Idea for pure flakes

[yajo]

I'm being affected by DeterminateSystems/flakehub-push#170.

Of course that's not devenv's fault. However, it made me wonder: why do we need a static value? This is how it's defined:

devenv/src/modules/top-level.nix

Lines 164 to 168 in 8f8c96b

	root = lib.mkOption {
	type = types.str;
	internal = true;
	default = builtins.getEnv "PWD";
	};

We can learn from other projects.

devshell uses environmental values from nix shell or direnv if available, and otherwise allows to provide a fallback script that can apply any custom logic that sets their PRJ_ROOT variable.

dream2nix allows setting path.projectRootFile, so that the script traverses the file tree up to finding this file, to find what is considered the project root.

That should let devenv flakes be evaluated purely, while still having a working dev shell with direnv, nix-shell or nix develop.

[gabyx]

Currently I use a custom flake input "variable" devenv-root to not need --no-pure-eval (hack/workaround)

 nix develop --accept-flake-config --override-input devenv-root "path:.devenv/state/pwd" "./tools/nix#default"

Where in my flake I have defined these inputs

    # The devenv module to create good development shells.
    devenv = {
      url = "github:cachix/devenv";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    devenv-root = {
      url = "file+file:///dev/null";
      flake = false;
    };

and makeShell to do this

  makeShell =
    toolchain:
    inputs.devenv.lib.mkShell {
      inherit pkgs inputs;
      modules = [
        (
          { ... }:
          {
            devenv.flakesIntegration = true;
            # This is currently needed for devenv to properly run in pure hermetic
            # mode while still being able to run processes & services and modify
            # (some parts) of the active shell.
            devenv.root = builtins.readFile inputs.devenv-root.outPath;
          }
        )
        ({ ... }: toolchain)
      ];
    };

In .envrc I do this to write the .devenv/state/pwd file

# This is currently needed for devenv to properly run in pure hermetic
# mode while still being able to run processes & services and modify
# (some parts) of the active shell.mkdir -p .devenv/state
# See: https://github.com/cachix/devenv/issues/1461
if [ ! -f .devenv/state/pwd ] ||
    [ "$(pwd)" != "$(cat .devenv/state/pwd)" ]; then
    mkdir -p .devenv/state && pwd >.devenv/state/pwd
fi

use flake ./tools/nix#default \
    --override-input devenv-root "path:.devenv/state/pwd"

This works but devenv up does not work once inside the shell. It would help a lot if we could get rid of this annoyance somehow better?

[sandydoo]

The long-term solution to this is #1548.

devshell uses environmental values from nix shell or direnv if available, and otherwise allows to provide a fallback script that can apply any custom logic that sets their PRJ_ROOT variable.

This only works for scripts that are executed after enterShell. There is a lot of value in knowing the project root path during the shell build, i.e. inside of Nix.

dream2nix allows setting path.projectRootFile, so that the script traverses the file tree up to finding this file, to find what is considered the project root.

devenv is currently implemented on top of Nix flakes. Flakes are always copied to the store before evaluation, so any path traversal would return the immutable store path. In fact, this root path is exposed as an API within flakes, but it's not useful to us for hooking up local state. What we want instead is the original path to your project.

[yajo]

There is a lot of value in knowing the project root path during the shell build, i.e. inside of Nix.

Could you give some examples? It'd help understand better.

[yajo]

Flakes are always copied to the store before evaluation

Relevant: NixOS/nix#3121

so any path traversal would return the immutable store path. In fact, this root path is exposed as an API within flakes, but it's not useful to us for hooking up local state. What we want instead is the original path to your project.

Yes, of course! And you can still get that when getting --impure. I just meant to not require passing --impure. For example, in CI environments, sometimes we care more about caching (for speed).

FWIW I found out that the current templates allow some kind of that:

devenv/templates/flake-parts/flake.nix

Lines 5 to 8 in 028c6a3

	devenv-root = {
	url = "file+file:///dev/null";
	flake = false;
	};

devenv/templates/flake-parts/flake.nix

Lines 38 to 42 in 028c6a3

	devenv.root =
	let
	devenvRootFileContent = builtins.readFile devenv-root.outPath;
	in
	pkgs.lib.mkIf (devenvRootFileContent != "") devenvRootFileContent;

devenv/templates/flake-parts/.envrc

Lines 8 to 10 in 028c6a3

	DEVENV_ROOT_FILE="$(mktemp)"
	printf %s "$PWD" > "$DEVENV_ROOT_FILE"
	if ! use flake . --override-input devenv-root "file+file://$DEVENV_ROOT_FILE"

It's not very ergonomic. I guess that could be added as a default value if an input named devenv-root is found in the flake. But I'll give it a shot.