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

with lib;

let
  cfg = config.services.telegraf;

  settingsFormat = pkgs.formats.toml {};
  configFile = settingsFormat.generate "config.toml" cfg.extraConfig;
in {
  ###### interface
  options = {
    services.telegraf = {
      enable = mkEnableOption "telegraf server";

      package = mkOption {
        default = pkgs.telegraf;
        defaultText = literalExpression "pkgs.telegraf";
        description = "Which telegraf derivation to use";
        type = types.package;
      };

      environmentFiles = mkOption {
        type = types.listOf types.path;
        default = [];
        example = [ "/run/keys/telegraf.env" ];
        description = ''
          File to load as environment file. Environment variables from this file
          will be interpolated into the config file using envsubst with this
          syntax: <literal>$ENVIRONMENT</literal> or <literal>''${VARIABLE}</literal>.
          This is useful to avoid putting secrets into the nix store.
        '';
      };

      extraConfig = mkOption {
        default = {};
        description = "Extra configuration options for telegraf";
        type = settingsFormat.type;
        example = {
          outputs.influxdb = {
            urls = ["http://localhost:8086"];
            database = "telegraf";
          };
          inputs.statsd = {
            service_address = ":8125";
            delete_timings = true;
          };
        };
      };
    };
  };


  ###### implementation
  config = mkIf config.services.telegraf.enable {
    systemd.services.telegraf = let
      finalConfigFile = if config.services.telegraf.environmentFiles == []
                        then configFile
                        else "/var/run/telegraf/config.toml";
    in {
      description = "Telegraf Agent";
      wantedBy = [ "multi-user.target" ];
      after = [ "network-online.target" ];
      serviceConfig = {
        EnvironmentFile = config.services.telegraf.environmentFiles;
        ExecStartPre = lib.optional (config.services.telegraf.environmentFiles != [])
          (pkgs.writeShellScript "pre-start" ''
            umask 077
            ${pkgs.envsubst}/bin/envsubst -i "${configFile}" > /var/run/telegraf/config.toml
          '');
        ExecStart="${cfg.package}/bin/telegraf -config ${finalConfigFile}";
        ExecReload="${pkgs.coreutils}/bin/kill -HUP $MAINPID";
        RuntimeDirectory = "telegraf";
        User = "telegraf";
        Group = "telegraf";
        Restart = "on-failure";
        # for ping probes
        AmbientCapabilities = [ "CAP_NET_RAW" ];
      };
    };

    users.users.telegraf = {
      uid = config.ids.uids.telegraf;
      group = "telegraf";
      description = "telegraf daemon user";
    };

    users.groups.telegraf = {};
  };
}