# Avahi daemon.
{pkgs, config, ...}:

###### interface
let
  inherit (pkgs.lib) mkOption;

  options = {
    services = {
      avahi = {

        enable = mkOption {
          default = false;
          description = ''
            Whether to run the Avahi daemon, which allows Avahi clients
            to use Avahi's service discovery facilities and also allows
            the local machine to advertise its presence and services
            (through the mDNS responder implemented by `avahi-daemon').
          '';
        };

        hostName = mkOption {
          default = "nixos";  # XXX: Would be nice to use `networking.hostName'.
          description = ''Host name advertised on the LAN.'';
        };

        browseDomains = mkOption {
          default = [ "0pointer.de" "zeroconf.org" ];
          description = ''
            List of non-local DNS domains to be browsed.
          '';
        };

        ipv4 = mkOption {
          default = true;
          description = ''Whether to use IPv4'';
        };

        ipv6 = mkOption {
          default = false;
          description = ''Whether to use IPv6'';
        };

        wideArea = mkOption {
          default = true;
          description = ''Whether to enable wide-area service discovery.'';
        };

        publishing = mkOption {
          default = true;
          description = ''Whether to allow publishing.'';
        };

        nssmdns = mkOption {
          default = false;
          description = ''
            Whether to enable the mDNS NSS (Name Service Switch) plug-in.
            Enabling it allows applications to resolve names in the `.local'
            domain by transparently querying the Avahi daemon.

            Warning: Currently, enabling this option breaks DNS lookups after
            a `nixos-rebuild'.  This is because `/etc/nsswitch.conf' is
            updated to use `nss-mdns' but `libnss_mdns' is not in
            applications' `LD_LIBRARY_PATH'.  The next time `/etc/profile' is
            sourced, it will set up an appropriate `LD_LIBRARY_PATH', though.
          '';
        };
      };
    };
  };
in

###### implementation
let
  cfg = config.services.avahi;
  inherit (pkgs.lib) mkIf mkThenElse;

  inherit (pkgs) avahi writeText lib;

  avahiDaemonConf = with cfg; writeText "avahi-daemon.conf" ''
    [server]
    host-name=${hostName}
    browse-domains=${lib.concatStringsSep ", " browseDomains}
    use-ipv4=${if ipv4 then "yes" else "no"}
    use-ipv6=${if ipv6 then "yes" else "no"}

    [wide-area]
    enable-wide-area=${if wideArea then "yes" else "no"}

    [publish]
    disable-publishing=${if publishing then "no" else "yes"}
  '';

  user = {
    name = "avahi";
    uid = config.ids.uids.avahi;
    description = "`avahi-daemon' privilege separation user";
    home = "/var/empty";
  };

  group = {
    name = "avahi";
    gid = config.ids.gids.avahi;
  };

  job = {
    name = "avahi-daemon";

    job = ''
      start on network-interfaces/started
      stop on network-interfaces/stop
      respawn
      script
        export PATH="${avahi}/bin:${avahi}/sbin:$PATH"

        # Make NSS modules visible so that `avahi_nss_support ()' can
        # return a sensible value.
        export LD_LIBRARY_PATH="${config.system.nssModules.path}"

        exec ${avahi}/sbin/avahi-daemon --daemonize -f "${avahiDaemonConf}"
      end script
    '';
  };
in

mkIf cfg.enable {
  require = [
    # ../upstart-jobs/default.nix # config.services.extraJobs
    # ../system/? # system.nssModules
    # ? # config.environment.etc
    # ../system/user.nix # users.*
    # ../upstart-jobs/udev.nix # services.udev.*
    # ../upstart-jobs/dbus.nix # services.dbus.*
    # ? # config.environment.extraPackages
    options
  ];

  system = {
    nssModules = pkgs.lib.optional cfg.nssmdns pkgs.nssmdns;
  };

  environment = {
    extraPackages = [avahi];
  };

  users = {
    extraUsers = [user];
    extraGroups = [group];
  };

  services = {
    extraJobs = [job];

    dbus = {
      enable = true;
      services = [avahi];
    };
  };
}