Don't pass the hole to SetElement when creating Array.observe change records

Also added comments to remind us why we were using the hole here in the first
place (it's used for the case where Object.observe, rather than Array.observe,
has been called on Array that's undergoing truncation).

BUG=356589
LOG=N
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/213823002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20316 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects.cc

@@ -11376,6 +11376,9 @@ Handle<Object> JSArray::SetElementsLength(Handle<JSArray> array,
   BeginPerformSplice(array);
 
   for (int i = 0; i < indices.length(); ++i) {
+    // For deletions where the property was an accessor, old_values[i]
+    // will be the hole, which instructs EnqueueChangeRecord to elide
+    // the "oldValue" property.
     JSObject::EnqueueChangeRecord(
         array, "delete", isolate->factory()->Uint32ToString(indices[i]),
         old_values[i]);
@@ -11392,6 +11395,9 @@ Handle<Object> JSArray::SetElementsLength(Handle<JSArray> array,
   Handle<JSArray> deleted = isolate->factory()->NewJSArray(0);
   if (delete_count > 0) {
     for (int i = indices.length() - 1; i >= 0; i--) {
+      // Skip deletions where the property was an accessor, leaving holes
+      // in the array of old values.
+      if (old_values[i]->IsTheHole()) continue;
       JSObject::SetElement(deleted, indices[i] - index, old_values[i], NONE,
                            SLOPPY);
     }

test/mjsunit/regress/regress-356589.js

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// This test passes if it does not crash in debug mode
+
+arr = ['a', 'b', 'c', 'd'];
+Object.defineProperty(arr.__proto__, '0', { get: function(){} });
+Object.defineProperty(arr, '2', {get: function(){} });
+Object.observe(arr, function() {});
+arr.length = 2;