857417 - Make it impossible to mis-order nsISupports and nsWrapperCache in WebIDL object inheritance

Status: RESOLVED FIXED

(Core :: DOM: Core & HTML, defect)

Description

[Boris Zbarsky [:bzbarsky]]

https://vargajan.wordpress.com/2013/04/02/moving-stuff-to-new-dom-bindings/ points out a known gotcha in our bindings code: For C++ objects that inherit from nsISupports, the nsISupports pointer needs to come first.

It's a bit of a pain, because the nsISupports the JS object is holding on to is not always the canonical nsISupports (e.g. nsContentList) and because we can't do the trick with static_cast and reinterpret_cast in cases when there is multiple inheritance from nsISupports, since the static_cast won't compile.

But what I think we _can_ do is to assert, when we're isupports and wrappercached, that static_cast and reinterpret_cast to nsWrapperCache give _different_ answers.  That would at least catch the case Jan ran into, I think.

Comment 1

[Boris Zbarsky [:bzbarsky]]

Attachment: Trigger a fatal assertion when wrapping a wrappercached and nsISupports WebIDL object which has the wrapper cache before the nsISupports in its object layout.

Comment 2

[Boris Zbarsky [:bzbarsky]]

This will obviously still let buggy code compile, but should fatally assert as soon as you try to wrap the object...

Comment 3

[Boris Zbarsky [:bzbarsky]]

Attachment: Even better

Maybe I don't need the assertISupportsInheritance bit anymore...

Comment 4

[Boris Zbarsky [:bzbarsky]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/d0d75e76d41f

Comment 5

[(no longer active)]

Backed out because of test failures: <https://tbpl.mozilla.org/?tree=Mozilla-Inbound&rev=b84b3d484636>

https://hg.mozilla.org/integration/mozilla-inbound/rev/d61fd3898e79

Comment 6

[Boris Zbarsky [:bzbarsky]]

Relanded as https://hg.mozilla.org/integration/mozilla-inbound/rev/ef9509eb9a4c now that I've fixed the classes that failed these asserts.

Comment 7

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/ef9509eb9a4c