Direct-style websockets with Eio

[patricoferris]

This PR adds direct-style websockets using eio and cohttp-eio (yet unreleased, but it has been merged in the main branch of mirage/ocaml-cohttp). It is WIP and probably will change, but I thought I would open this PR in case I don't have time to come back to it later.

cc: @bikallem @talex5 who might be able to comment on the core IO implementation a little :))

[talex5]

I don't know anything about web-sockets, but the Eio stuff looks reasonable to me. I suspect you don't need most of the Buf_write.flush calls, though. The buffer gets flushed automatically when you block; flush just makes the caller wait until the data has finished flushing but doesn't speed it up (any more than calling Fiber.yield () would, anyway).

[ryanslade]

Is there anything holding back this PR for merge? Anything I can perhaps do to help push it over the line?

[patricoferris]

Is there anything holding back this PR for merge?

Thanks for the interest and offer for help @ryanslade. I would say this PR is blocked on landing a proper release of cohttp-eio. Right now, it is an alpha release and constrains us to eio.0.6 which means we also don't have access to Eio.Buf_read.BE for example. There is some experimentation on what a good HTTP library might look like, see mirage/ocaml-cohttp#969 but I think this implies it's going to be a while before we see a full release of cohttp-eio.

[ushitora-anqou]

Hi. Cohttp 6.0.0~beta1 is out and it has Cohttp_eio.Private.IO module, which implements Cohttp.S.IO. As the Lwt version of ocaml-websocket does, we can feed it to Websocket.Make to get the necessary functions for WS server and client implementations. Personally, I needed an Eio version of a WS client implementation and tried this strategy, which worked just fine. I have left the code here for anyone who needs it.

[Kakadu]

I tried to build this PR with cohttp-eio.6.0.0~alpha2 but stuck with compilation error.

File "ocaml-websocket/eio/websocket_eio.ml", line 61, characters 23-29:
61 |       Cohttp_eio.Body.(Custom f) ) in
                            ^^^^^^
Error: Unbound constructor Custom

I would say this PR is blocked on landing a proper release of cohttp-eio.

@patricoferris have things become better in last year?

[patricoferris]

Quickly bump to work with cohttp-eio.6.0.0~beta2 should make this work with the beta2 branch of cohttp-eio -- I don't have time to work on this in any detail so please do take the code and run with it :)) Good luck !