nixos-config/nixos/modules/system/on-failure.nix

{ config, lib, pkgs, ... }:
with lib;
let

  cfg = config.on-failure;

  api = {

    enable = mkEnableOption "krebs.on-failure" // {
      default = cfg.plans != { };
    };

    url = mkOption {
      type = types.str;
      description = "url on where to send the message to";
    };

    plans = mkOption {
      default = { };
      type = with types;
        attrsOf (submodule ({ config, ... }: {
          options = {
            enable = mkEnableOption "on-failure.${config.name}" // {
              default = true;
            };
            name = mkOption {
              type = types.str;
              default = config._module.args.name;
              description = "Name of the to-be-monitored service.";
            };
          };
        }));
    };
  };

  enabled-plans = filter (getAttr "enable") (attrValues cfg.plans);

  to-services = plan: {
    "${plan.name}".unitConfig.OnFailure = "on-failure.${plan.name}.service";
    "on-failure.${plan.name}".serviceConfig = rec {
      ExecStart = mattermostStart plan;
      SyslogIdentifier = ExecStart.name;
      Type = "oneshot";
    };
  };

  # todo this output must be better
  mattermostStart = plan:
    pkgs.writers.writeDash "on-failure.${plan.name}" ''
      ${pkgs.curl}/bin/curl \
      --include \
      --request POST \
      --data-urlencode \
      'payload={"text": "<!channel> :fire: Service Failed ${plan.name} on ${config.networking.hostName}"}' \
      ${cfg.url}
    '';

in {

  options.on-failure = api;

  config = lib.mkIf cfg.enable {
    systemd.services = foldl (a: b: a // b) { } (map to-services enabled-plans);
  };
}