Roadmap to the proper support of finalisation

[wks]

Task list:

• Properly support non-resurrecting finalisation (Ruby ObjectSpace.define_finalizer)
  • Treat "zombies" like PhantomReference
    We will stop using mmtk-core's Java-style reference handling. Instead we'll use the new binding-side reference/finaliser processing API.
    • PhantomReference support in mmtk-core (Done: Weak reference processing mmtk-core#564)
  • Create "zombie" "finalisation job" when a finaliser is added
    • I choose not to call it "zombie" because they do not reuse the space of dead objects, and I invented my own structure that is very different to RZombie.
  • Execute "finalisation jobs" periodically
  • Execute "finalisation jobs" at exit
• Properly support resurrecting finalisation (any non-trivial actions in obj_free, especially T_DATA, T_FILE and T_SYMBOL)
  • mmtk-core supports getting object registered for finalisation before the object dies. (Get finalizable object before object dies mmtk-core#578)
    We will stop using mmtk-core's Java-style reference handling. Instead we'll use the new binding-side reference/finaliser processing API. Generalise weak reference processing for other languages mmtk-core#700
  • Decouple the Finalization work packet fromProcessEdgesWork to support object enqueuing.
    We don't need special handling in Finalization. The current Java-style finalization mechanism supports Ruby well.
    - https://github.com/wks/mmtk-ruby/tree/gen-weakref-api
    - https://github.com/wks/ruby/tree/gen-weakref-api
    • mmtk-core support ProcessObjectsWork in addition to ProcessEdgesWork.
      • Issue: Add ProcessObjectsWork in addition to ProcessEdgesWork mmtk-core#581
      • PR: Node-enqueuing tracing. mmtk-core#628 (merged)
  • Add a hook on the Ruby side. When GC triggered during heap allocation (mmtk_alloc), we run non-resurrecting finalizers (obj_free).

[wks]

While working on supporting VM binding-side reference/finaliser handling, I managed to handle finalisers (ObjectSpace.define_finalizer) by traversing the finalizer_table and creating "finalisation job" (our own alternative to zombies) for dead entries. Now those finalisers can be properly executed after GC is triggered.

[wks]

I think we now fully support finalisers, at least on my experimental branch.

• https://github.com/wks/mmtk-core/tree/gen-weakref-api
• https://github.com/wks/mmtk-ruby/tree/gen-weakref-api
• https://github.com/wks/ruby/tree/gen-weakref-api

[wks]

The gen-weakref-api branches listed above have been merged. Closing this issue...