Boris and I talked more 1:1 -- it is not clear when a direct proxy can
be safely "cast" to its target. The internal proxies Gecko uses are
known implementations where this is safe (with a security check). An
arbitrary scripted direct proxy? Not safe in general, and stripping the
proxy to its target may break the abstraction of that certain scripted
proxy.
/be
Boris Zbarsky wrote:
François REMY wrote:
> In case you hit the "invalid cast" case, you can check if you
> received a Proxy. If you received a proxy, you retry the cast using
> its target instead.
For what it's worth, this is exactly how Gecko's WebIDL bindings work.
We use something much like a Proxy to enforce cross-window security
membranes, so a call to a WebIDL binding method may in fact have a
"this" object or arguments that are not actual WebIDL objects but
wrappers for them. This is a rare case, so we do exactly what you
describe: check for a WebIDL object, then if what we have is not a
WebIDL object check for a security wrapper and unwrap it as needed
(the unwrapping performs a security check, which can of course fail).
So we could in fact add support for something like scripted Proxies
for WebIDL objects without too much trouble in Gecko, I think. Using
them would deoptimize things, of course, not only because of the extra
code after the "WebIDL object" check fails, but also because it would
force runtime typechecks on "this", as opposed to compile-time ones,
at all callsites that used Proxies as the this object. That would not
be that big a hit compared to the other things going on with Proxies,
though, or so I suspect.
-Boris
P.S. Please cc me on replies; I'm not subscribed to this list. I was
just pointed to this thread.
P.P.S. WebIDL objects are also normal ECMAScript objects in Gecko,
except the ones that aren't because they're actually "proxies" without
a target object. JSC uses objects with a custom
getOwnPropertyDescriptor for similar purposes, last I looked. That
said, clearly we have a way to test whether a given object is a WebIDL
object, and so do other implementations.
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