# ==============================================================================
# PROJECT SKELETON — NixOS Service Module
# ==============================================================================
#
# USAGE:
#   1. Copy this file into your project repo, e.g. ~/projects/my-project/my-project.nix
#   2. Replace every occurrence of:
#        MY_PROJECT      → your project name in camelCase   (e.g. myRssReader)
#        my-project      → your project name in kebab-case  (e.g. my-rss-reader)
#        my-project.example.com  → your actual domain
#        127.0.0.1:3000  → the address your app listens on internally
#   3. In /etc/nixos/configuration.nix, add:
#        imports = [ /path/to/my-project.nix ];
#        services.MY_PROJECT.enable = true;
#
# ASSUMPTIONS:
#   - Nginx global settings (recommendedTlsSettings etc.) and ACME email are
#     configured centrally in configuration.nix (see bottom of this file).
#   - Your app runs as a systemd service on a local port (reverse proxy setup).
#   - You want HTTPS via Let's Encrypt.
#
# ==============================================================================

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

let
  # The internal port your application listens on.
  # Nginx will forward public traffic here.
  appPort = 3000;

  # Shorthand so you can reference the merged config of this module cleanly.
  cfg = config.services.MY_PROJECT;

in
{
  # ============================================================================
  # OPTIONS
  # Define the knobs that other files (or configuration.nix) can turn.
  # Nothing in `config` below runs until `enable` is set to true.
  # ============================================================================

  options.services.MY_PROJECT = {

    enable = lib.mkEnableOption "MY_PROJECT service";
    # Adds a boolean option `services.MY_PROJECT.enable` that defaults to false.
    # Set it to true in configuration.nix to activate this module.

    domain = lib.mkOption {
      type        = lib.types.str;
      default     = "my-project.example.com";
      description = "The public domain name Nginx will serve and ACME will certify.";
    };

    port = lib.mkOption {
      type        = lib.types.port;
      default     = appPort;
      description = "Internal port the application process listens on.";
    };

    dataDir = lib.mkOption {
      type        = lib.types.path;
      default     = "/var/lib/my-project";
      description = "Directory for persistent application data (database, uploads, etc.).";
    };

    # Optional: expose an environment file path for secrets.
    # The file should contain KEY=VALUE pairs and never be committed to git.
    # Example content:
    #   DATABASE_URL=postgres://user:password@localhost/mydb
    #   SECRET_KEY=supersecret
    environmentFile = lib.mkOption {
      type        = lib.types.nullOr lib.types.path;
      default     = null;
      description = ''
        Path to a file containing secret environment variables.
        This file is read by systemd at runtime and never stored in the Nix store.
        Example: /run/secrets/my-project.env
      '';
    };

  };

  # ============================================================================
  # CONFIG
  # Everything below only takes effect when `services.MY_PROJECT.enable = true`.
  # ============================================================================

  config = lib.mkIf cfg.enable {

    # --------------------------------------------------------------------------
    # Systemd Service
    # Runs your application as a background process.
    # --------------------------------------------------------------------------

    systemd.services.my-project = {
      description = "MY_PROJECT application service";

      # Start after the network and (if used) PostgreSQL are ready.
      after    = [ "network.target" "postgresql.service" ];
      wantedBy = [ "multi-user.target" ];

      serviceConfig = {
        # --- Replace this with your actual start command ---
        # Examples:
        #   Node.js:  "${pkgs.nodejs}/bin/node ${cfg.dataDir}/server.js"
        #   Python:   "${pkgs.python3}/bin/python ${cfg.dataDir}/app.py"
        #   Binary:   "${pkgs.my-package}/bin/my-binary"
        ExecStart = "${pkgs.nodejs}/bin/node ${cfg.dataDir}/server.js";

        # Working directory for the process.
        WorkingDirectory = cfg.dataDir;

        # DynamicUser: systemd creates an unprivileged user automatically.
        # The user has no fixed UID and cannot log in. Good for most services.
        # Set to false if you need a persistent user (e.g. for file ownership).
        DynamicUser = true;

