Improve internal stringifcation for custom Error objects.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index b586d24882b02a5e46528efef9dded04ac510540..de6921bafa57c5979c94cb9554ba77c0112c7b1c 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -228,7 +228,10 @@ function NoSideEffectToString(obj) {
       }
     }
   }
-  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);
 }
 
@@ -257,7 +260,8 @@ function IsNativeErrorObject(obj) {
 // 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);