# NixOS module for Transmission BitTorrent daemon

{ config, pkgs, ... }:

with pkgs.lib;

let

  cfg = config.services.transmission;
  homeDir = "/var/lib/transmission";
  settingsDir = "${homeDir}/.config/transmission-daemon";
  settingsFile = "${settingsDir}/settings.json";

  # Strings must be quoted, ints and bools must not (for settings.json).
  toOption = x:
    if x == true then "true"
    else if x == false then "false"
    else if builtins.isInt x then toString x
    else toString ''\"${x}\"'';

  # All lines in settings.json end with a ',' (comma), except for the last
  # line. This is standard JSON. But a comma can also appear *inside* some
  # fields, notably the "rpc-whitelist" field. This is difficult to handle in
  # sed so we simply ignore it and say that if you want to change the option at
  # the last line of settings.json, you have to do it manually. At this time of
  # writing, the last option is "utp-enable":true.
  attrsToSedArgs = as:
    concatStrings (concatLists (mapAttrsToList (name: value:
      #map (x: '' -e 's=\(\"${name}\":\)[^,]*\(.*\)=\1 ${toOption x}\2=' '') # breaks if comma inside value field
      map (x: '' -e 's=\(\"${name}\":\).*=\1 ${toOption x},=' '') # always append ',' (breaks last line in settings.json)
        (if isList value then value else [value]))
        as));

in

{

  ### configuration

  options = {

    services.transmission = {

      enable = mkOption {
        type = types.uniq types.bool;
        default = false;
        description = ''
          Whether or not to enable the headless Transmission BitTorrent daemon.

          Transmission daemon can be controlled via the RPC interface using
          transmission-remote or the WebUI (http://localhost:9091/ by default).

          Torrents are downloaded to ${homeDir}/Downloads/ by default and are
          accessible to users in the "transmission" group.
        '';
      };

      settings = mkOption {
        type = types.attrs;
        default =
          {
            # for users in group "transmission" to have access to torrents
            umask = 2;
          }
        ;
        example =
          {
            download-dir = "/srv/torrents/";
            incomplete-dir = "/srv/torrents/.incomplete/";
            incomplete-dir-enabled = true;
            rpc-whitelist = "127.0.0.1,192.168.*.*";
            # for users in group "transmission" to have access to torrents
            umask = 2;
          }
        ;
        description = ''
          Attribute set whos fields overwrites fields in settings.json (each
          time the service starts). String values must be quoted, integer and
          boolean values must not.

          See https://trac.transmissionbt.com/wiki/EditConfigFiles for documentation
          and/or look at ${settingsFile}."
        '';
      };

      rpc_port = mkOption {
        type = types.uniq types.int;
        default = 9091;
        description = "TCP port number to run the RPC/web interface.";
      };

    };

  };

  ### implementation

  config = mkIf cfg.enable {

    systemd.services.transmission = {
      description = "Transmission BitTorrent Daemon";
      after = [ "network.target" ];
      wantedBy = [ "multi-user.target" ];
      # 1) Only the "transmission" user and group have access to torrents.
      # 2) Optionally update/force specific fields into the configuration file.
      serviceConfig.ExecStartPre =
        if config.services.transmission.settings != {} then ''
          ${pkgs.stdenv.shell} -c "chmod 770 ${homeDir} && mkdir -p ${settingsDir} && ${pkgs.transmission}/bin/transmission-daemon -d |& sed ${attrsToSedArgs config.services.transmission.settings} > ${settingsFile}.tmp && mv ${settingsFile}.tmp ${settingsFile}"
        ''
        else ''
          ${pkgs.stdenv.shell} -c "chmod 770 ${homeDir}"
        '';
      serviceConfig.ExecStart = "${pkgs.transmission}/bin/transmission-daemon -f --port ${toString config.services.transmission.rpc_port}";
      serviceConfig.ExecReload = "kill -HUP $MAINPID";
      serviceConfig.User = "transmission";
      # NOTE: transmission has an internal umask that also must be set (in settings.json)
      serviceConfig.UMask = "0002";
    };

    # It's useful to have transmission in path, e.g. for remote control
    environment.systemPackages = [ pkgs.transmission ];

    users.extraUsers.transmission = {
      group = "transmission";
      description = "Transmission BitTorrent user";
      home = homeDir;
      createHome = true;
    };

    users.extraGroups.transmission = {};

  };

}