config/modules/base/unitstatus/default.nix

{
  config,
  lib,
  pkgs,
  ...
}: {
  # TODO:
  # - assert postfix is configured
  options.mj.base.unitstatus = with lib.types; {
    enable = lib.mkEnableOption "alert by email on unit failure";
    email = lib.mkOption {type = str;};
    units = lib.mkOption {
      type = listOf str;
      default = [];
    };
  };

  config = lib.mkIf config.mj.base.unitstatus.enable {
    systemd.services =
      {
        "unit-status-mail@" = let
          # https://northernlightlabs.se/2014-07-05/systemd-status-mail-on-unit-failure.html
          script = pkgs.writeShellScript "unit-status-mail" ''
            set -e
            MAILTO="${config.mj.base.unitstatus.email}"
            UNIT=$1
            EXTRA=""
            for e in "''${@:2}"; do
              EXTRA+="$e"$'\n'
            done
            UNITSTATUS=$(${pkgs.systemd}/bin/systemctl status "$UNIT" || :)
            ${pkgs.postfix}/bin/sendmail $MAILTO <<EOF
            Subject:Status mail for unit: $UNIT

            Status report for unit: $UNIT
            $EXTRA

            $UNITSTATUS
            EOF

            echo -e "Status mail sent to: $MAILTO for unit: $UNIT"
          '';
        in {
          description = "Send an email on unit failure";
          serviceConfig = {
            Type = "simple";
            ExecStart = ''${script} "%i" "Hostname: %H" "Machine ID: %m" "Boot ID: %b" '';
          };
        };
      }
      // lib.genAttrs config.mj.base.unitstatus.units (
        unit: {
          unitConfig = {OnFailure = "unit-status-mail@${unit}.service";};
        }
      );
  };
}