class FinalExceptionSubscriber

class FinalExceptionSubscriber implements EventSubscriberInterface {
  use StringTranslationTrait;
  
  /**
   * @var string
   *
   * One of the error level constants defined in bootstrap.inc.
   */
  protected $errorLevel;
  
  /**
   * The config factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;
  
  /**
   * Constructs a new FinalExceptionSubscriber.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The configuration factory.
   */
  public function __construct(ConfigFactoryInterface $config_factory) {
    $this->configFactory = $config_factory;
  }
  
  /**
   * Gets the configured error level.
   *
   * @return string
   */
  protected function getErrorLevel() {
    if (!isset($this->errorLevel)) {
      $this->errorLevel = $this->configFactory
        ->get('system.logging')
        ->get('error_level');
    }
    return $this->errorLevel;
  }
  
  /**
   * Handles exceptions for this subscriber.
   *
   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
   *   The event to process.
   */
  public function onException(ExceptionEvent $event) {
    $exception = $event->getThrowable();
    $error = Error::decodeException($exception);
    // Display the message if the current error reporting level allows this type
    // of message to be displayed, and unconditionally in update.php.
    $message = '';
    if ($this->isErrorDisplayable($error)) {
      // If error type is 'User notice' then treat it as debug information
      // instead of an error message.
      if ($error['%type'] == 'User notice') {
        $error['%type'] = 'Debug';
      }
      $error = $this->simplifyFileInError($error);
      unset($error['backtrace'], $error['exception'], $error['severity_level']);
      if (!$this->isErrorLevelVerbose()) {
        // Without verbose logging, use a simple message.
        // We use \Drupal\Component\Render\FormattableMarkup directly here,
        // rather than use t() since we are in the middle of error handling, and
        // we don't want t() to cause further errors.
        $message = new FormattableMarkup(Error::DEFAULT_ERROR_MESSAGE, $error);
      }
      else {
        // With verbose logging, we will also include a backtrace.
        $backtrace_exception = $exception;
        while ($backtrace_exception->getPrevious()) {
          $backtrace_exception = $backtrace_exception->getPrevious();
        }
        $backtrace = $backtrace_exception->getTrace();
        // First trace is the error itself, already contained in the message.
        // While the second trace is the error source and also contained in the
        // message, the message doesn't contain argument values, so we output it
        // once more in the backtrace.
        array_shift($backtrace);
        // Generate a backtrace containing only scalar argument values.
        $error['@backtrace'] = Error::formatBacktrace($backtrace);
        $message = new FormattableMarkup(Error::DEFAULT_ERROR_MESSAGE . ' <pre class="backtrace">@backtrace</pre>', $error);
      }
    }
    $content_type = $event->getRequest()
      ->getRequestFormat() == 'html' ? 'text/html' : 'text/plain';
    $content = $this->t('The website encountered an unexpected error. Please try again later.');
    $content .= $message ? '<br><br>' . $message : '';
    $response = new Response($content, 500, [
      'Content-Type' => $content_type,
    ]);
    if ($exception instanceof HttpExceptionInterface) {
      $response->setStatusCode($exception->getStatusCode());
      $response->headers
        ->add($exception->getHeaders());
    }
    else {
      $response->setStatusCode(Response::HTTP_INTERNAL_SERVER_ERROR, '500 Service unavailable (with message)');
    }
    $event->setResponse($response);
  }
  
  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    // Run as the final (very late) KernelEvents::EXCEPTION subscriber.
    $events[KernelEvents::EXCEPTION][] = [
      'onException',
      -256,
    ];
    return $events;
  }
  
  /**
   * Checks whether the error level is verbose or not.
   *
   * @return bool
   */
  protected function isErrorLevelVerbose() {
    return $this->getErrorLevel() === ERROR_REPORTING_DISPLAY_VERBOSE;
  }
  
  /**
   * Wrapper for error_displayable().
   *
   * @param $error
   *   Optional error to examine for ERROR_REPORTING_DISPLAY_SOME.
   *
   * @return bool
   *
   * @see \error_displayable
   */
  protected function isErrorDisplayable($error) {
    return error_displayable($error);
  }
  
  /**
   * Attempts to reduce error verbosity in the error message's file path.
   *
   * Attempts to reduce verbosity by removing DRUPAL_ROOT from the file path in
   * the message. This does not happen for (false) security.
   *
   * @param $error
   *   Optional error to examine for ERROR_REPORTING_DISPLAY_SOME.
   *
   * @return array
   *   The updated $error.
   */
  protected function simplifyFileInError($error) {
    // Attempt to reduce verbosity by removing DRUPAL_ROOT from the file path
    // in the message. This does not happen for (false) security.
    $root_length = strlen(DRUPAL_ROOT);
    if (substr($error['%file'], 0, $root_length) == DRUPAL_ROOT) {
      $error['%file'] = substr($error['%file'], $root_length + 1);
    }
    return $error;
  }

}