1494121 - Check whether all Debugger-related CrossCompartmentKey variants are really needed

Status: NEW

(Core :: JavaScript Engine, enhancement, P3)

Description

[Jason Orendorff [:jorendorff]]

We have special CrossCompartmentKeys for entries in the Compartment's WrapperMap that represent Debugger.Objects. We need it because each Debugger.Object has a ⚡Dirëct Pøinter⚡ to its referent, compartment boundaries be damned.

We also have special CrossCompartmentKeys for Debugger.Source, Debugger.Environment, and so on: <https://searchfox.org/mozilla-central/rev/881a3c5664ede5e08ee986d76433bc5c4b5680e6/js/src/vm/Compartment.h#33-37>
Those would be unnecessary, if instead of using setPrivateGCThing to store the cross-compartment edge, we can just use the normal CCW machinery.

So, for example, instead of a Debugger.Source object having a private slot that points directly to a debuggee ScriptSourceObject, it would have a reserved slot that points to the debugger-compartment CCW whose target is the ScriptSourceObject.

Can we do that?

(The reason we don't do this for Debugger.Object is that the referent of a Debugger.Object can be a cross-compartment wrapper whose target is in a non-debuggee compartment. The normal CCW machinery would make a mess of that. I'm saying I don't think Source objects, scripts, or the other referent types have that special case.)

Comment 1

[Jim Blandy :jimb]

How could we ensure that a debugger-compartment CCW for the referent (ScriptSourceObject, environment object, whatever) would be a fully transparent wrapper? Or, it doesn't matter because we'd just always UnsafeDereference it before using it from the C++ code?

Comment 2

[Jason Orendorff [:jorendorff]]

The latter. I think UncheckedUnwrap would be fine, though a CheckedUnwrap should always succeed too.

Comment 3

[Jim Blandy :jimb]

What's the overhead on UncheckedUnwrap? If that's acceptable, then making those pointers less magic seems like the right approach. 

I guess we must already dealing with the fact that the ultimate referent is in a separate compartment.