Allow Flake inputs to accept boolean and integer attributes

[DanilaFe]

Looking through the code base for Nix, I found that it creates fetchers::Input from the attributes given to Flakes' input refs (see also lines 108 through 133 in flake.cc):

nix/src/libexpr/flake/flakeref.cc

Lines 191 to 198 in 920e6a6

	FlakeRef FlakeRef::fromAttrs(const fetchers::Attrs & attrs)
	{
	auto attrs2(attrs);
	attrs2.erase("dir");
	return FlakeRef(
	fetchers::Input::fromAttrs(std::move(attrs2)),
	fetchers::maybeGetStrAttr(attrs, "dir").value_or(""));
	}

The attributes accepted by the fetches can be strings, integers, or booleans. However, currently, the code in flake.cc rejects all attributes that are not strings. It does, however, forward all extra string arguments, no matter what they are, to the fetchers::Input. This seems inconsistent, and prevents input types like Git from accepting certain arguments, including submodules.

This PR expands the code in flake.cc to accept non-string arguments. Combined with this, I was able to make non-flake inputs with recursive git modules work as follows:

inputs = {
    blog-source = {
      flake = false;
      url = "https://my.example.com/repo.git";
      type = "git";
      submodules = true;
    }
};

Note that type = "git" is already supported by Nix; this PR allows the submodules = true argument to be passed in alongside type. I believe this would close #4357 (and its duplicate, #4423).

So far, the only non-integer attributes are submodules (bool), revCount (int), and lastModified (int). While the latter two are not particularly useful, I think the benefits for git submodules and consistency make this a worthwhile change.

[infinisil]

I'd probably go for a switch here

[DanilaFe]

I didn't realize it was an enum! Yes, switch works much better here.

[DanilaFe]

I pushed a commit that switches this to a switch.

[knedlsepp]

Do you know how one would make sure that nix fetches submodules for self with this?

[DanilaFe]

@knedlsepp unfortunately, I'm not sure. This change affects only the inputs, which self is not part of. So, to make sure a flake has its submodules initialized and cloned, it must be that whatever is invoking the flake has submodules enabled. With this change, it's impossible to force the current flake to have its own submodules resolved, only its dependencies. I think this is still consistent, though - a flake's own flake.nix does not have control over the fetcher used to retrieve it, as far as I know.

[L-as]

@knedlsepp unfortunately, I'm not sure. This change affects only the inputs, which self is not part of. So, to make sure a flake has its submodules initialized and cloned, it must be that whatever is invoking the flake has submodules enabled. With this change, it's impossible to force the current flake to have its own submodules resolved, only its dependencies. I think this is still consistent, though - a flake's own flake.nix does not have control over the fetcher used to retrieve it, as far as I know.

What you say makes sense, but when you consider how it's actually used, it's very inconvenient. A flake will either always want its submodules initialized, or never want to. A flake that has submodules will not work if you don't enable them, so there is no point in allowing a user to not fetch the submodules for a flake (if they are required to build it).

Since whether submodules should be fetched depends on the flake, I propose that the flake set for itself whether its submodules should be fetched. You could also just unconditionally fetch them for flakes, since I can't see any scenario where you wouldn't want them. It's not foreign repositories being fetched for nixpkgs; they're flakes fit for nix. If the owner has added a submodule, it's likely required for using the flake.

Would a PR enabling fetching of submodules unconditionally for all flakes be accepted?

[nrdxp]

I'm getting a very strange error when trying to use this. After successfully fetching the module and all submodules, nix gave an error that disallowed usage saying:

submodule is not a Boolean

when it fact it is. I was able to work around by manually removing the submodule keys from the flake.lock and it works without it, since the repo has already been fetched.

[hurricanehrndz]

I am also getting submodule is not a Boolean.

[mhuesch]

I am running into the same issue that @L-as nicely described, where my self flake has submodules and I am stuck in the inconvenient situation of not being able to fetch them.

I would be interested in contributing to a flake-submodule-fetcher PR if such a thing would be accepted.

[kenranunderscore]

I'm also in the same situation as @mhuesch and @L-as.
Here is a post by someone showing how to correct this when invoking from the command line. Is there an update on how to fetch submodules for self? As prefixing every nix build target with .?submodules=1 seems quite inconvenient for me.

Ofc I could try converting the submodule to a flake input, but that creates different inconveniences for us.

[nrdxp]

I could try converting the submodule to a flake input, but that creates different inconveniences for us.

It would be inconvenient to do manually, but if Nix itself could gain the power of locking the submodules as flake inputs so that Nix can handle the fetching of them lazily, that might be the best solution in the long run.