Resource-management for resumptions

[gadmm]

As a follow-up to the discussion at https://discuss.ocaml.org/t/multi-shot-continuations-gone-forever/9072/13, I think the usual resource-management issues inside resumptions apply to resumptions themselves: currently they have to be dropped manually, which prevents us from referring to them inside other resumptions since it becomes impossible to track when they should be freed.

As suggested in the discussion, one solution is to make resumptions purely GC-managed values. To do so, one can represent a resumption either as a heap value obtained by copying a fiber, or as a fiber which is only discontinued by the GC using a finaliser and which is never reinstated/converted back to a continuation. The interface gets simplified by removal of demote, drop and Resumption_already_dropped.

The distinction between linear continuations, which are allowed to own resources, and multi-shot continuations, which are not, could be explained in the documentation, to underline the issues arising from promoting arbitrary continuations.

(cc @gasche)

[gasche]

I agree, and/but I suspect that sending a PR to propose the new implementation may be more effective, in case @dhil rather prefers to keep this as a low-effort project on the side.

[gadmm]

Let me know what you think @dhil!

[dhil]

I think it is a good idea! I am interested in exploring such designs. My initial plan was just to replicate clone_continuation, such that I can continue working on my research software which is written in an older version of Multicore OCaml.

[dhil]

@gadmm I have managed to get byte code compilation going now. I will look into experimenting with GC'ed resumptions next.

[dhil]

Commits 2ac6158 and 1f7fc26 add initial support for experimenting with GC managed resumptions/continuations.