt"
+	"reflect"
 	"time"
 
 	"k8s.io/apimachinery/pkg/api/meta"
@@ -111,21 +112,25 @@ func newDefaultFieldManager(f Manager, objectCreater runtime.ObjectCreater, kind
 func (f *FieldManager) Update(liveObj, newObj runtime.Object, manager string) (object runtime.Object, err error) {
 	// If the object doesn't have metadata, we should just return without trying to
 	// set the managedFields at all, so creates/updates/patches will work normally.
-	if _, err = meta.Accessor(newObj); err != nil {
+	newAccessor, err := meta.Accessor(newObj)
+	if err != nil {
 		return newObj, nil
 	}
 
 	// First try to decode the managed fields provided in the update,
 	// This is necessary to allow directly updating managed fields.
 	var managed Managed
-	if managed, err = internal.DecodeObjectManagedFields(newObj); err != nil || len(managed.Fields()) == 0 {
+	if isResetManagedFields(newAccessor.GetManagedFields()) {
+		managed = internal.NewEmptyManaged()
+	} else if managed, err = internal.DecodeObjectManagedFields(newAccessor.GetManagedFields()); err != nil || len(managed.Fields()) == 0 {
+		liveAccessor, err := meta.Accessor(liveObj)
+		if err != nil {
+			return newObj, nil
+		}
 		// If the managed field is empty or we failed to decode it,
 		// let's try the live object. This is to prevent clients who
 		// don't understand managedFields from deleting it accidentally.
-		managed, err = internal.DecodeObjectManagedFields(liveObj)
-		if err != nil {
-			// If we also can't decode the liveObject, then
-			// just restart managedFields from scratch.
+		if managed, err = internal.DecodeObjectManagedFields(liveAccessor.GetManagedFields()); err != nil {
 			managed = internal.NewEmptyManaged()
 		}
 	}
@@ -163,17 +168,33 @@ func (f *FieldManager) UpdateNoErrors(liveObj, newObj runtime.Object, manager st
 	return obj
 }
 
+// Returns true if the managedFields indicate that the user is trying to
+// reset the managedFields, i.e. if the list is non-nil but empty, or if
+// the list has one empty item.
+func isResetManagedFields(managedFields []metav1.ManagedFieldsEntry) bool {
+	if len(managedFields) == 0 {
+		return managedFields != nil
+	}
+
+	if len(managedFields) == 1 {
+		return reflect.DeepEqual(managedFields[0], metav1.ManagedFieldsEntry{})
+	}
+
+	return false
+}
+
 // Apply is used when server-side apply is called, as it merges the
 // object and updates the managed fields.
 func (f *FieldManager) Apply(liveObj, appliedObj runtime.Object, manager string, force bool) (object runtime.Object, err error) {
 	// If the object doesn't have metadata, apply isn't allowed.
-	if _, err = meta.Accessor(liveObj); err != nil {
+	accessor, err := meta.Accessor(liveObj)
+	if err != nil {
 		return nil, fmt.Errorf("couldn't get accessor: %v", err)
 	}
 
 	// Decode the managed fields in the live object, since it isn't allowed in the patch.
-	var managed Managed
-	if managed, err = internal.DecodeObjectManagedFields(liveObj); err != nil {
+	managed, err := internal.DecodeObjectManagedFields(accessor.GetManagedFields())
+	if err != nil {
 		return nil, fmt.Errorf("failed to decode managed fields: %v", err)
 	}
 

staging/src/k8s.io/apiserver/pkg/endpoints/handlers/fieldmanager/fieldmanager_test.go

