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 (IsNativeErrorObject(obj) ||
+      (obj instanceof $Error && obj.toString === $Error.prototype.toString)) {

[Michael Starzinger, 2013/08/02 11:47:04]

As discussed offline: I am a little bit worried that the "obj.toString" access
here is observable and might throw an exception, or even worse return different
values every other time. Better use "GetPropertyWithoutInvokingMonkeyGetters" to
get to the value of the "toString" property.

Also "$Error.prototype.toString" is configurable and might not point to the
primordial implementation, it would be better to compare against "ErrorToString"
instead.

[Mike West, 2013/08/02 12:04:50]

Both these suggestions make sense, and are in the latest patchset.

Thanks for taking a look!

+    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) {
   switch (%_ClassOf(obj)) {
     case 'Error':
     case 'EvalError':
     case 'RangeError':
     case 'ReferenceError':
     case 'SyntaxError':
     case 'TypeError':
     case 'URIError':
       return true;
   }
   return false;
 }
 
 
 // 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 (IsNativeErrorObject(obj) ||
+      (obj instanceof $Error && obj.toString === $Error.prototype.toString)) {
     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();