Hanging Lwt_main.run case

[koen-struyve]

The following code snippet

let test () =
  let waiter, wakener = Lwt.task () in
  let wrapped_t =
    begin
      let%lwt () = Lwt_unix.yield () in
      Lwt.return ()
    end
    [%finally
      let () = Lwt.wakeup wakener () in
      Lwt.return ()
    ]
  in
  let%lwt () = waiter in
  wrapped_t

let () = Lwt_main.run (test ())

hangs indefinitely, with wrapped_t remaining in a Sleep state. We noted that binding on the result of "test ()" first seems to resolve this.

This behavior is present from version 3.1.0 onwards.

[aantron]

Thanks for reporting. The attached commit fixes the issue. It is a bug in bind, catch, etc., that I accidentally introduced while reorganizing lwt.ml (#354). All the functions are fixed, and I also added test cases for all of them based on your snippet (thank you again).

I will look at couple more issues, and then make a release to make sure this fix reaches people soon.

[hcarty]

I see something similar with 3.2.1 - haven't tested against master yet to know if it's the same issue but the hanging effect is the same:

let l : unit Lwt.t list = []

let _, _ = Lwt_main.run (Lwt.nchoose_split [])

will hang indefinitely. Tested in utop and compiled with ocamlopt, 64bit Linux w/libev.

[hcarty]

Looks like this is a separate issue that exists after opam pin add lwt --dev-repo.

[hcarty]

Issue opened in #551

[aantron]

@koen-struyve the fix is now out in 3.3.0. Thanks again for reporting.