Expose is_emergency_collection to VM bindings

[wks]

is_emergency_collection is useful during weak reference processing. JVMs can choose not to retain referents of SoftReference during emergency GC.

The is_emergency_collection function was moved from Plan to GlobalState in 57af17f. After that, the MMTK:state field is private and inaccessible to VM bindings. VM bindings that depend on the to-be-deprecated built-in ReferenceProcessor still work because it is part of the mmtk crate, and can mmtk.state.is_emergency_collection. However, VM bindings that do weak reference processing on the VM side using the Scanning::process_weak_refs API can no longer call that function. This makes this PR unable to merge. In the future, the OpenJDK binding will also need it when it off-loads weak reference processing to the binding side.

This PR adds a public API function MMTK::is_emergency_collection which can be called by the VM bindings.

[k-sareen]

What about exposing GlobalState itself? Being global state, shouldn't the binding also be able to access it?

[wks]

What about exposing GlobalState itself? Being global state, shouldn't the binding also be able to access it?

Yes. That makes sense, too. Although not all states in the GlobalState struct is public, we can select what is public by annotating fields and methods with pub.

[qinsoon]

LGTM

[qinsoon]

What about exposing GlobalState itself? Being global state, shouldn't the binding also be able to access it?

Yes. That makes sense, too. Although not all states in the GlobalState struct is public, we can select what is public by annotating fields and methods with pub.

I agree. Being global doesn't mean they are public.

This PR looks good. Providing getter/setter is generally better than exposing public fields.