cohttp-eio: confusing use of the `~sw` argument

[smondet]

The Cohttp_eio.Client.call function takes a ~sw and returns a Body.t. calling Eio.Flow.read_all on the body after getting out of the Switch.run can cause a “use-after-close” exception from Eio:

Invalid-argument: readv: file descriptor used after calling close!

It gives the impression of defeating the purpose of eio-swtches, right?

PS: the documentation does say "Consume [response_body] in a timely fashion." which is a bit vague :)

[talex5]

The purpose of the switch is to make sure that the connection gets freed, even if an error occurs. Ideally the type system would also make sure you don't use things after they're closed, but OCaml can't do that yet.

[yawaramin]

Is this issue actionable?

[talex5]

The only action I can see is improving the documentation, which is just the generic cohttp docs and is also present in the Lwt version, e.g. https://ocaml.org/p/cohttp-lwt-unix/6.1.1/doc/cohttp-lwt-unix/Cohttp_lwt_unix/Client/index.html#val-call says:

val call : [...]

call ?headers ?body ?chunked meth uri

    returns

    (response, response_body) Consume response_body in a timely fashion.
    Please see [call](https://ocaml.org/p/cohttp-lwt-unix/6.1.1/doc/cohttp-lwt-unix/Cohttp_lwt_unix/Client/index.html#val-call) about how and why.

Having the call documentation tell you to look at the call documentation for more information isn't very helpful.