Lwt.protected listeners are not removed from the protected thread on cancel

[mfp]

It is rather easy to expect Lwt.protected to work like choose or pick (which remove listeners) and keep adding listeners accidentally to an existing thread when wrapping it with Lwt.protected. This could happen if the user does for example something resembling match_lwt Lwt.pick [Lwt.protected t; Lwt_unix.sleep t0 >> raise_lwt Timeout] (where the t thread is not canceled on timeout, but the timeout thread is on completion).

Iow., is there a reason for the following not to run in constant mem?

let () =
  Lwt_main.run begin
    let t, _ = Lwt.wait () in
    Lwt.on_cancel t (fun () -> print_endline "CANCEL 1");

    let rec loop () =
      let t' = Lwt.bind (Lwt.protected t) @@ fun () ->
               Lwt.return @@ print_endline "X"
      in
        Lwt.cancel t';
        loop ()
    in
      loop ()
  end

[aantron]

Thanks, this was a bug. If interested, see the message of the attached commit for details.

[mfp]

On Tue, Jun 28, 2016 at 06:26:48PM -0700, Anton wrote:

Closed #181 via 616168f.

Thank you.

I'm adding for the record (to help future readers looking for help on memory
issues with the currently released Lwt versions) that this also solves
memleaks in Lwt_stream (peek, and probably more functions).

Mauricio Fernández

[aantron]

Thanks. Indeed. See, for instance, #56 (also fixed by the commit).