8234423: Modifying ArrayList.subList().subList() resets modCount of subList

Reviewed-by: rriggs

Author: Ivan Gerasimov

src/java.base/share/classes/java/util/ArrayList.java

@@ -1133,7 +1133,7 @@ private SubList(SubList<E> parent, int fromIndex, int toIndex) {
             this.parent = parent;
             this.offset = parent.offset + fromIndex;
             this.size = toIndex - fromIndex;
-            this.modCount = root.modCount;
+            this.modCount = parent.modCount;
         }
 
         public E set(int index, E element) {
@@ -1286,7 +1286,7 @@ public ListIterator<E> listIterator(int index) {
             return new ListIterator<E>() {
                 int cursor = index;
                 int lastRet = -1;
-                int expectedModCount = root.modCount;
+                int expectedModCount = SubList.this.modCount;
 
                 public boolean hasNext() {
                     return cursor != SubList.this.size;
@@ -1330,7 +1330,7 @@ public void forEachRemaining(Consumer<? super E> action) {
                         final Object[] es = root.elementData;
                         if (offset + i >= es.length)
                             throw new ConcurrentModificationException();
-                        for (; i < size && modCount == expectedModCount; i++)
+                        for (; i < size && root.modCount == expectedModCount; i++)
                             action.accept(elementAt(es, offset + i));
                         // update once at end to reduce heap write traffic
                         cursor = i;
@@ -1356,7 +1356,7 @@ public void remove() {
                         SubList.this.remove(lastRet);
                         cursor = lastRet;
                         lastRet = -1;
-                        expectedModCount = root.modCount;
+                        expectedModCount = SubList.this.modCount;
                     } catch (IndexOutOfBoundsException ex) {
                         throw new ConcurrentModificationException();
                     }
@@ -1382,7 +1382,7 @@ public void add(E e) {
                         SubList.this.add(i, e);
                         cursor = i + 1;
                         lastRet = -1;
-                        expectedModCount = root.modCount;
+                        expectedModCount = SubList.this.modCount;
                     } catch (IndexOutOfBoundsException ex) {
                         throw new ConcurrentModificationException();
                     }

test/jdk/java/util/ArrayList/SubListModCount.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2019, 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.
+ *
+ * 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.
+ */
+
+/**
+ * @test
+ * @bug 8234423
+ * @summary Modifying ArrayList.subList().subList() resets modCount of subList
+ */
+
+import java.util.ArrayList;
+import java.util.ConcurrentModificationException;
+
+public class SubListModCount {
+    public static void main(String[] args) {
+        int failures = 0;
+        var root = new ArrayList<Integer>();
+        var subList = root.subList(0, 0);
+        root.add(42);
+        try {
+            subList.size();
+            failures++;
+            System.out.println("Accessing subList");
+        } catch (ConcurrentModificationException expected) {
+        }
+        var subSubList = subList.subList(0, 0);
+        try {
+            subSubList.size();
+            failures++;
+            System.out.println("Accessing subList.subList");
+        } catch (ConcurrentModificationException expected) {
+        }
+        try {
+            subSubList.add(42);
+            failures++;
+            System.out.println("Modifying subList.subList");
+        } catch (ConcurrentModificationException expected) {
+        }
+        try {
+            subList.size();
+            failures++;
+            System.out.println("Accessing subList again");
+        } catch (ConcurrentModificationException expected) {
+        }
+        if (failures > 0) {
+            throw new AssertionError(failures + " tests failed");
+        }
+    }
+}