labmcp/nix/module.nix

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

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

  # Determine database URL based on configuration
  # For postgres with connectionStringFile, the URL is set at runtime via script
  useConnectionStringFile = cfg.database.type == "postgres" && cfg.database.connectionStringFile != null;

  databaseUrl = if cfg.database.type == "sqlite"
    then "sqlite://${cfg.dataDir}/${cfg.database.name}"
    else if useConnectionStringFile
    then "" # Will be set at runtime from file
    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"

          WARNING: This value will be stored in the Nix store, which is world-readable.
          For production use with sensitive credentials, use connectionStringFile instead.
        '';
      };

      connectionStringFile = lib.mkOption {
        type = lib.types.nullOr lib.types.path;
        default = null;
        description = ''
          Path to a file containing the PostgreSQL connection string.
          The file should contain just the connection string, e.g.:
          postgres://user:password@localhost/nixos_options?sslmode=disable

          This is the recommended way to configure PostgreSQL credentials
          as the file is not stored in the world-readable Nix store.
          The file must be readable by the service user.
        '';
        example = "/run/secrets/nixos-options-mcp-db";
      };
    };

    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 != ""
          || cfg.database.connectionStringFile != null;
        message = "services.nixos-options-mcp.database: when using postgres backend, either connectionString or connectionStringFile must be set";
      }
      {
        assertion = cfg.database.connectionString == "" || cfg.database.connectionStringFile == null;
        message = "services.nixos-options-mcp.database: connectionString and connectionStringFile are mutually exclusive";
      }
    ];

    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 = lib.mkIf (!useConnectionStringFile) {
        NIXOS_OPTIONS_DATABASE = databaseUrl;
      };

      path = [ cfg.package ];

      script = let
        indexCommands = lib.optionalString (cfg.indexOnStart != []) ''
          ${lib.concatMapStringsSep "\n" (rev: ''
            echo "Indexing revision: ${rev}"
            nixos-options index "${rev}" || true
          '') cfg.indexOnStart}
        '';
      in
        if useConnectionStringFile then ''
          # Read database connection string from file
          if [ ! -f "${cfg.database.connectionStringFile}" ]; then
            echo "Error: connectionStringFile not found: ${cfg.database.connectionStringFile}" >&2
            exit 1
          fi
          export NIXOS_OPTIONS_DATABASE="$(cat "${cfg.database.connectionStringFile}")"

          ${indexCommands}
          exec nixos-options serve
        '' else ''
          ${indexCommands}
          exec nixos-options serve
        '';

      serviceConfig = {
        Type = "simple";
        User = cfg.user;
        Group = cfg.group;
        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";
      };
    };
  };
}