eio_luv: Losing effect handler scope

[patricoferris]

This is an issue formed out of https://discuss.ocaml.org/t/how-to-compose-effect-handlers-with-eio/11279 and is reproducible with this small program.

module Echo = struct
  type _ Effect.t += Echo : string -> unit Effect.t

  let run f =
    Effect.Deep.try_with f ()
      {
        effc =
          (fun (type b) (eff : b Effect.t) ->
            match eff with
            | Echo string ->
                Some
                  (fun (k : (b, unit) Effect.Deep.continuation) ->
                    print_endline string;
                    Effect.Deep.continue k ())
            | _ -> None);
      }
end

let () =
  Echo.run @@ fun () ->
  Eio_main.run @@ fun _ ->
  Eio.Fiber.yield ();
  Effect.perform (Echo.Echo "world")

We define a new effect, Echo, and provide a simple handler (this is a contrived example). We would expect that all effects in this program would be handled but actually using Eio_luv the Echo effect is unhandled. I think this is due to the wakeup happening as a callback in the Luv.Async object.

[talex5]

It's because you can't perform an effect over a C function call. So effects performed inside libuv's run function can't be handled by an effect handler outside it.

[wokalski]

I think an explicit API to inject handlers either on the Fiber or Switch level would be great here.

[patricoferris]

I think an explicit API to inject handlers either on the Fiber or Switch level would be great here

Note that this issue is not about that problem, this is just a bug in Eio_luv due to an implementation detail. If you have time I would create a separate issue for the API for injecting handlers. I think it would also be super helpful to have a non-contrived example that can't be achieved with functions and fiber local storage or functors as part of that issue which will help with getting a design in mind :))