Lwt.npick is broken

[aantron]

It seems that trying to resolve Lwt.npick, when the argument promises are initially pending, results in a promise that fails with Lwt.Canceled:

open Lwt.Infix

let () =
  let p1, r = Lwt.task () in
  let p2, _ = Lwt.task () in

  let p3 = Lwt.npick [p1; p2] in
  assert (Lwt.state p3 = Lwt.Sleep);

  Lwt.wakeup r ();

  (* Expected: *)
  (* assert (Lwt.state p3 = Lwt.Return [()]) *)

  (* Actual: *)
  assert (Lwt.state p3 = Lwt.Fail Lwt.Canceled);

(* ocamlfind opt -linkpkg -package lwt npick.ml && ./a.out *)

By comparison, if you replace npick with nchoose, you get the Expected behavior.

Since nobody has complained about this, I guess nobody is actually using npick with pending promises :)

[aantron]

Probably best fixed after #354 is in.