nix-config/modules-hm/impermanence.nix

{ config, lib, ... }:
let
  inherit (lib)
    flip
    mapAttrs
    attrNames
    mkOption
    types
    mkMerge
    isAttrs
    ;
in
{
  # Expose a home manager module for each user that allows extending
  # environment.persistence.${sourceDir}.users.${userName} simply by
  # specifying home.persistence.${sourceDir} in home manager.
  home-manager.sharedModules = [
    {
      options.home.persistence = mkOption {
        description = "Additional persistence config for the given source path";
        default = { };
        type = types.attrsOf (
          types.submodule {
            options = {
              files = mkOption {
                description = "Additional files to persist via NixOS impermanence.";
                type = types.listOf (types.either types.attrs types.str);
                default = [ ];
              };

              directories = mkOption {
                description = "Additional directories to persist via NixOS impermanence.";
                type = types.listOf (types.either types.attrs types.str);
                default = [ ];
              };
            };
          }
        );
      };
    }
  ];

  # For each user that has a home-manager config, merge the locally defined
  # persistence options that we defined above.
  imports =
    let
      mkUserFiles = map (
        x: { parentDirectory.mode = "700"; } // (if isAttrs x then x else { file = x; })
      );
      mkUserDirs = map (x: { mode = "700"; } // (if isAttrs x then x else { directory = x; }));
    in
    [
      {
        environment.persistence = mkMerge (
          flip map (attrNames config.home-manager.users) (
            user:
            let
              hmUserCfg = config.home-manager.users.${user};
            in
            flip mapAttrs hmUserCfg.home.persistence (
              _: sourceCfg: {
                users.${user} = {
                  # This needs to be set for allo users with non
                  # standart home (not /home/<userName>
                  # due to nixpkgs it
                  # can't be deduced from homeDirectory
                  # as there will be infinite recursion
                  # If this setting is forgotten there
                  # are assertions in place warning you
                  home = { root = "/root"; }.${user} or "/home/${user}";
                  files = mkUserFiles sourceCfg.files;
                  directories = mkUserDirs sourceCfg.directories;
                };
              }
            )
          )
        );
      }
    ];
}