[BUG] Some variables escaped: <v151362>

[nojb]

I am testing js_of_ocaml 6.2.0 and some of our code triggers (only when built in release mode) the following fatal error:

Some variables escaped (#1). Use [--debug js_assign] for more info.
/home/nojebar/mlfi/mlfi-ins/bin/js_of_ocaml: You found a bug. Please report it at https://github.com/ocsigen/js_of_ocaml/issues :
Error: File "js_of_ocaml/compiler/lib/js_assign.ml", line 503, characters 5-11: Assertion failed

The file being built looks like:

(executable
 (name foo)
 (libraries lib)
 (modes js))

where lib is a large-ish unwrapped library built with (library_flags -linkall). Unfortunately, I haven't been able to produce a small reproduction case so far. In case it is helpful, the output of --debug js_assign is attached

log.txt

[nojb]

If you have any suggestions or tips to debug this, I am happy to hear them :) Thanks.

[nojb]

It is somehow related to passing -linkall because the error goes away if I remove it.

[nojb]

I was able to produce a smaller example:

Put the following in a file js_UI.ml

external fun_to_js: (unit -> unit) -> unit = "foo"

let[@tail_mod_cons] rec aux f = f () :: aux f

let map_to_list f = aux (fun x -> f x)

let embedded_input_file_handler _ =
  fun_to_js
    (fun _ ->
       let _ = map_to_list (fun _ -> assert false) in
       ()
    )

let global = ref []

let () = global := List.concat [[embedded_input_file_handler]; !global]

with an empty js_UI.mli and use the following Dune file:

(library
 (name js_core)
 (wrapped false)
 (library_flags -linkall)
 (modules js_UI))

(rule (with-stdout-to foo.ml (progn)))

(executable
 (name foo)
 (modules foo)
 (libraries js_core)
 (js_of_ocaml (flags (--debug js_assign)))
 (modes js))

Then dune build --profile release foo.bc.js fails with the following error log:

js_log.txt

[hhugo]

My hunch is that it comes from #1935.
@vouillon, can you take a look ? It seems that we do not update live_vars of free variables when` duplicating closures.

[hhugo]

@nojb, can you try #2107 ?

[nojb]

@nojb, can you try #2107 ?

Thanks for the quick response. Will do, and report back on the PR itself.