Builds fail with (lang dune 3.17)

[henrytill]

Describe the bug
When dune-project contains (lang dune 3.17), builds fail with:

error: builder for '/nix/store/6i5di8cfaxp8fjzh0xp177l5zfkiphip-repro-dev.drv' failed with exit code 1;
       last 12 log lines:
       > Running phase: unpackPhase
       > unpacking source archive /nix/store/yr0mkpxm5bpzi33br6q6jf0gb7dd8m49-01klbdnhwh0s9j5km1pnb030px9angz5-bk9izj5svxgyvcmmvjhcjc0wfazifacn-source
       > source root is 01klbdnhwh0s9j5km1pnb030px9angz5-bk9izj5svxgyvcmmvjhcjc0wfazifacn-source
       > Running phase: patchPhase
       > Running phase: updateAutotoolsGnuConfigScriptsPhase
       > Running phase: configurePhase
       > patching script interpreter paths in .
       > Running phase: buildPhase
       > File "repro.opam", line 1, characters 10-25:
       > 1 | # This file is generated by dune, edit dune-project instead
       >               ^^^^^^^^^^^^^^^
       > Error: repository does not contain any version information
       For full logs, run 'nix-store -l /nix/store/6i5di8cfaxp8fjzh0xp177l5zfkiphip-repro-dev.drv'.

To Reproduce

dune init proj repro
cd repro
dune build @all
nix flake init -t github:tweag/opam-nix
sed -i 's/throw "Put the package name here!"/"repro"/' flake.nix
nix build # fails
sed -i 's/3.17/3.16/' dune-project 
nix build # succeeds

Expected behavior
Build succeeds.

Environment

$ uname -a
Linux thalassa 6.12.12-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.12-1 (2025-02-02) x86_64 GNU/Linux
$ dune --version
3.17.2
$ nix --version
nix (Nix) 2.25.3

[balsoft]

This is a weird one. The command that fails with that error is `dune subst`, which is called because dune automatically adds it to the generated `repro.opam` with `dev` condition. `opam-nix` sets `dev = true` because the package comes from a source repository and not from `opam-repository`. The reason for the failure is that when `dune_version` is `3.17`, it tries to call `git describe` to get the package version, even though there is no templates to substitute; this fails because there's no `git` in the environment, and even if there was there's no `.git` subdir for it to work. This is in contrast with the behaviour when `dune_version` is `3.16`, in which case it is lazy and doesn't call `git` since there's nothing to substitute. I'm not yet sure the reason for this change or what the proper fix would be. For now, you can add `(subst false)` stanza to your `dune-project` to prevent it from generating the `dune subst` command in `repro.opam`; this at least circumvents the issue.

[balsoft]

There's a couple of similar issues in dune's bug tracker: ocaml/dune#11200 ocaml/dune#11290

[balsoft]

Aha, there's actually this exact issue already reported as well: ocaml/dune#11045

I think it makes sense to close this issue in favor of that one, do you agree?

[henrytill]

Thanks for looking into this, much appreciated.

Indeed this works around the problem for now:

echo "(subst disabled)" >> dune-project
dune build repro.opam
nix build # succeeds

I agree with your assertion that this is seems to be an upstream issue.

At the same time, anyone starting a fresh project bringing in opam-nix or wanting to upgrade to use newer features like wasm_of_ocaml support (like #113) is going to hit this issue until it's addressed upstream. In fact, I discovered this bug trying to use wasm_of_ocaml in an existing project and only discovered then that it could be reproduced with a fresh project.

So it may make sense to leave this open for visibility, so that other users can find out what is happening when they hit it.