nixpkgs/modules/config/guest-users.nix

{pkgs, config, ...}:

let
  inherit (pkgs.lib) mkOption;

  options = {
    services = {
      guestUsers = {
        enable = mkOption {
	  default = false;
	  description = "
	    Whether to enable automatic addition of users with empty passwords
	  ";
	};
	users = mkOption {
	  default = ["guest"];
	  description = "
	    List of usernames to add
	  ";
	};
	includeRoot = mkOption {
	  default = false;
	  description = "
	    LEAVE THAT ALONE; whether to reset root password
	  ";
	};
	extraGroups = mkOption {
	  default = ["audio"];
	  description = "
	    Extra groups to grant
	  ";
	};
      };
    };
  };
  
  inherit (pkgs.lib) concatStringsSep optionalString;

  cfg = config.services.guestUsers;

  userEntry = user:
  {
    name = user;
    description = "NixOS guest user";
    home = "/home/${user}";
    createHome = true;
    group = "users";
    extraGroups = cfg.extraGroups;
    shell = "/bin/sh";
  };

  nameString = (concatStringsSep " " cfg.users) + optionalString cfg.includeRoot " root";

in 

pkgs.lib.mkIf cfg.enable {
  require = options;

  system.activationScripts = {

    clearPasswords = pkgs.lib.fullDepEntry
      ''
        for i in ${nameString}; do
            echo | ${pkgs.pwdutils}/bin/passwd --stdin $i
        done
      '' ["defaultPath" "users" "groups"];

  };

  services.mingetty.helpLine = "\nThese users have empty passwords: ${nameString}";
  
  users.extraUsers = map userEntry cfg.users;
}