Improve internal stringifcation for custom Error objects.

src/messages.js

 // Copyright 2012 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 210 matching lines @@
   if (IS_FUNCTION(obj)) return  %_CallFunction(obj, FunctionToString);
   if (IS_OBJECT(obj) && %GetDataProperty(obj, "toString") === ObjectToString) {
     var constructor = %GetDataProperty(obj, "constructor");
     if (typeof constructor == "function") {
       var constructorName = constructor.name;
       if (IS_STRING(constructorName) && constructorName !== "") {
         return "#<" + constructorName + ">";
       }
     }
   }
-  if (IsNativeErrorObject(obj)) return %_CallFunction(obj, ErrorToString);
+  if (CanBeSafelyTreatedAsAnErrorObject(obj)) {
+    return %_CallFunction(obj, ErrorToString);
+  }
   return %_CallFunction(obj, ObjectToString);
 }
 
-
-// To check if something is a native error we need to check the
-// concrete native error types. It is not sufficient to use instanceof
-// since it possible to create an object that has Error.prototype on
-// its prototype chain. This is the case for DOMException for example.
-function IsNativeErrorObject(obj) {
+// To determine whether we can safely stringify an object using ErrorToString
+// without the risk of side-effects, we need to check whether the object is
+// either an instance of a native error type (via '%_ClassOf'), or has $Error
+// in its prototype chain and hasn't overwritten 'toString' with something
+// strange and unusual.
+function CanBeSafelyTreatedAsAnErrorObject(obj) {
   switch (%_ClassOf(obj)) {
     case 'Error':
     case 'EvalError':
     case 'RangeError':
     case 'ReferenceError':
     case 'SyntaxError':
     case 'TypeError':
     case 'URIError':
       return true;
   }
-  return false;
+
+  var objToString = %GetDataProperty(obj, "toString");
+  return obj instanceof $Error && objToString === ErrorToString;
 }
 
 
 // When formatting internally created error messages, do not
 // invoke overwritten error toString methods but explicitly use
 // the error to string method. This is to avoid leaking error
 // objects between script tags in a browser setting.
 function ToStringCheckErrorObject(obj) {
-  if (IsNativeErrorObject(obj)) {
+  if (CanBeSafelyTreatedAsAnErrorObject(obj)) {
     return %_CallFunction(obj, ErrorToString);
   } else {
     return ToString(obj);
   }
 }
 
 
 function ToDetailString(obj) {
   if (obj != null && IS_OBJECT(obj) && obj.toString === ObjectToString) {
     var constructor = obj.constructor;
@@ skipping 1066 matching lines @@
     %GetAndClearOverflowedStackTrace(this);
   };
 
   %DefineOrRedefineAccessorProperty(
       boilerplate, 'stack', getter, setter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();