Dune 3.21.0 unable to lock project depending on ocaml-system

[gridbugs]

Dune 3.21.0 can't find a solution if a project depends on the package ocaml-system. This is a regression from 3.20.2 where Dune could successfully find a solution.

Expected Behavior

Dune should be able to generate lockdirs for projects depending on the ocaml-system package.

Actual Behavior

The command dune pkg lock fails with this error:

Error:
Unable to solve dependencies while generating lock directory: dune.lock

The dependency solver failed to find a solution for the following platforms:
- arch = x86_64; os = linux
- arch = arm64; os = linux
- arch = x86_64; os = macos
- arch = arm64; os = macos
...with this error:
Couldn't solve the package dependency formula.
Selected candidates: foo.dev
- ocaml-system -> (problem)
    No usable implementations:
      ocaml-system.5.4.0: Availability condition not satisfied
      ocaml-system.5.3.0: Availability condition not satisfied
      ocaml-system.5.2.1: Availability condition not satisfied
      ocaml-system.5.2.0: Availability condition not satisfied
      ocaml-system.5.1.1: Availability condition not satisfied
      ...

Reproduction

Build this dockerfile:

FROM archlinux

RUN pacman -Syu --noconfirm
RUN pacman -S --noconfirm ocaml git

RUN curl -4fsSL https://github.com/ocaml-dune/dune-bin-install/releases/download/v3/install.sh | sh -s 3.21.0 --install-root /usr --no-update-shell-config

RUN useradd --create-home --shell /usr/sbin/bash user
USER user
WORKDIR /home/user

RUN mkdir project
WORKDIR project

COPY <<EOF dune-project
(lang dune 3.20)

(package
 (name foo)
 (depends ocaml-system))
EOF

COPY <<EOF foo.ml
let () = ()
EOF

COPY <<EOF dune
(executable (public_name foo))
EOF

RUN dune pkg lock

Change the version to 3.20.2 in the curl command and it will succeed.

[Alizter]

This is due to portable lock directories. You can fix it by including a solver env variable like:

COPY <<EOF dune-workspace
(lang dune 3.20)
(lock_dir
 (solver_env
  (sys-ocaml-version 5.4.0)))
EOF

or disabling portable lock directories all together with:

ENV DUNE_CONFIG__PORTABLE_LOCK_DIR=disabled

Unfortunately, dune is stripping system-dependent solver environment variables like the system OCaml version which makes it difficult to choose ocaml-system.

[gridbugs]

Ah that makes sense! I guess it would be a bit weird if a user could check in a lockdir with a version of ocaml derived from whichever version of ocaml they had installed on their system at the time. Maybe in this case Dune could print a warning suggesting to add an appropriate section to dune-workspace?