machine_setup/nix/configuration/util/install_files/default.nix

{
  config,
  lib,
  pkgs,
  home-manager,
  ...
}:

let
  cfg = config.me.install;
  inherit (lib)
    filter
    attrNames
    ;

  install_user_file =
    let
      constructors = {
        "overwrite" = install_user_file_overwrite;
        "symlink" = install_user_file_symlink;
      };
    in
    stage: target: (constructors."${target.method}"."${stage}" target);
  old_install_user_file =
    let
      constructors = {
        "overwrite" = install_user_file_overwrite;
        "symlink" = install_user_file_symlink;
      };
    in
    (target: (constructors."${target.method}" target));
  install_user_file_overwrite = {
    "check" = (target: "");
    "install" = (
      target:
      let
        source = lib.strings.escapeShellArg "${target.source}";
        destination = lib.strings.escapeShellArg "${target.target}";
        mode = lib.strings.escapeShellArg "${target.mode}";
      in
      ''
        $DRY_RUN_CMD install $VERBOSE_ARG -D --compare -m ${mode} ${source} ${destination}
      ''
    );
    "uninstall" = (target: "");
  };
  install_user_file_symlink = {
    "check" = (target: "");
    "install" = (
      target:
      let
        source = lib.strings.escapeShellArg "${target.source}";
        destination = lib.strings.escapeShellArg "${target.target}";
      in
      ''
        $DRY_RUN_CMD ln $VERBOSE_ARG -s ${source} ${destination}
      ''
    );
    "uninstall" = (target: "");
  };
in
{
  imports = [ ];

  options.me.install = {
    user = lib.mkOption {
      type = lib.types.attrsOf (
        lib.types.submodule (
          { name, config, ... }:
          let
            username = name;
          in
          {
            options = {
              enable = lib.mkOption {
                type = lib.types.bool;
                default = true;
                defaultText = "enable";
                example = lib.literalExpression false;
                description = "Whether we want to install files in this user's home directory.";
              };

              file = lib.mkOption {
                type = lib.types.attrsOf (
                  lib.types.submodule (
                    { name, config, ... }:
                    let
                      path = name;
                    in
                    {
                      options = {
                        enable = lib.mkOption {
                          type = lib.types.bool;
                          default = true;
                          defaultText = "enable";
                          example = lib.literalExpression false;
                          description = "Whether we want to install this file in this user's home directory.";
                        };
                        username = lib.mkOption {
                          type = lib.types.str;
                          defaultText = "username";
                          example = "root";
                          description = "The username for the user whose home directory will contain the file.";
                        };
                        target = lib.mkOption {
                          type = lib.types.str;
                          defaultText = "target";
                          example = ".local/share/foo/bar.txt";
                          description = "The path where the file should be written.";
                        };
                        method = lib.mkOption {
                          type = lib.types.enum [
                            "symlink"
                            "overwrite"
                            # "bind_mount" TODO: for directories?
                          ];
                          default = "symlink";
                          defaultText = "me.install.file.‹path›.method";
                          example = "overwrite";
                          description = "The way in which the file should be installed.";
                        };
                        mode = lib.mkOption {
                          type = lib.types.str;
                          default = "0444";
                          defaultText = "me.install.file.‹path›.mode";
                          example = "0750";
                          description = "The read, write, execute permission flags.";
                        };
                        source = lib.mkOption {
                          type = lib.types.path;
                          defaultText = "me.install.file.‹path›.source";
                          example = ./files/foo.txt;
                          description = "The source file to install into the destination.";
                        };
                      };

                      config = {
                        username = lib.mkDefault username;
                        target = lib.mkDefault path;
                      };
                    }
                  )
                );
              };
            };
          }
        )
      );
    };
  };

  config =
    let
      all_users = builtins.map (username: cfg.user."${username}") (attrNames cfg.user);
      enabled_users = filter (user: user.enable) all_users;
      all_file_targets = lib.flatten (
        builtins.map (user: (builtins.map (path: user.file."${path}") (attrNames user.file))) enabled_users
      );
      enabled_file_targets = filter (target: target.enable) all_file_targets;
      check_commands = builtins.map (install_user_file "check") enabled_file_targets;
      install_commands = builtins.map (install_user_file "install") enabled_file_targets;
      uninstall_commands = builtins.map (install_user_file "uninstall") enabled_file_targets;
    in
    {
      systemd.services.me-install-file = {
        enable = true;
        description = "me-install-file";
        wantedBy = [ "multi-user.target" ];
        wants = [ "multi-user.target" ];
        after = [ "multi-user.target" ];
        # path = with pkgs; [
        #   zfs
        # ];
        unitConfig.DefaultDependencies = "no";
        serviceConfig = {
          Type = "oneshot";
          RemainAfterExit = "yes";
        };
        script = (lib.strings.concatStringsSep "\n" (check_commands ++ install_commands));
        preStop = (lib.strings.concatStringsSep "\n" uninstall_commands);
      };

      environment.etc."install_out".text = config.systemd.services.me-install-file.script;
    };
}