OCaml Unix.select bug impacts Lwt_engine.select

[gabelevi]

OCaml's Unix.select takes three list of fds. If any fd in any list is < 0 or > FD_SETSIZE then it will raise Unix.Unix_error(EINVAL, "select", ""). This is probably not quite the right behavior, and I've reported it here: https://caml.inria.fr/mantis/view.php?id=7700

Lwt_engine.select_based explicitly handles EBADF, but doesn't handle EINVAL.. Ideally, OCaml will be fixed to only throw EBADF for bad fds, but in the meantime it might make sense to catch EINVAL and filter the fds.

I found this issue via a stack trace. I'm using OCaml 4.05 and Lwt 3.1.0.

Unhandled exception: Unix.Unix_error(Unix.EINVAL, "select", "")
Raised by primitive operation at file "src/unix/lwt_engine.ml", line 410, characters 26-60
Called from file "src/unix/lwt_engine.ml", line 351, characters 8-19
Called from file "src/unix/lwt_main.ml", line 40, characters 4-78
Called from file "flow/src/flow.ml", line 81, characters 6-34
Called from file "flow/src/flow.ml", line 117, characters 4-21

[gabelevi]

I'm guessing that my particular issue is just having a more than 1024 fds open, and passing through an fd > 1024 (FD_SETSIZE). I'm guessing this is the most common reason anyone will see Unix.Unix_error(Unix.EINVAL, "select", ""), even though I think that is a non-standard use of EINVAL

I think I'm going to try something like this in the meantime, which should make it clear in my logs what's going on.

class my_unix_select = object
  inherit Lwt_engine.select_based

  method private select fds_r fds_w timeout =
    let fds_r, fds_w, _ =
      try Unix.select fds_r fds_w [] timeout
      with
      | Unix.Unix_error (Unix.EINVAL, fn, params) -> begin
        (* Ok, so either one of the fds is an invalid fd, or maybe it's a valid fd but too large
         * for select *)
        begin try
          let explode_if_bad fd = Unix.fstat fd |> ignore in
          List.iter explode_if_bad fds_r;
          List.iter explode_if_bad fds_w
        with Unix.Unix_error (_, _, _) ->
          raise (Unix.Unix_error (Unix.EBADF, fn, params))
        end;
        (* Oh boy. So it looks like all the fds are valid. This likely means that one fd is larger
         * than FD_SETSIZE (which is probably 1024). select() stops working for large fds like this
         *)
        let string_of_fd fd = string_of_int ((Obj.magic fd): int) in
        let string_of_fds fds = String.concat ";" (List.map string_of_fd fds) in
        let params = spf "[%s] [%s] []" (string_of_fds fds_r) (string_of_fds fds_w) in
        raise (Unix.Unix_error (Unix.EINVAL, "select", params))
      end
    in
    (fds_r, fds_w)
end

[aantron]

Yes, this is most likely due to an fd over the limit for select. I do prefer EBADF to EINVAL, it seems more consistent with the (still ambiguous) verbiage here: http://pubs.opengroup.org/onlinepubs/9699919799/functions/select.html.

It might be useful to expose a comparison with FD_SETSIZE to OCaml to avoid all those fstat calls, on the other hand this is only done in an exceptional situation, and we already do them for EBADF anyway.

It's normally recommended to use Lwt with libev (opam install conf-libev lwt) to avoid this problem. Is that an option for you?

[aantron]

We may want to merge code like your check into the select engine, to get nicer error messages. If the Mantis ticket results in Unix.select throwing EBADF instead, we'll have to run this check in both cases to support OCaml down to 4.02.

[gabelevi]

It's normally recommended to use Lwt with libev (opam install conf-libev lwt) to avoid this problem. Is that an option for you?

My project runs also runs on Windows, so probably can't use libev. For the moment, it's easier for me to try and avoid having too many fds. In the future, though, if I need more fds I may try and build a libuv lwt_engine. If I do, I'll definitely send a PR.