Implement aria2 service for controlling a daemon via rpc.

      rpc = 271;

      geoip = 272;

      fcron = 273;

      aria2 = 274;

      # When adding a uid, make sure it doesn't match an existing gid. And don't use uids above 399!

      #rpc = 271; # unused

      #geoip = 272; # unused

      fcron = 273;

	  aria2 = 274;

      # When adding a gid, make sure it doesn't match an existing

      # uid. Users and groups with the same name should have equal

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

with lib;

let

  cfg = config.services.aria2;

  homeDir = "/var/lib/aria2";

  settingsDir = "${homeDir}";

  sessionFile = "${homeDir}/aria2.session";

  downloadDir = "${homeDir}/Downloads";

  rangesToStringList = map (x: builtins.toString x.from +"-"+ builtins.toString x.to);

  settingsFile = pkgs.writeText "aria2.conf"

  ''

    dir=${cfg.downloadDir}

    listen-port=${concatStringsSep "," (rangesToStringList cfg.listenPortRange)}

    rpc-listen-port=${toString cfg.rpcListenPort}

    rpc-secret=${cfg.rpcSecret}

  '';

in

{

  options = {

    services.aria2 = {

      enable = mkOption {

        type = types.bool;

        default = false;

        description = ''

          Whether or not to enable the headless Aria2 daemon service.

          Aria2 daemon can be controlled via the RPC interface using

          one of many WebUI (http://localhost:6800/ by default).

          Targets are downloaded to ${downloadDir} by default and are

          accessible to users in the "aria2" group.

          '';

      };

      openPorts = mkOption {

        type = types.bool;

        default = true;

        description = ''

          Open the ports in the firewall. Defaults are:

          - UDP 6881-6999 for transfers

          - TCP 6800 for RPC

          '';

      };

      downloadDir = mkOption {

        type = types.string;

        default = "${downloadDir}";

        description = ''

          Directory to store downloaded files.

          '';

      };

      listenPortRange = mkOption {

        type = types.listOf types.attrs;

        default = [ { from = 6881; to = 6999; } ];

        description = ''

          Set UDP listening port range used by DHT(IPv4, IPv6) and UDP tracker.

          '';

      };

      rpcListenPort = mkOption {

        type = types.int;

        default = 6800;

        description = "Specify a port number for JSON-RPC/XML-RPC server to listen to. Possible Values: 1024-65535";

      };

      rpcSecret = mkOption {

        type = types.string;

        default = "aria2rpc";

        description = ''

          Set RPC secret authorization token.

          Read https://aria2.github.io/manual/en/html/aria2c.html#rpc-auth to know how this option value is used.

          '';

      };

      extraArguments = mkOption {

        type = types.string;

        example = "--rpc-secret=12345 --enable-rpc --rpc-listen-all";

        default = "";

        description = ''

          Additional arguments to be passed to Aria2.

          '';

      };

    };

  };

  config = mkIf cfg.enable {

    # Need to open ports for proper functioning

    networking.firewall = mkIf cfg.openPorts {

      allowedUDPPortRanges = config.services.aria2.listenPortRange;

      allowedTCPPorts = [ config.services.aria2.rpcListenPort ];

    };

    users.extraUsers.aria2 = {

      group = "aria2";

      uid = config.ids.uids.aria2;

      description = "aria2 user";

      home = homeDir;

      createHome = false;

    };

    users.extraGroups.aria2.gid = config.ids.gids.aria2;

    systemd.services.aria2 = {

      description = "aria2 Service";

      after = [ "local-fs.target" "network.target" ];

      wantedBy = [ "multi-user.target" ];

      preStart = ''

        mkdir -m 0770 -p "${homeDir}"

				chown aria2:aria2 "${homeDir}"

        if [[ ! -d "${config.services.aria2.downloadDir}" ]]

				then 

					mkdir -m 0770 -p "${config.services.aria2.downloadDir}"

					chown aria2:aria2 "${config.services.aria2.downloadDir}"

				fi

        if [[ ! -e "${sessionFile}" ]]

				then 

					touch "${sessionFile}"

					chown aria2:aria2 "${sessionFile}"

				fi

        cp -f "${settingsFile}" "${settingsDir}/aria2.conf"

      '';

      serviceConfig = {

        Restart = "on-abort";

        ExecStart = "${pkgs.aria2}/bin/aria2c --enable-rpc --conf-path=${settingsDir}/aria2.conf ${config.services.aria2.extraArguments} --save-session=${sessionFile}";

        ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";

        User = "aria2";

        Group = "aria2";

				PermissionsStartOnly = true;

      };

    };

  };

}