labmcp/nix/module.nix

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

let
  cfg = config.services.nixos-options-mcp;

  databaseUrl = if cfg.database.type == "sqlite"
    then "sqlite://${cfg.dataDir}/${cfg.database.name}"
    else cfg.database.connectionString;
in
{
  options.services.nixos-options-mcp = {
    enable = lib.mkEnableOption "NixOS Options MCP server";

    package = lib.mkPackageOption pkgs "nixos-options-mcp" { };

    user = lib.mkOption {
      type = lib.types.str;
      default = "nixos-options-mcp";
      description = "User account under which the service runs.";
    };

    group = lib.mkOption {
      type = lib.types.str;
      default = "nixos-options-mcp";
      description = "Group under which the service runs.";
    };

    dataDir = lib.mkOption {
      type = lib.types.path;
      default = "/var/lib/nixos-options-mcp";
      description = "Directory to store data files.";
    };

    database = {
      type = lib.mkOption {
        type = lib.types.enum [ "sqlite" "postgres" ];
        default = "sqlite";
        description = "Database backend to use.";
      };

      name = lib.mkOption {
        type = lib.types.str;
        default = "nixos-options.db";
        description = "SQLite database filename (when using sqlite backend).";
      };

      connectionString = lib.mkOption {
        type = lib.types.str;
        default = "";
        description = ''
          PostgreSQL connection string (when using postgres backend).
          Example: "postgres://user:password@localhost/nixos_options?sslmode=disable"
        '';
      };
    };

    indexOnStart = lib.mkOption {
      type = lib.types.listOf lib.types.str;
      default = [ ];
      example = [ "nixos-unstable" "nixos-24.11" ];
      description = ''
        List of nixpkgs revisions to index on service start.
        Can be channel names (nixos-unstable) or git hashes.
        Indexing is skipped if the revision is already indexed.
      '';
    };

    openFirewall = lib.mkOption {
      type = lib.types.bool;
      default = false;
      description = ''
        Whether to open the firewall for the MCP server.
        Note: MCP typically runs over stdio, so this is usually not needed.
      '';
    };
  };

  config = lib.mkIf cfg.enable {
    assertions = [
      {
        assertion = cfg.database.type == "sqlite" || cfg.database.connectionString != "";
        message = "services.nixos-options-mcp.database.connectionString must be set when using postgres backend";
      }
    ];

    users.users.${cfg.user} = lib.mkIf (cfg.user == "nixos-options-mcp") {
      isSystemUser = true;
      group = cfg.group;
      home = cfg.dataDir;
      description = "NixOS Options MCP server user";
    };

    users.groups.${cfg.group} = lib.mkIf (cfg.group == "nixos-options-mcp") { };

    systemd.tmpfiles.rules = [
      "d ${cfg.dataDir} 0750 ${cfg.user} ${cfg.group} -"
    ];

    systemd.services.nixos-options-mcp = {
      description = "NixOS Options MCP Server";
      wantedBy = [ "multi-user.target" ];
      after = [ "network.target" ]
        ++ lib.optional (cfg.database.type == "postgres") "postgresql.service";

      environment = {
        NIXOS_OPTIONS_DATABASE = databaseUrl;
      };

      preStart = lib.mkIf (cfg.indexOnStart != [ ]) ''
        ${lib.concatMapStringsSep "\n" (rev: ''
          echo "Indexing revision: ${rev}"
          ${cfg.package}/bin/nixos-options index "${rev}" || true
        '') cfg.indexOnStart}
      '';

      serviceConfig = {
        Type = "simple";
        User = cfg.user;
        Group = cfg.group;
        ExecStart = "${cfg.package}/bin/nixos-options serve";
        Restart = "on-failure";
        RestartSec = "5s";

        # Hardening
        NoNewPrivileges = true;
        ProtectSystem = "strict";
        ProtectHome = true;
        PrivateTmp = true;
        PrivateDevices = true;
        ProtectKernelTunables = true;
        ProtectKernelModules = true;
        ProtectControlGroups = true;
        RestrictNamespaces = true;
        RestrictRealtime = true;
        RestrictSUIDSGID = true;
        MemoryDenyWriteExecute = true;
        LockPersonality = true;

        ReadWritePaths = [ cfg.dataDir ];
        WorkingDirectory = cfg.dataDir;
        StateDirectory = lib.mkIf (cfg.dataDir == "/var/lib/nixos-options-mcp") "nixos-options-mcp";
      };
    };
  };
}