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

with lib;

let

  inherit (pkgs) ifplugd;

  cfg = config.networking.interfaceMonitor;

  # The ifplugd action script, which is called whenever the link
  # status changes (i.e., a cable is plugged in or unplugged).
  plugScript = pkgs.writeScript "ifplugd.action"
    ''
      #! ${pkgs.stdenv.shell}
      iface="$1"
      status="$2"
      ${cfg.commands}
    '';

in

{

  ###### interface

  options = {

    networking.interfaceMonitor.enable = mkOption {
      type = types.bool;
      default = false;
      description = ''
        If <literal>true</literal>, monitor Ethernet interfaces for
        cables being plugged in or unplugged.  When this occurs, the
        commands specified in
        <option>networking.interfaceMonitor.commands</option> are
        executed.
      '';
    };

    networking.interfaceMonitor.beep = mkOption {
      type = types.bool;
      default = false;
      description = ''
        If <literal>true</literal>, beep when an Ethernet cable is
        plugged in or unplugged.
      '';
    };

    networking.interfaceMonitor.commands = mkOption {
      type = types.lines;
      default = "";
      description = ''
        Shell commands to be executed when the link status of an
        interface changes.  On invocation, the shell variable
        <varname>iface</varname> contains the name of the interface,
        while the variable <varname>status</varname> contains either
        <literal>up</literal> or <literal>down</literal> to indicate
        the new status.
      '';
    };

  };


  ###### implementation

  config = mkIf cfg.enable {
    systemd.services.ifplugd = {
      description = "Network interface connectivity monitor";
      after = [ "network-interfaces.target" ];
      wantedBy = [ "multi-user.target" ];
      script = ''
        ${ifplugd}/sbin/ifplugd --no-daemon --no-startup --no-shutdown \
          ${if config.networking.interfaceMonitor.beep then "" else "--no-beep"} \
          --run ${plugScript}
      '';
    };

    environment.systemPackages = [ ifplugd ];
  };
}