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

let
  inherit (lib) mkEnableOption mkIf mkOption optionalString types;

  generic = variant:
    let
      cfg = config.services.${variant};
      pkg = pkgs.${variant};
      birdBin = if variant == "bird6" then "bird6" else "bird";
      birdc = if variant == "bird6" then "birdc6" else "birdc";
      descr =
        { bird = "1.9.x with IPv4 suport";
          bird6 = "1.9.x with IPv6 suport";
          bird2 = "2.x";
        }.${variant};
    in {
      ###### interface
      options = {
        services.${variant} = {
          enable = mkEnableOption "BIRD Internet Routing Daemon (${descr})";
          config = mkOption {
            type = types.lines;
            description = ''
              BIRD Internet Routing Daemon configuration file.
              <link xlink:href='http://bird.network.cz/'/>
            '';
          };
          checkConfig = mkOption {
            type = types.bool;
            default = true;
            description = ''
              Whether the config should be checked at build time.
              Disabling this might become necessary if the config includes files not present during build time.
            '';
          };
        };
      };

      ###### implementation
      config = mkIf cfg.enable {
        environment.systemPackages = [ pkg ];

        environment.etc."bird/${variant}.conf".source = pkgs.writeTextFile {
          name = "${variant}.conf";
          text = cfg.config;
          checkPhase = optionalString cfg.checkConfig ''
            ${pkg}/bin/${birdBin} -d -p -c $out
          '';
        };

        systemd.services.${variant} = {
          description = "BIRD Internet Routing Daemon (${descr})";
          wantedBy = [ "multi-user.target" ];
          reloadIfChanged = true;
          restartTriggers = [ config.environment.etc."bird/${variant}.conf".source ];
          serviceConfig = {
            Type = "forking";
            Restart = "on-failure";
            ExecStart = "${pkg}/bin/${birdBin} -c /etc/bird/${variant}.conf -u ${variant} -g ${variant}";
            ExecReload = "/bin/sh -c '${pkg}/bin/${birdBin} -c /etc/bird/${variant}.conf -p && ${pkg}/bin/${birdc} configure'";
            ExecStop = "${pkg}/bin/${birdc} down";
            CapabilityBoundingSet = [ "CAP_CHOWN" "CAP_FOWNER" "CAP_DAC_OVERRIDE" "CAP_SETUID" "CAP_SETGID"
                                      # see bird/sysdep/linux/syspriv.h
                                      "CAP_NET_BIND_SERVICE" "CAP_NET_BROADCAST" "CAP_NET_ADMIN" "CAP_NET_RAW" ];
            ProtectSystem = "full";
            ProtectHome = "yes";
            SystemCallFilter="~@cpu-emulation @debug @keyring @module @mount @obsolete @raw-io";
            MemoryDenyWriteExecute = "yes";
          };
        };
        users = {
          users.${variant} = {
            description = "BIRD Internet Routing Daemon user";
            group = variant;
          };
          groups.${variant} = {};
        };
      };
    };

in

{
  imports = map generic [ "bird" "bird6" "bird2" ];
}