Symfony\Component\HttpKernel\HttpCache\HttpCache::lock PHP Method

    protected function lock(Request $request, Response $entry)
    {
        // try to acquire a lock to call the backend
        $lock = $this->store->lock($request);
        // there is already another process calling the backend
        if (true !== $lock) {
            // check if we can serve the stale entry
            if (null === ($age = $entry->headers->getCacheControlDirective('stale-while-revalidate'))) {
                $age = $this->options['stale_while_revalidate'];
            }
            if (abs($entry->getTtl()) < $age) {
                $this->record($request, 'stale-while-revalidate');
                // server the stale response while there is a revalidation
                return true;
            }
            // wait for the lock to be released
            $wait = 0;
            while ($this->store->isLocked($request) && $wait < 5000000) {
                usleep(50000);
                $wait += 50000;
            }
            if ($wait < 5000000) {
                // replace the current entry with the fresh one
                $new = $this->lookup($request);
                $entry->headers = $new->headers;
                $entry->setContent($new->getContent());
                $entry->setStatusCode($new->getStatusCode());
                $entry->setProtocolVersion($new->getProtocolVersion());
                foreach ($new->headers->getCookies() as $cookie) {
                    $entry->headers->setCookie($cookie);
                }
            } else {
                // backend is slow as hell, send a 503 response (to avoid the dog pile effect)
                $entry->setStatusCode(503);
                $entry->setContent('503 Service Unavailable');
                $entry->headers->set('Retry-After', 10);
            }
            return true;
        }
        // we have the lock, call the backend
        return false;
    }