impermanence.nix

{
  config,
  lib,
  ...
}:
let
  inherit (lib)
    attrNames
    flip
    isAttrs
    mapAttrs
    mkMerge
    mkOption
    optionals
    types
    ;
in
{
  # Give agenix access to the hostkey independent of impermanence activation
  age.identityPaths = [ "/persist/etc/ssh/ssh_host_ed25519_key" ];

  # 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} = {
                  files = mkUserFiles sourceCfg.files;
                  directories = mkUserDirs sourceCfg.directories;
                };
              }
            )
          )
        );
      }
    ];

  # State that should be kept across reboots, but is otherwise
  # NOT important information in any way that needs to be backed up.
  fileSystems."/state".neededForBoot = true;
  environment.persistence."/state" = {
    hideMounts = true;
    directories =
      [
        "/var/lib/systemd"
        "/var/log"
        "/var/spool"
        #{ directory = "/tmp"; mode = "1777"; }
        #{ directory = "/var/tmp"; mode = "1777"; }
      ]
      ++ optionals config.networking.wireless.iwd.enable [
        {
          directory = "/var/lib/iwd";
          mode = "0700";
        }
      ];
  };

  # State that should be kept forever, and backed up accordingly.
  fileSystems."/persist".neededForBoot = true;
  environment.persistence."/persist" = {
    hideMounts = true;
    files = [
      # For ephemeral nixos-containers we cannot link the /etc/machine-id file,
      # because it will be generated based on a stable container uuid.
      (lib.mkIf (!config.boot.isContainer) "/etc/machine-id")
      "/etc/ssh/ssh_host_ed25519_key"
      "/etc/ssh/ssh_host_ed25519_key.pub"
    ];
    directories =
      [
        "/var/lib/nixos"
      ]
      ++ optionals config.security.acme.acceptTerms [
        {
          directory = "/var/lib/acme";
          user = "acme";
          group = "acme";
          mode = "0755";
        }
      ]
      ++ optionals config.services.printing.enable [
        {
          directory = "/var/lib/cups";
          mode = "0700";
        }
      ]
      ++ optionals config.services.postgresql.enable [
        {
          directory = "/var/lib/postgresql";
          user = "postgres";
          group = "postgres";
          mode = "0700";
        }
      ];
  };
}