@@ -783,3 +783,86 @@ func TestNoOpChanges(t *testing.T) {
 		t.Fatalf("No-op apply has changed the object:\n%v\n---\n%v", before, f.liveObj)
 	}
 }
+
+// Tests that one can reset the managedFields by sending either an empty
+// list
+func TestResetManagedFieldsEmptyList(t *testing.T) {
+	f := NewTestFieldManager(schema.FromAPIVersionAndKind("v1", "Pod"))
+
+	obj := &unstructured.Unstructured{Object: map[string]interface{}{}}
+	if err := yaml.Unmarshal([]byte(`{
+		"apiVersion": "v1",
+		"kind": "Pod",
+		"metadata": {
+			"labels": {
+				"a": "b"
+			},
+		}
+	}`), &obj.Object); err != nil {
+		t.Fatalf("error decoding YAML: %v", err)
+	}
+	if err := f.Apply(obj, "fieldmanager_test_apply", false); err != nil {
+		t.Fatalf("failed to apply object: %v", err)
+	}
+
+	if err := yaml.Unmarshal([]byte(`{
+		"apiVersion": "v1",
+		"kind": "Pod",
+		"metadata": {
+			"managedFields": [],
+			"labels": {
+				"a": "b"
+			},
+		}
+	}`), &obj.Object); err != nil {
+		t.Fatalf("error decoding YAML: %v", err)
+	}
+	if err := f.Update(obj, "update_manager"); err != nil {
+		t.Fatalf("failed to update with empty manager: %v", err)
+	}
+
+	if len(f.ManagedFields()) != 0 {
+		t.Fatalf("failed to reset managedFields: %v", f.ManagedFields())
+	}
+}
+
+// Tests that one can reset the managedFields by sending either a list with one empty item.
+func TestResetManagedFieldsEmptyItem(t *testing.T) {
+	f := NewTestFieldManager(schema.FromAPIVersionAndKind("v1", "Pod"))
+
+	obj := &unstructured.Unstructured{Object: map[string]interface{}{}}
+	if err := yaml.Unmarshal([]byte(`{
+		"apiVersion": "v1",
+		"kind": "Pod",
+		"metadata": {
+			"labels": {
+				"a": "b"
+			},
+		}
+	}`), &obj.Object); err != nil {
+		t.Fatalf("error decoding YAML: %v", err)
+	}
+	if err := f.Apply(obj, "fieldmanager_test_apply", false); err != nil {
+		t.Fatalf("failed to apply object: %v", err)
+	}
+
+	if err := yaml.Unmarshal([]byte(`{
+		"apiVersion": "v1",
+		"kind": "Pod",
+		"metadata": {
+			"managedFields": [{}],
+			"labels": {
+				"a": "b"
+			},
+		}
+	}`), &obj.Object); err != nil {
+		t.Fatalf("error decoding YAML: %v", err)
+	}
+	if err := f.Update(obj, "update_manager"); err != nil {
+		t.Fatalf("failed to update with empty manager: %v", err)
+	}
+
+	if len(f.ManagedFields()) != 0 {
+		t.Fatalf("failed to reset managedFields: %v", f.ManagedFields())
+	}
+}

staging/src/k8s.io/apiserver/pkg/endpoints/handlers/fieldmanager/internal/managedfields.go

@@ -78,16 +78,8 @@ func RemoveObjectManagedFields(obj runtime.Object) {
 }
 
 // DecodeObjectManagedFields extracts and converts the objects ManagedFields into a fieldpath.ManagedFields.
-func DecodeObjectManagedFields(from runtime.Object) (ManagedInterface, error) {
-	if from == nil {
-		return &managedStruct{}, nil
-	}
-	accessor, err := meta.Accessor(from)
-	if err != nil {
-		panic(fmt.Sprintf("couldn't get accessor: %v", err))
-	}
-
-	managed, err := decodeManagedFields(accessor.GetManagedFields())
+func DecodeObjectManagedFields(from []metav1.ManagedFieldsEntry) (ManagedInterface, error) {
+	managed, err := decodeManagedFields(from)
 	if err != nil {
 		return nil, fmt.Errorf("failed to convert managed fields from API: %v", err)
 	}

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