Refactor `overrides.nix` using `pkgs.formats`

[PJungkamp]

I'd like to see a more flexible overrides mechanism using pkgs.formats. The current implementation only supports the bare minimum of overrides. The format of the overrides file is, according to flatpak-metadata(5), just plain ini with list that are concatenated with ';'.

Here is an example of how one might use the upstream nixpkgs ini format:

{ pkgs ? import <nixpkgs> {}, lib ? pkgs.lib }:
let
  ini = pkgs.formats.ini {
    listToValue = lib.concatMapStringsSep ";" (lib.generators.mkValueStringDefault {});
  };
in ini.generate "flatpak-override" {
  "Context" = {
    "filesystems" = ["~/Games/Heroic" "/media/Data"];
    "sockets" = "!x11";
  };
}

I'll give it a try myself when I get around to tweaking my NixOS configuration again (might take some time), I just wanted to drop it here for anyone who's interested in improving the override mechanism. :D

It shouldn't be hard to make this as flexible as the upstream nixpkgs systemd service configuration, which I'd take as inspiration and design guideline.

@GermanBread Danke für dieses NixOS Modul! Ein Hoch auf das deutsche Brot!

[GermanBread]

Now I get it. But the issue remains that I cannot implement the type checks properly.

I have no idea how to tell Nix that the overrride attribute can support different submodules (types.oneOf [ <list of submodules> ] for example). It just errors out in a weird way.

Thanks for the tip though.

[PJungkamp]

I'd suggest something like the following for the options. (Just from the top of my head, not actually tested. I'm pretty much guaranteed to miss some semicolon)

  overrides = mkOption {
    type = types.attrsOf (types.submodule ({config, ...}: {
      options = {
        source = mkOption {
          type = types.path;
          internal = true;
        };
        metadata = mkOption {
          type = with types; attrsOf (attrsOf anything);
          internal = true;
        };
        filesystems = mkOption {
          type = with types; listOf str;
          default = [];
        };
        sockets = mkOption {
          type = with types; listOf str;
          default = [];
        };
        environment = mkOption {
          type = with types; attrsOf str;
          default = {};
        };
      };

      config.metadata = {
        "Context" = {
          inherit (config) filesystems sockets;
        };
        "Environment" = config.environment;
      };

      config.source = ini.generate config.metadata;
    });
    default = {};
  };

Note that I changed the type of overrides.<name>.environment and removed some unnecessary nullOr types. You can then pretty much generate the INI file using the ini.generate from above.

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The nice thing here is that it also supports overriding the generated metadata file with another one which might also resolve issue #14. He'd be able to use builtins.readDir and map in combination with the internal overrides.<name>.source attribute to realize his wish.

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I also quite like using internal options in NixOS modules since they can be inspected easily using nix eval and nix build.

# here's how I'd format my laptops global flatpak overrides as nice json using `jq`
$ nix eval --json .#nixosConfigurations.yoga9.config.services.flatpak.overrides.global.metadata | jq

# here's how I'd build and inpect the generated metadata file
$ nix build .#nixosConfigurations.yoga9.config.services.flatpak.overrides.global.source | cat ./result

It makes working with generated files so much easier.

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I'm currently not using the declarative-flatpak module, I tried to refactor a bit so that it won't touch my system flatpak installation and uses a custom one instead, but I haven't got it working yet. See e.g. https://blogs.gnome.org/mclasen/2018/08/26/about-flatpak-installations/

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Just some thoughts!

I hoffe du bist gut ins neue Jahr gekommen! :D

EDIT: I did miss a semicolon.