nixpkgs/modules/system/boot/loader/generations-dir/generations-dir.nix

{pkgs, config, ...}:

###### interface
let
  inherit (pkgs.lib) mkOption mkIf;

  options = {
    boot = {
      loader = {
        generationsDir = {

          enable = mkOption {
            default = false;
            description = ''
              Whether to create symlinks to the system generations under
              <literal>/boot</literal>.  When enabled,
              <literal>/boot/default/kernel</literal>,
              <literal>/boot/default/initrd</literal>, etc., are updated to
              point to the current generation's kernel image, initial RAM
              disk, and other bootstrap files.

              This optional is not necessary with boot loaders such as GNU GRUB
              for which the menu is updated to point to the latest bootstrap
              files.  However, it is needed for U-Boot on platforms where the
              boot command line is stored in flash memory rather than in a
              menu file.
            '';
          };

          copyKernels = mkOption {
            default = false;
            description = "
              Whether copy the necessary boot files into /boot, so
              /nix/store is not needed by the boot loadear.
            ";
          };
        };
      };
    };
  };

in

###### implementation
let
  generationsDirBuilder = pkgs.substituteAll {
    src = ./generations-dir-builder.sh;
    isExecutable = true;
    inherit (pkgs) bash;
    path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
    inherit (config.boot.loader.generationsDir) copyKernels;
  };

  # Temporary check, for nixos to cope both with nixpkgs stdenv-updates and trunk
  platform = pkgs.stdenv.platform;
in
{
  require = [
    options

    # config.system.build
    # ../system/system-options.nix
  ];

  system = mkIf config.boot.loader.generationsDir.enable {
    build.installBootLoader = generationsDirBuilder;
    boot.loader.id = "generationsDir";
    boot.loader.kernelFile = platform.kernelTarget;
  };
}