`#use_output "dune ocaml top"` fails in a project where `dune utop` works

[Khady]

In ahrefs' codebase we can't use #use_output "dune ocaml top" in utop because it fails with an error. This is surprising as dune utop works flawlessly (if we don't consider the long build time)

Expected Behavior

#use_output "dune ocaml top" should work in all projects where dune utop is working.

Actual Behavior

utop # #use_output "dune ocaml top";;
File "(command-output)", line 1:
Error: Reference to undefined global `Dune_site__Dune_site_data'
^CFatal error: exception Stdlib.Sys.Break
^CFatal error: exception Stdlib.Sys.Break

Reproduction

Unfortunately I don't have an open source project to reproduce the error currently.

Specifications

• Version of dune (output of dune --version): e1f3e19
• Version of ocaml (output of ocamlc --version): 4.14.0
• Operating system (distribution and version): debian 11

[nojb]

Thanks. Relevant to the discussion in #6894

[zbaylin]

This also happens (with a different error) when any library requiring dune-build-info is used (or if it is #required itself). I imagine this is because the Build_info_data module has no strict implementation.

This can be easily tested by running:

$ utop
utop # #require "dune-build-info"
Error: Reference to undefined global `Build_info__Build_info_data'

[Khady]

I just found out that the failure is only in utop, it works "fine" in ocaml or in a jupyter kernel. There's still a log about Error: Reference to undefined global `Dune_site__Dune_site_data' but it doesn't block the repl.

$ ocaml
OCaml version 4.14.0
Enter #help;; for help.

Findlib has been successfully loaded. Additional directives:
  #require "package";;      to load a package
  #list;;                   to list the available packages
  #camlp4o;;                to load camlp4 (standard syntax)
  #camlp4r;;                to load camlp4 (revised syntax)
  #predicates "p,q,...";;   to set these predicates
  Topfind.reset();;         to force that packages will be reloaded
  #thread;;                 to enable threads

# #use_output "dune ocaml top .";;
Entering directory '/home/me/Code/ahrefs/monorepo2'
File "(command-output)", line 1:       
Error: Reference to undefined global `Dune_site__Dune_site_data'
# #show Url;;
module Url :
  sig
    type scheme = Http | Https
    val show_scheme : scheme -> string
  end

[Khady]

I was actually too optimistic. I can show the module but not use the function defined in it.

# Url.show_scheme;;
Error: Reference to undefined global `Url'
# Url.show_scheme Http;;
Error: Reference to undefined global `Url'
# Http;;
Error: Unbound constructor Http
# Url.Http;;
- : Url.scheme = Url.Http
# let a = Url.Http;;
val a : Url.scheme = Url.Http
# let a = Url.show_scheme Url.Http;;
Error: Reference to undefined global `Url'