dev/configs/nixos/modules/services/steamcmd.nix

{
  config,
  lib,
  pkgs,
  ...
}:
{
  options.modules.services.steamcmd = {
    enable = lib.mkEnableOption "SteamCMD-managed game server";

    steamDir = lib.mkOption {
      type = lib.types.str;
      example = "/data/steamcmd";
      description = "Directory where SteamCMD stores metadata and cache files.";
    };

    serviceName = lib.mkOption {
      type = lib.types.str;
      example = "valheim";
      description = "Name of the systemd service that runs the game server.";
    };

    appId = lib.mkOption {
      type = lib.types.ints.positive;
      example = 896660;
      description = "Steam app ID for the dedicated server package.";
    };

    installDir = lib.mkOption {
      type = lib.types.str;
      example = "/data/servers/valheim/main";
      description = "Directory where SteamCMD installs the server files and where the game service starts.";
    };

    stateDir = lib.mkOption {
      type = lib.types.str;
      example = "/data/state/valheim/main";
      description = "Writable directory for durable game server state.";
    };

    executable = lib.mkOption {
      type = lib.types.str;
      example = "./valheim_server.x86_64";
      description = "Server executable, relative to installDir or absolute.";
    };

    arguments = lib.mkOption {
      type = lib.types.listOf lib.types.str;
      default = [ ];
      example = [
        "-name"
        "main"
      ];
      description = "Arguments passed to the server executable.";
    };

    udpPorts = lib.mkOption {
      type = lib.types.listOf lib.types.port;
      default = [ ];
      description = "UDP ports opened for the game service.";
    };

    tcpPorts = lib.mkOption {
      type = lib.types.listOf lib.types.port;
      default = [ ];
      description = "TCP ports opened for the game service.";
    };

    autoStart = lib.mkOption {
      type = lib.types.bool;
      default = true;
      description = "Start the game service after the guest reaches network readiness.";
    };
  };

  config = lib.mkMerge [
    {
      registry."stable-uid".${config.networking.hostName}.steamcmd = 20003;
    }

    (lib.mkIf config.modules.services.steamcmd.enable (
      let
        service = config.modules.services.steamcmd;
        user = "steamcmd";
        group = "storage";

        prepareScript = pkgs.writeShellScript "steamcmd-${service.serviceName}-prepare" ''
          set -eu

          ${pkgs.coreutils}/bin/install -d \
            -o ${user} \
            -g ${group} \
            -m 0770 \
            ${lib.escapeShellArg service.steamDir} \
            ${lib.escapeShellArg service.installDir} \
            ${lib.escapeShellArg service.stateDir}
        '';

        updateScript = pkgs.writeShellScript "steamcmd-${service.serviceName}-update" ''
          set -eu

          export HOME=${lib.escapeShellArg service.steamDir}
          exec ${lib.getExe pkgs.steamcmd} \
            +force_install_dir ${lib.escapeShellArg service.installDir} \
            +login anonymous \
            +app_update ${toString service.appId} validate \
            +quit
        '';

        startScript = pkgs.writeShellScript "steamcmd-${service.serviceName}-start" ''
          set -eu

          cd ${lib.escapeShellArg service.installDir}
          export LD_LIBRARY_PATH=linux64:''${LD_LIBRARY_PATH-}
          exec ${lib.getExe pkgs.steam-run} ${lib.escapeShellArg service.executable} ${lib.escapeShellArgs service.arguments}
        '';
      in
      {
        users.users.${user} = {
          isSystemUser = true;
          uid = config.registry."stable-uid".${config.networking.hostName}.steamcmd;
          inherit group;
          home = service.steamDir;
        };

        systemd.tmpfiles.rules = [
          "d ${service.steamDir} 0770 ${user} ${group} - -"
          "Z ${service.steamDir} 0770 ${user} ${group} - -"
          "d ${service.installDir} 0770 ${user} ${group} - -"
          "Z ${service.installDir} 0770 ${user} ${group} - -"
          "d ${service.stateDir} 0770 ${user} ${group} - -"
          "Z ${service.stateDir} 0770 ${user} ${group} - -"
        ];

        systemd.services = {
          "steamcmd-${service.serviceName}-update" = {
            description = "Update ${service.serviceName} with SteamCMD";
            after = [ "network-online.target" ];
            wants = [ "network-online.target" ];
            serviceConfig = {
              Type = "oneshot";
              User = user;
              Group = group;
              WorkingDirectory = "-${service.steamDir}";
              ExecStartPre = "+${prepareScript}";
              ExecStart = updateScript;
              TimeoutStartSec = "15min";
            };
          };

          "${service.serviceName}-autostart" = {
            description = "Queue ${service.serviceName} game server startup";
            after = [ "network-online.target" ];
            wants = [ "network-online.target" ];
            wantedBy = lib.mkIf service.autoStart [ "multi-user.target" ];
            serviceConfig = {
              Type = "oneshot";
              ExecStart = "${pkgs.systemd}/bin/systemctl --no-block start ${service.serviceName}.service";
            };
          };

          ${service.serviceName} = {
            description = "${service.serviceName} game server";
            after = [
              "network-online.target"
              "steamcmd-${service.serviceName}-update.service"
            ];
            wants = [
              "network-online.target"
              "steamcmd-${service.serviceName}-update.service"
            ];
            serviceConfig = {
              Type = "simple";
              User = user;
              Group = group;
              WorkingDirectory = "-${service.installDir}";
              ExecStartPre = "+${prepareScript}";
              ExecStart = startScript;
              Restart = "on-failure";
              TimeoutStartSec = "15min";
            };
          };
        };

        networking.firewall.allowedUDPPorts = service.udpPorts;
        networking.firewall.allowedTCPPorts = service.tcpPorts;
      }
    ))
  ];
}