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";
        };
      });
  };
}