8259046: ViewPainter.ROOT_PATHS holds reference to Scene causing memory leak

Reviewed-by: pbansal, arapte

Author: kevinrushforth

modules/javafx.graphics/src/main/java/com/sun/javafx/tk/quantum/ViewPainter.java

@@ -319,6 +319,7 @@ protected void paintImpl(final Graphics backBufferGraphics) {
                     g.setClipRect(dirtyRect);
                     g.setClipRectIndex(i);
                     doPaint(g, getRootPath(i));
+                    getRootPath(i).clear();
                 }
             }
         } else {

tests/system/src/test/java/test/com/sun/javafx/tk/quantum/ViewPainterLeakTest.java

@@ -0,0 +1,115 @@
+/*
+ * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package test.com.sun.javafx.tk.quantum;
+
+import java.lang.ref.WeakReference;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import javafx.application.Application;
+import javafx.application.Platform;
+import javafx.scene.Scene;
+import javafx.scene.control.ScrollPane;
+import javafx.scene.layout.StackPane;
+import javafx.scene.text.Text;
+import javafx.scene.text.TextFlow;
+import javafx.stage.Stage;
+import javafx.stage.WindowEvent;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import test.util.Util;
+import test.util.memory.JMemoryBuddy;
+
+import static org.junit.Assert.*;
+
+public class ViewPainterLeakTest {
+
+    private static CountDownLatch startupLatch;
+    private static Stage stage;
+    private static ScrollPane scrollPane;
+    private static WeakReference<Scene> sceneRef;
+
+    private static final String text =
+            "The quick brown fox jumps over the lazy dog." +
+            " " +
+            "The quick brown fox jumps over the lazy dog.";
+
+    public static class TestApp extends Application {
+        @Override
+        public void start(Stage stage) {
+            ViewPainterLeakTest.stage = stage;
+
+            Platform.setImplicitExit(false);
+
+            TextFlow content = new TextFlow(new Text(text));
+            scrollPane = new ScrollPane(content);
+            StackPane root = new StackPane(scrollPane);
+
+            Scene scene = new Scene(root, 200, 100);
+            sceneRef = new WeakReference<>(scene);
+            stage.setScene(scene);
+
+            stage.addEventHandler(WindowEvent.WINDOW_SHOWN, e -> {
+                Platform.runLater(() -> {
+                    startupLatch.countDown();
+                });
+            });
+            stage.show();
+        }
+    }
+
+    @BeforeClass
+    public static void setupOnce() throws Exception {
+        startupLatch = new CountDownLatch(1);
+        new Thread(() -> Application.launch(TestApp.class, (String[]) null)).start();
+        assertTrue("Timeout waiting for FX runtime to start",
+                startupLatch.await(15, TimeUnit.SECONDS));
+    }
+
+    @AfterClass
+    public static void teardown() {
+        Platform.exit();
+    }
+
+    @Test
+    public void testViewPainterLeak() {
+        // Wait for some time to make sure the initial content is displayed
+        Util.sleep(500);
+
+        // This will trigger the leak
+        Util.runAndWait(() -> scrollPane.setHvalue(0.5));
+
+        // Wait for some time to make sure the scrolled content is displayed
+        Util.sleep(500);
+        Util.runAndWait(() -> {
+            stage.hide();
+            stage = null;
+            scrollPane = null;
+        });
+        JMemoryBuddy.assertCollectable(sceneRef);
+    }
+
+}