Fix issue 1152: temporary JS array invariant violation in ArrayConcat.

src/runtime.cc

 // Copyright 2010 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 8338 matching lines @@
         }
         if (JSObject::kMaxElementCount - result_length < length_estimate) {
           result_length = JSObject::kMaxElementCount;
           break;
         }
         result_length += length_estimate;
       }
     }
   }
 
-  // Allocate an empty array, will set length and content later.
+  // Allocate an empty array, will set map, length, and content later.
   Handle<JSArray> result = Factory::NewJSArray(0);
 
   uint32_t estimate_nof_elements = IterateArguments(arguments, NULL);
   // If estimated number of elements is more than half of length, a
   // fixed array (fast case) is more time and space-efficient than a
   // dictionary.
   bool fast_case = (estimate_nof_elements * 2) >= result_length;
 
+  Handle<Map> map;
   Handle<FixedArray> storage;
   if (fast_case) {
     // The backing storage array must have non-existing elements to
     // preserve holes across concat operations.
+    map = Factory::GetFastElementsMap(Handle<Map>(result->map()));
     storage = Factory::NewFixedArrayWithHoles(result_length);
-    Handle<Map> fast_map =
-        Factory::GetFastElementsMap(Handle<Map>(result->map()));
-    result->set_map(*fast_map);
   } else {
+    map = Factory::GetSlowElementsMap(Handle<Map>(result->map()));
     // TODO(126): move 25% pre-allocation logic into Dictionary::Allocate
     uint32_t at_least_space_for = estimate_nof_elements +
                                   (estimate_nof_elements >> 2);
     storage = Handle<FixedArray>::cast(
-                  Factory::NewNumberDictionary(at_least_space_for));
-    Handle<Map> slow_map =
-        Factory::GetSlowElementsMap(Handle<Map>(result->map()));
-    result->set_map(*slow_map);
+        Factory::NewNumberDictionary(at_least_space_for));
   }
 
   Handle<Object> len = Factory::NewNumber(static_cast<double>(result_length));
 
   ArrayConcatVisitor visitor(storage, result_length, fast_case);
 
   IterateArguments(arguments, &visitor);
 
+  // Please note:
+  // - the storage might have been changed in the visitor;
+  // - the map and the storage must be set together to avoid breaking
+  //   the invariant that the map describes the array's elements.
+  result->set_map(*map);
   result->set_length(*len);
-  // Please note the storage might have changed in the visitor.
   result->set_elements(*visitor.storage());
 
   return *result;
 }
 
 
 // This will not allocate (flatten the string), but it may run
 // very slowly for very deeply nested ConsStrings.  For debugging use only.
 static MaybeObject* Runtime_GlobalPrint(Arguments args) {
   NoHandleAllocation ha;
@@ skipping 2790 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     Counters::gc_last_resort_from_js.Increment();
     Heap::CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal