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;
    }

}