class PathProcessorFront

class PathProcessorFront implements InboundPathProcessorInterface {
    
    /**
     * A config factory for retrieving required config settings.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $config;
    
    /**
     * Constructs a PathProcessorFront object.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config
     *   A config factory for retrieving the site front page configuration.
     */
    public function __construct(ConfigFactoryInterface $config) {
        $this->config = $config;
    }
    
    /**
     * {@inheritdoc}
     */
    public function processInbound($path, Request $request) {
        if ($path === '/') {
            $path = $this->config
                ->get('system.site')
                ->get('page.front');
            if (empty($path)) {
                // We have to return a valid path but / won't be routable and config
                // might be broken so stop execution.
                throw new NotFoundHttpException();
            }
            $components = parse_url($path);
            // Remove query string and fragment.
            $path = $components['path'];
            // Merge query parameters from front page configuration value
            // with URL query, so that actual URL takes precedence.
            if (!empty($components['query'])) {
                parse_str($components['query'], $parameters);
                array_replace($parameters, $request->query
                    ->all());
                $request->query
                    ->replace($parameters);
            }
        }
        return $path;
    }

}