<?php

…
/** @var AckStamp $ackStamp */
if ($batchHandler && $ackStamp = $envelope->last(AckStamp::class)) {
    $ack = new Acknowledger(get_debug_type($batchHandler), static function (\Throwable $e = null, $result = null) use ($envelope, $ackStamp, $handlerDescriptor) {
        if (null !== $e) {
            $e = new HandlerFailedException($envelope, [$e]);
        } else {
            $envelope = $envelope->with(HandledStamp::fromDescriptor($handlerDescriptor, $result));
        }

        $ackStamp->ack($envelope, $e);
    });

    $result = $handler($message, $ack);
…