class ExceptionLoggingSubscriber

class ExceptionLoggingSubscriber implements EventSubscriberInterface {

  /**
   * The logger channel factory.
   *
   * @var \Drupal\Core\Logger\LoggerChannelFactoryInterface
   */
  protected $logger;

  /**
   * Constructs a new ExceptionLoggingSubscriber.
   *
   * @param \Drupal\Core\Logger\LoggerChannelFactoryInterface $logger
   *   The logger channel factory.
   */
  public function __construct(LoggerChannelFactoryInterface $logger) {
    $this->logger = $logger;
  }

  /**
   * Log 403 errors.
   *
   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
   *   The event to process.
   */
  public function on403(ExceptionEvent $event) {

    // Log the exception with the page where it happened so that admins know
    // why access was denied.
    $exception = $event
      ->getThrowable();
    $error = Error::decodeException($exception);
    unset($error['@backtrace_string']);
    $error['@uri'] = $event
      ->getRequest()
      ->getRequestUri();
    $this->logger
      ->get('access denied')
      ->warning('Path: @uri. ' . Error::DEFAULT_ERROR_MESSAGE, $error);
  }

  /**
   * Log 404 errors.
   *
   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
   *   The event to process.
   */
  public function on404(ExceptionEvent $event) {
    $request = $event
      ->getRequest();
    $this->logger
      ->get('page not found')
      ->warning('@uri', [
      '@uri' => $request
        ->getRequestUri(),
    ]);
  }

  /**
   * Log not-otherwise-specified errors, including HTTP 500.
   *
   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
   *   The event to process.
   */
  public function onError(ExceptionEvent $event) {
    $exception = $event
      ->getThrowable();
    $error = Error::decodeException($exception);
    $this->logger
      ->get('php')
      ->log($error['severity_level'], Error::DEFAULT_ERROR_MESSAGE, $error);
    $is_critical = !$exception instanceof HttpExceptionInterface || $exception
      ->getStatusCode() >= 500;
    if ($is_critical) {
      error_log(sprintf('Uncaught PHP Exception %s: "%s" at %s line %s', get_class($exception), $exception
        ->getMessage(), $exception
        ->getFile(), $exception
        ->getLine()));
    }
  }

  /**
   * Log 4xx errors that are not 403 or 404.
   *
   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
   *   The event to process.
   */
  public function onClientError(ExceptionEvent $event) {
    $exception = $event
      ->getThrowable();
    $error = Error::decodeException($exception);
    $error += [
      'status_code' => $exception
        ->getStatusCode(),
    ];
    unset($error['@backtrace_string']);
    $this->logger
      ->get('client error')
      ->warning(Error::DEFAULT_ERROR_MESSAGE, $error);
  }

  /**
   * Log all exceptions.
   *
   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
   *   The event to process.
   */
  public function onException(ExceptionEvent $event) {
    $exception = $event
      ->getThrowable();
    $method = 'onError';

    // Treat any non-HTTP exception as if it were one, so we log it the same.
    if ($exception instanceof HttpExceptionInterface) {
      $status_code = $exception
        ->getStatusCode();
      $possible_method = 'on' . $status_code;
      if (method_exists($this, $possible_method)) {
        $method = $possible_method;
      }
      elseif ($status_code >= 400 && $status_code < 500) {
        $method = 'onClientError';
      }
    }
    $this
      ->{$method}($event);
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {
    $events[KernelEvents::EXCEPTION][] = [
      'onException',
      50,
    ];
    return $events;
  }

}