Allow clock to be configured in Uring.wait

[bikallem]

This PR allows specifying clocks for timeout (as timeout is relative to a clock) when waiting for cqe events; specifically Uring.wait now accepts clock argument along with the timeout value in nanoseconds.

Summary:

• Changes timeout from seconds to nanoseconds (float -> int64)
• Add Clock_mono, Clock_sys and Clock_default to Uring.wait

Required by ocaml-multicore/eio#249

[talex5]

I think the wait API should match the C library. You should be able to add io_uring_prep_timeout as a separate operation, like the other jobs.

[bikallem]

I think the wait API should match the C library. You should be able to add io_uring_prep_timeout as a separate operation, like the other jobs.

Does that mean we need another version of wait_cqe_timeout - for eg. wait_cqe_timeout2. Something like this:

let wait ?clock ?timeout t =
  match timeout with
  | None -> fn_on_ring Uring.wait_cqe t
  | Some timeout -> begin
    assert(timeout >= 0L);
    match clock with 
    | Some c -> fn_on_ring (Uring.wait_cqe_timeout2 c timeout) t
    | None ->  fn_on_ring (Uring.wait_cqe_timeout timeout) t
 end

AFAIK there is no corresponding liburing API which matches what I am attempting to do here, i.e wait cqe with clock source specified.

[talex5]

There shouldn't be any need to change anything about wait to support different clocks. Just add a new function for timeouts. Something like:

val timeout : 'a t -> [`Clock_mono |`Clock_sys | `Clock_default] -> int64 -> 'a -> 'a job option

Then users can submit (and cancel) timeout operations just like any other job.

[bikallem]

@talex5 created a new function timeout as suggested.

[talex5]

Needs a test-case added to tests/main.md.

[bikallem]

@talex5 Uring.submit on Uring.timeout job returns -62. Is this optimal/ergonomic? Should it perhaps return [ETIME | Int of int]? Or perhaps a helper function val is_timeout : int -> boolto decode the return value fromUring.submit`?

EDIT: Aargh, just realized changing Uring.submit would incur huge backwards incompatibility change on eio, so disregard my above comment on changing Uring.submit signature.

[talex5]

Looks good to me, once the flag(s) are added.