8246788: ZoneRules invariants can be broken

src/java.base/share/classes/java/time/zone/ZoneRules.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 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
@@ -259,10 +259,12 @@ public static ZoneRules of(ZoneOffset offset) {
         }
 
         // last rules
-        if (lastRules.size() > 16) {
+        Object[] temp = lastRules.toArray();
+        ZoneOffsetTransitionRule[] rulesArray = Arrays.copyOf(temp, temp.length, ZoneOffsetTransitionRule[].class);
+        if (rulesArray.length > 16) {
             throw new IllegalArgumentException("Too many transition rules");
         }
-        this.lastRules = lastRules.toArray(new ZoneOffsetTransitionRule[lastRules.size()]);
+        this.lastRules = rulesArray;
     }
 
     /**

test/jdk/java/time/test/java/time/zone/TestMutableZoneRules.java

@@ -0,0 +1,85 @@
+/*
+ * 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.
+ *
+ * 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.java.time.zone;
+
+import java.time.*;
+import java.time.zone.*;
+import java.util.*;
+
+import org.testng.annotations.Test;
+import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertThrows;
+
+/**
+ * @summary ZoneRules invariants can be broken.
+ *
+ * @bug 8246788
+ */
+@Test
+public class TestMutableZoneRules {
+    static final ZoneOffset offset = ZoneOffset.ofHoursMinutes(1, 30);
+
+    static final ZoneOffsetTransitionRule rule1 =
+        ZoneOffsetTransitionRule.of(Month.APRIL, 2, DayOfWeek.TUESDAY, LocalTime.MIN, true,
+                ZoneOffsetTransitionRule.TimeDefinition.UTC, offset, offset, offset);
+
+    static final ZoneOffsetTransitionRule rule2 =
+        ZoneOffsetTransitionRule.of(Month.MARCH, 2, DayOfWeek.MONDAY, LocalTime.MIN, true,
+                ZoneOffsetTransitionRule.TimeDefinition.UTC, offset, offset, offset);
+
+    public void testMutation() {
+        ZoneOffsetTransitionRule[] array = { rule1 };
+        ZoneRules zr1 = ZoneRules.of(offset, offset, List.of(), List.of(), List.of(rule1));
+        ZoneRules zr2 = ZoneRules.of(offset, offset, List.of(), List.of(), new TestList(array, array.length));
+
+        assertEquals(zr2, zr1);
+        array[0] = rule2;
+        assertEquals(zr2, zr1);
+    }
+
+    public void testLength() {
+        ZoneOffsetTransitionRule[] array = new ZoneOffsetTransitionRule[17];
+        Arrays.setAll(array, i -> rule1);
+
+        assertThrows(IllegalArgumentException.class,
+            () -> ZoneRules.of(offset, offset, List.of(), List.of(), new TestList(array, 1)));
+    }
+
+    static class TestList extends AbstractList<ZoneOffsetTransitionRule> {
+        final ZoneOffsetTransitionRule[] array;
+        final int size;
+
+        TestList(ZoneOffsetTransitionRule[] array, int size) {
+            this.array = array;
+            this.size = size;
+        }
+
+        public int size()                           { return size; }
+        public ZoneOffsetTransitionRule get(int i)  { return array[i]; }
+        public Object[] toArray()                   { return array; }
+
+        @SuppressWarnings("unchecked")
+        public <T> T[] toArray(T[] a)               { return (T[]) array; }
+    }
+}