On Windows (Cygwin), lwt version >= 6.1.0 causes "utop" to fail immediately with "segmentation fault"

[bdodrem]

I've recently upgraded the ocaml packages :
ocaml to version 5.4.1
utop to version 2.16.0
lwt to version 6.1.1

Once the upgrade was performed, utop command failed immediately with "segmentation fault".

Downgrading lwt to 6.0.0 solved the utop issue.

This issue is also present on Windows MSYS2 MINGW64

[raphael-proust]

Your setup is too specific for me to reproduce, but I'll look into whether this happens in other setups too.

[roboticarm2000]

Similar thing happened with ocaml 4.14.3 and utop 2.16.0 on windows. Downgrading lwt 6.0.0 fixed it.

[kandu]

steps to reproduce:

1. https://ocaml.org/install#windows
2. opam install lwt
3. compile and run Lwt_main.run (Lwt_io.printl "hi")

[raphael-proust]

The difference between 6.0.0 and 6.1.1 is rather thin (https://github.com/ocsigen/lwt/compare/6.0.0..6.1.1), and not much would be a possible candidate for such an error.

Maybe the lock for starting a job?? @kandu do you observe the failure whenever you replace Lwt_io... with any system call? and do you not get it if you only do non-system work (e.g., just do Lwt.pause)?

[raphael-proust]

can you check on top of #1103 ?

you'd need to opam pin add lwt.dev git+https://github.com/mtelvers/lwt.git#windows-fixes-v2 (or something along this line) to install the correct lwt version

[kandu]

This is another testing:

open Lwt

let t1 ()=
  Lwt_unix.sleep 1. >>= Lwt.pause >>= fun ()-> print_endline "t1 end" |> return

let t2 ()=
  Lwt_unix.sleep 0.5 >>= fun ()-> Lwt_io.(read_line stdin) >|= print_endline

let ()=
  Lwt.all [t1(); t2()] |> Lwt_main.run |> ignore

What I did was:

1. start the program
2. wait for 't1 end' to be printed
3. press keystroke 'a'

and its 'screenshot':

kandu@DESKTOP-2EJF9H7 ~
$ ./l
t1 end
Segmentation fault

kandu@DESKTOP-2EJF9H7 ~
$ a

As you can see, Lwt_unix.sleep and the thread scheduler work as expected. But Lwt_io.read_line causes the segment fault and it didn't consume the 'a' keystroke.

After checking out the windows-fixes-v2 branch, this is the 'screenshot' on my windows 10:

kandu@DESKTOP-2EJF9H7 ~
$ opam pin add lwt ./lwt
[lwt.6.1.0] synchronised (git+file://C:/Users/kandu/AppData/Local/opam/.cygwin/root/home/kandu/lwt#windows-fixes-v2)
lwt is now pinned to git+file://C:/Users/kandu/AppData/Local/opam/.cygwin/root/home/kandu/lwt#windows-fixes-v2 (version
6.1.1)

The following actions will be performed:
=== recompile 4 packages
  ↕ lambda-term 3.3.3                   [uses lwt]
  ↕ logs        0.10.0                  [uses lwt]
  ↕ lwt_react   1.2.0                   [uses lwt]
  ↕ utop        2.16.0                  [uses lwt]
=== upgrade 1 package
  ↑ lwt         6.1.0 to 6.1.1 (pinned)

Proceed with ↕ 4 recompilations and ↑ 1 upgrade? [Y/n]
...
...
kandu@DESKTOP-2EJF9H7 ~
$ ocamlfind ocamlopt -thread -linkpkg -package lwt.unix l.ml -o l

kandu@DESKTOP-2EJF9H7 ~
$ ./l
t1 end
some keystrokes
some keystrokes

kandu@DESKTOP-2EJF9H7 ~
$

The issue is fixed. This program and utop both work properly.

[raphael-proust]

thanks for the tests

so it is the lock and @mtelvers did fix it, i'll get that released (no committement on date)