Recheck if transformed data is stale when doing live lookup during update

staging/src/k8s.io/apiserver/pkg/storage/etcd3/store.go

@@ -332,13 +332,16 @@ func (s *store) GuaranteedUpdate(
 				if err != nil {
 					return err
 				}
+				mustCheckData = false
 				if !bytes.Equal(data, origState.data) {
 					// original data changed, restart loop
-					mustCheckData = false
 					continue
 				}
 			}
-			return decode(s.codec, s.versioner, origState.data, out, origState.rev)
+			// recheck that the data from etcd is not stale before short-circuiting a write
+			if !origState.stale {
+				return decode(s.codec, s.versioner, origState.data, out, origState.rev)
+			}
 		}
 
 		newData, err := s.transformer.TransformToStorage(data, transformContext)

staging/src/k8s.io/apiserver/pkg/storage/etcd3/store_test.go

@@ -526,6 +526,8 @@ func TestGuaranteedUpdateChecksStoredData(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	store.transformer = prefixTransformer{prefix: []byte(defaultTestPrefix)}
+
 	// this update should write the canonical value to etcd because the new serialization differs
 	// from the stored serialization
 	input.ResourceVersion = strconv.FormatInt(resp.Header.Revision, 10)
@@ -540,6 +542,36 @@ func TestGuaranteedUpdateChecksStoredData(t *testing.T) {
 	if out.ResourceVersion == strconv.FormatInt(resp.Header.Revision, 10) {
 		t.Errorf("guaranteed update should have updated the serialized data, got %#v", out)
 	}
+
+	lastVersion := out.ResourceVersion
+
+	// this update should not write to etcd because the input matches the stored data
+	input = out
+	out = &example.Pod{}
+	err = store.GuaranteedUpdate(ctx, key, out, true, nil,
+		func(_ runtime.Object, _ storage.ResponseMeta) (runtime.Object, *uint64, error) {
+			return input, nil, nil
+		}, input)
+	if err != nil {
+		t.Fatalf("Update failed: %v", err)
+	}
+	if out.ResourceVersion != lastVersion {
+		t.Errorf("guaranteed update should have short-circuited write, got %#v", out)
+	}
+
+	store.transformer = prefixTransformer{prefix: []byte(defaultTestPrefix), stale: true}
+
+	// this update should write to etcd because the transformer reported stale
+	err = store.GuaranteedUpdate(ctx, key, out, true, nil,
+		func(_ runtime.Object, _ storage.ResponseMeta) (runtime.Object, *uint64, error) {
+			return input, nil, nil
+		}, input)
+	if err != nil {
+		t.Fatalf("Update failed: %v", err)
+	}
+	if out.ResourceVersion == lastVersion {
+		t.Errorf("guaranteed update should have written to etcd when transformer reported stale, got %#v", out)
+	}
 }
 
 func TestGuaranteedUpdateWithConflict(t *testing.T) {