Fix non-idempotent modification in JSObject::AddFastProperty.

src/objects.cc

 // Copyright 2013 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
@@ skipping 1806 matching lines @@
     // Make room for the new value
     MaybeObject* maybe_values =
         properties()->CopySize(properties()->length() + kFieldsAdded);
     if (!maybe_values->To(&values)) return maybe_values;
   }
 
   TransitionFlag flag = INSERT_TRANSITION;
 
   Heap* heap = isolate->heap();
 
-  Map* new_map;
-  MaybeObject* maybe_new_map = map()->CopyAddDescriptor(&new_field, flag);
-  if (!maybe_new_map->To(&new_map)) return maybe_new_map;
-
   Object* storage;
   MaybeObject* maybe_storage =
       value->AllocateNewStorageFor(heap, representation);
   if (!maybe_storage->To(&storage)) return maybe_storage;
 
+  // Note that Map::CopyAddDescriptor has side-effects, the new map is already
+  // inserted in the transition tree. No more allocations that might fail are
+  // allowed after this point.
+  Map* new_map;
+  MaybeObject* maybe_new_map = map()->CopyAddDescriptor(&new_field, flag);
+  if (!maybe_new_map->To(&new_map)) return maybe_new_map;
+
   if (map()->unused_property_fields() == 0) {
     ASSERT(values != NULL);
     set_properties(values);
     new_map->set_unused_property_fields(kFieldsAdded - 1);
   } else {
     new_map->set_unused_property_fields(map()->unused_property_fields() - 1);
   }
 
   set_map(new_map);
 
@@ skipping 13791 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal