WindowProxy and violations of internal method invariants

[annevk]

Basically, the problem raised by @bzbarsky in https://esdiscuss.org/topic/figuring-out-the-behavior-of-windowproxy-in-the-face-of-non-configurable-properties two years ago now was resolved, but the resolution could not be implemented per https://bugzilla.mozilla.org/show_bug.cgi?id=1197958#c4 and therefore the HTML Standard's WindowProxy object definition contradicts ECMAScript here after I discussed the situation with bz and @ajklein.

See https://html.spec.whatwg.org/multipage/browsers.html#windowproxy-getownproperty (step 3) and https://html.spec.whatwg.org/multipage/browsers.html#windowproxy-defineownproperty (also step 3) for the violations.

(I missed this when reporting the document.all violation as this one was not properly tagged in HTML.)

[bzbarsky]

I think the other relevant links showing why this could not be implemented are https://bugzilla.mozilla.org/show_bug.cgi?id=1178638#c1 and https://bugzilla.mozilla.org/show_bug.cgi?id=1178638#c2 with the latter being a third-party library.

[erights]

Regarding the proposal in the first bullet at https://bugzilla.mozilla.org/show_bug.cgi?id=1197958#c4 comment 4:

• Redefine the contract of Object.defineProperty so that it can signal failure either by throwing (as now) or by returning false. Returning false rather than throwing is hazardous, because existing clients that rely only on normal control flow to determine success can be misled. Thus, we would allow false only on the narrowest case we can viably specify, such as non-writable, non-configurable on the WindowProxy when the underlying property on the Window (the real but unreifiable global object) has indeed been successfully made non-writable, non-configurable. Object.getOwnPropertydescriptor would still do what the draft spec says it should do -- report the property on the WindowProxy to be configurable. (Crucially, we do not alter the contract of Reflect.defineProperty or the contract of the [[DefineOwnProperty]] trap.)

Do we agree that this would fix all reported breakages? On rereading this, I would be prepared to go even a bit further on weakening how Object.defineProperty reports failure, given that the Reflect.defineProperty and [[DefineOwnProperty]] contracts were not weakened, nor the invariants upheld by these contracts. From the reported breakages, I see no reason to weaken any of those. We need only weaken how Object.defineProperty reports failure.

[bzbarsky]

Do we agree that this would fix all reported breakages?

I believe it would, yes. So, importantly, the magic would live in Object.defineProperty, and hence in the ES spec, not in the [[DefineOwnProperty]] of WindowProxy objects?

[erights]

yes, I think that's correct.

[allenwb]

I think a cleaner way to allow for this behavior is to add a new paragraph to https://tc39.github.io/ecma262/#sec-error-handling-and-language-extensions that says something like:

"An implementation my define failure behavior other than throwing TypeError for Object.defineProperty when it's first argument is an implementation provided exotic object."

The HTML spec. would then state that Object.defineObject returns false rather than throwing when its first argument is a Windows Proxy object.

This would keep all of the details out of the ES spec. and put them back into the HTML spec, which is where they probably belong.

[domenic]

@allenwb's version seems OK but I'd prefer to formalize it with something like a HostObjectDefinePropertyFailureBehavior abstract op which is called as part of the algorithm.

[allenwb]

@domenic I think we should avoid adding attractive nuisance hooks the to the ES spec. that could be taken as a general invitation for hosts to do unanticipated bizarre things. In that light, I would further refine my proposed addition to clause 16 to says:

"An implementation my define failure behavior other than throwing TypeError for Object.defineProperty when it's first argument is an implementation provided exotic object and the throwing behavior would cause legacy code to fail."

The HTML spec. would not need to include any algorithmic specifications for this behavior. All it needs to say is: When a WindowProxy is passed as the first argument to Object.defineProperty failure is reported by returning false rather than throwing a TypeError exception.

[domenic]

Sure. I think our proposals are equivalent except stylistically. I think it's much nicer to have the host-defined hooks called out by specific names and as explicit modifications to the algorithm steps, instead of hidden in paragraphs afterward.

[domenic]

I'll put up a pull request that we can discuss more concretely, using much of your phrasing.

[bzbarsky]

The HTML spec. would not need to include any algorithmic specifications for this behavior.

It totally would, because of the following two constraints:

1. We want this code:

Object.defineProperty(window, "gotcha", { configurable: false, value: 5 });

to indicate failure (to make the property non-configurable) by returning false.
2. We want this code:

Object.defineProperty(window, "gotcha", { configurable: false, writable: false, value: 5 });
Object.defineProperty(window, "gotcha", { configurable: false, writable: false, value: 6 });

to throw.

[ljharb]

@bzbarsky what's the property name you're defining in those examples?

[bzbarsky]

Er, right. Doesn't matter, as long as it's consistent. I'll edit the examples.