ExceptionTestSiteSubscriber.php

<?php

namespace Drupal\Core\EventSubscriber;

use Drupal\Core\Utility\Error;
use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;

/**
 * Custom handling of errors when in a system-under-test.
 */
class ExceptionTestSiteSubscriber extends HttpExceptionSubscriberBase {
    
    /**
     * {@inheritdoc}
     */
    protected static function getPriority() {
        return 3;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getHandledFormats() {
        return [
            'html',
        ];
    }
    
    /**
     * Checks for special handling of errors inside Simpletest.
     *
     * @todo The $headers array appears to not actually get used at all in the
     *   original code. It's quite possible that this entire method is now
     *   vestigial and can be removed.
     *
     * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
     */
    public function on500(GetResponseForExceptionEvent $event) {
        $exception = $event->getException();
        $error = Error::decodeException($exception);
        $headers = [];
        // When running inside the testing framework, we relay the errors
        // to the tested site by the way of HTTP headers.
        if (DRUPAL_TEST_IN_CHILD_SITE && !headers_sent() && (!defined('SIMPLETEST_COLLECT_ERRORS') || SIMPLETEST_COLLECT_ERRORS)) {
            // $number does not use drupal_static as it should not be reset
            // as it uniquely identifies each PHP error.
            static $number = 0;
            $assertion = [
                $error['@message'],
                $error['%type'],
                [
                    'function' => $error['%function'],
                    'file' => $error['%file'],
                    'line' => $error['%line'],
                ],
            ];
            $headers['X-Drupal-Assertion-' . $number] = rawurlencode(serialize($assertion));
            $number++;
        }
    }

}