667047 - ArrayBuffer: __proto__ inconsistent behaviour

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Nikhil Marathe [:nsm] (No longer reading bugmail, please needinfo?)]

User-Agent:       Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:5.0) Gecko/20100101 Firefox/5.0
Build Identifier: 

fixes for bug 665355 cause inconsistent behaviour of __proto__ on ArrayBuffers

Other JS objects treat __proto__ as a normal property once it has been set to null (the prototype chain is destroyed).

test case:
var d = new Date();
d.__proto__ = null
d.__proto__ = {a:2}
assertEq(d.a, undefined) // PASS

var x = new ArrayBuffer();
x.__proto__ = null
x.__proto__ = {a:2}
assertEq(x.a, undefined) // FAIL

Reproducible: Always



Expected Results:  
x.a should be undefined.

Comment 1

[Nikhil Marathe [:nsm] (No longer reading bugmail, please needinfo?)]

Attachment: Identical behaviour to native JS objects.

Comment 2

[Blake Kaplan (:mrbkap) (inactive, use needinfo)]

Comment on attachment 542195 [details] [diff] [review]
Identical behaviour to native JS objects.

Review of attachment 542195 [details] [diff] [review]:
-----------------------------------------------------------------

::: js/src/jstypedarray.cpp
@@ +279,5 @@
>  
>      if (JSID_IS_ATOM(id, cx->runtime->atomState.protoAtom)) {
> +        JSObject *proto = obj->getProto();
> +        if (proto) {
> +            if (!SetProto(cx, obj, vp->toObjectOrNull(), true))

This is close, but it isn't quite sufficient. For example consider:

var proto = Object.create(null);
var normal = {};
var ab = new ArrayBuffer([]);
ab.__proto__ = proto;
ab.__proto__ = { a: 42 };

normal.__proto__ = proto;
normal.__proto__ = { a: 42 };

is(ab.a, normal.a);

So the mere existence of a prototype doesn't mean that you have a __proto__ property.

How about always unconditionally calling js_SetProperty on the delegate and then detecting whether or not its proto changes and, if it does, changing the arraybuffer's proto?

@@ +292,2 @@
>          }
> +        else {

Uber-nit: the else should be on the same line as the closing brace for the if.

Comment 3

[Nikhil Marathe [:nsm] (No longer reading bugmail, please needinfo?)]

Attachment: Identical behaviour to native JS objects.

Alternative fix for bug 667047 to deal with cases 
where the prototype is set to null in the JS sense 
but not in the C++ sense                                  
eg.
var x = Object.create(null)

x is still null in the JS sense but in C++ the JSObject pointer is not NULL, but a valid JSObject.

I hope this explanation is valid.

Comment 4

[Blake Kaplan (:mrbkap) (inactive, use needinfo)]

(In reply to comment #3) 
> eg.
> var x = Object.create(null)
> 
> x is still null in the JS sense but in C++ the JSObject pointer is not NULL,
> but a valid JSObject.

I don't understand what this means. x isn't null; it's an object that you can stick properties on and then look those properties up later. It just happens to have a null prototype.

Comment 5

[Nikhil Marathe [:nsm] (No longer reading bugmail, please needinfo?)]

that's what I meant, that for truthiness purposes x is null, but its not really null

Comment 6

[Michael Wu [:mwu]]

Backed out while investigating a webgl permaorange on mozilla-inbound.

Comment 7

[Marco Bonardo [::mak]]

http://hg.mozilla.org/mozilla-central/rev/96f52c040d20