        # Give the dynamic user write access to the data directory.
        StateDirectory = "my-project";
        StateDirectoryMode = "0750";

        # Load secrets from a file at runtime (optional).
        # The contents are injected as environment variables.
        EnvironmentFile = lib.mkIf (cfg.environmentFile != null) cfg.environmentFile;

        # Hardening — remove lines you don't need, but keep as many as possible.
        NoNewPrivileges        = true;
        PrivateTmp             = true;   # /tmp is isolated from other services
        ProtectSystem          = "strict";
        ProtectHome            = true;
        RestrictSUIDSGID       = true;
        LockPersonality        = true;
        RestrictNamespaces     = true;

        # Restart automatically if the process crashes.
        Restart   = "on-failure";
        RestartSec = "5s";
      };

      # Environment variables that are safe to be in the Nix store.
      # For secrets, use environmentFile above instead.
      environment = {
        NODE_ENV = "production";
        PORT     = toString cfg.port;
        DATA_DIR = cfg.dataDir;
      };
    };

    # --------------------------------------------------------------------------
    # Nginx Virtual Host
    # Terminates TLS and proxies requests to the local application port.
    # --------------------------------------------------------------------------

    services.nginx = {
      # Nginx is enabled automatically when virtualHosts is non-empty,
      # but being explicit is clearer.
      enable = true;

      virtualHosts.${cfg.domain} = {

        # Redirect all plain HTTP to HTTPS.
        forceSSL = true;

        # Let NixOS manage the Let's Encrypt certificate automatically.
        # Renewal is handled by a systemd timer — no manual cron needed.
        # Requires: security.acme.acceptTerms = true and defaults.email set
        # in your central configuration.nix.
        enableACME = true;

        # --- Main reverse proxy location ---
        locations."/" = {
          proxyPass = "http://127.0.0.1:${toString cfg.port}";

          # Needed for apps that use WebSockets (e.g. live reload, chat).
          # Remove these two lines if your app does not use WebSockets.
          proxyWebsockets = true;
          extraConfig = ''
            proxy_read_timeout 60s;
          '';
        };

        # --- Optional: serve static files directly from Nginx ---
        # Uncomment and adjust if your app has a public assets directory.
        # Serving static files from Nginx is faster than going through Node/Python.
        #
        # locations."/static/" = {
        #   alias = "${cfg.dataDir}/static/";
        #   extraConfig = ''
        #     expires 30d;
        #     add_header Cache-Control "public, immutable";
        #   '';
        # };

        # --- Optional: block access to sensitive paths ---
        # locations."~ /\\." = {
        #   extraConfig = "deny all;";
        # };
      };
    };

    # --------------------------------------------------------------------------
    # PostgreSQL Database (optional)
    # Uncomment this block if your project needs a database.
    # --------------------------------------------------------------------------

    # services.postgresql = {
    #   enable = true;
    #
    #   # Creates the database and user if they don't exist yet.
    #   ensureDatabases = [ "my-project" ];
    #   ensureUsers = [{
    #     name = "my-project";
    #     ensureDBOwnership = true;
    #   }];
    # };

    # --------------------------------------------------------------------------
    # Firewall
    # Ports 80 and 443 must be open for Nginx and Let's Encrypt to work.
    # If these are already opened in configuration.nix, remove this block.
    # --------------------------------------------------------------------------

    networking.firewall.allowedTCPPorts = [ 80 443 ];

  };

  # ============================================================================
  # CENTRAL CONFIGURATION.NIX REQUIREMENTS
  #
  # Make sure these are set somewhere in your configuration.nix (not here,
  # so they are shared across all project modules without duplication):
  #
  #   services.nginx = {
  #     recommendedGzipSettings  = true;
  #     recommendedOptimisation   = true;
  #     recommendedTlsSettings    = true;
  #     recommendedProxySettings  = true;
  #   };
  #
  #   security.acme = {
  #     acceptTerms = true;
  #     defaults.email = "your@email.com";
  #   };
  #
  # ============================================================================

}