nixos-modules/modules/nix.nix

{ config, lib, pkgs, ... }:

let
  cfg = config.nix;
in
{
  options.nix = {
    deleteChannels = lib.mkEnableOption "" // { description = "Whether to delete all channels on a system switch."; };

    deleteUserProfiles = lib.mkEnableOption "" // { description = "Whether to delete all user profiles on a system switch."; };

    remoteBuilder = {
      enable = lib.mkEnableOption "restricted nix remote builder";

      sshPublicKeys = lib.mkOption {
        description = "SSH public keys accepted by the remote build user.";
        type = lib.types.listOf lib.types.str;
      };

      name = lib.mkOption {
        description = "Name of the user used for remote building.";
        type = lib.types.str;
        readOnly = true;
        default = "nix-remote-builder";
      };
    };
  };

  config = {
    # based on https://github.com/numtide/srvos/blob/main/nixos/roles/nix-remote-builder.nix
    # and https://discourse.nixos.org/t/wrapper-to-restrict-builder-access-through-ssh-worth-upstreaming/25834
    nix.settings.trusted-users = lib.mkIf cfg.remoteBuilder.enable [ cfg.remoteBuilder.name ];

    users.users.${cfg.remoteBuilder.name} = lib.mkIf cfg.remoteBuilder.enable {
      group = "nogroup";
      isNormalUser = true;
      openssh.authorizedKeys.keys = map
        (key:
          let
            wrapper-dispatch-ssh-nix = pkgs.writeShellScriptBin "wrapper-dispatch-ssh-nix" /* bash */ ''
              case $SSH_ORIGINAL_COMMAND in
                "nix-daemon --stdio")
                  exec ${config.nix.package}/bin/nix-daemon --stdio
                  ;;
                "nix-store --serve --write")
                  exec ${config.nix.package}/bin/nix-store --serve --write
                  ;;
                *)
                  echo "Access is only allowed for the nix remote builder" 1>&2
                  exit 1
              esac
            '';

          in
          "restrict,pty,command=\"${wrapper-dispatch-ssh-nix}/bin/wrapper-dispatch-ssh-nix\" ${key}"
        )
        config.nix.remoteBuilder.sshPublicKeys;
    };

    system.activationScripts = {
      deleteChannels = lib.mkIf cfg.deleteChannels ''
        echo "Deleting all channels..."
        rm -rf /root/.nix-channels /home/*/.nix-channels /nix/var/nix/profiles/per-user/*/channels* || true
      '';

      deleteUserProfiles = lib.mkIf cfg.deleteUserProfiles ''
        echo "Deleting all user profiles..."
        rm -rf /root/.nix-profile /home/*/.nix-profile /nix/var/nix/profiles/per-user/*/profile* || true
      '';
    };
  };
}