ContentLength.php

<?php

namespace Drupal\Core\StackMiddleware;

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\HttpKernelInterface;

/**
 * Adds a Content-Length HTTP header to responses.
 */
class ContentLength implements HttpKernelInterface {
    
    /**
     * Constructs a new ContentLength instance.
     *
     * @param \Symfony\Component\HttpKernel\HttpKernelInterface $httpKernel
     *   The wrapped HTTP kernel.
     */
    public function __construct(HttpKernelInterface $httpKernel) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function handle(Request $request, $type = self::MAIN_REQUEST, $catch = TRUE) : Response {
        $response = $this->httpKernel
            ->handle($request, $type, $catch);
        if ($response->isInformational() || $response->isEmpty()) {
            return $response;
        }
        if ($response->headers
            ->has('Transfer-Encoding')) {
            return $response;
        }
        // Drupal cannot set the correct content length header when there is a
        // server error.
        if ($response->isServerError()) {
            return $response;
        }
        $content = $response->getContent();
        if ($content === FALSE) {
            return $response;
        }
        $response->headers
            ->set('Content-Length', strlen($content), TRUE);
        return $response;
    }

}