Add missing SubRequestBuffer Class.

src/Controller/SubrequestExtractionController.php

@@ -2,9 +2,9 @@
 
 namespace Drupal\graphql\Controller;
 
-use Drupal\Core\Cache\CacheableMetadata;
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\Core\Render\RenderContext;
 use Drupal\Core\Render\RendererInterface;
 use Drupal\graphql\SubRequestResponse;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -70,9 +70,19 @@ public function extract() {
     $request = $this->requestStack->getCurrentRequest();
     $callback = $request->attributes->get('_graphql_subrequest');
 
-    $metadata = new CacheableMetadata();
-    $response = new SubRequestResponse($callback($metadata));
-    $response->addCacheableDependency($metadata);
+    // @todo Remove this once https://www.drupal.org/project/drupal/issues/2940036#comment-12479912 is resolved.
+    $this->languageManager->reset();
+
+    // Collect any potentially leaked cache metadata released by the callback.
+    $context = new RenderContext();
+    $result = $this->renderer->executeInRenderContext($context, function () use ($callback) {
+      return $callback();
+    });
+
+    $response = new SubRequestResponse($result);
+    if (!$context->isEmpty()) {
+      $response->addCacheableDependency($context->pop());
+    }
 
     return $response;
   }

src/GraphQL/Buffers/SubRequestBuffer.php

@@ -0,0 +1,152 @@
+<?php
+
+namespace Drupal\graphql\GraphQL\Buffers;
+
+use Drupal\Core\Cache\CacheableDependencyInterface;
+use Drupal\Core\Url;
+use Drupal\Core\Routing\LocalRedirectResponse;
+use Drupal\graphql\SubRequestResponse;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\RequestStack;
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+
+/**
+ * SubRequestBuffer class provided a buffer for sub-requests.
+ */
+class SubRequestBuffer extends BufferBase {
+
+  /**
+   * The http kernel service.
+   *
+   * @var \Symfony\Component\HttpKernel\HttpKernelInterface
+   */
+  protected $httpKernel;
+
+  /**
+   * The request stack service.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * SubrequestBuffer constructor.
+   *
+   * @param \Symfony\Component\HttpKernel\HttpKernelInterface $httpKernel
+   *   The http kernel service.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $requestStack
+   *   The request stack service.
+   */
+  public function __construct(HttpKernelInterface $httpKernel, RequestStack $requestStack) {
+    $this->httpKernel = $httpKernel;
+    $this->requestStack = $requestStack;
+  }
+
+  /**
+   * Add an item to the buffer.
+   *
+   * @param \Drupal\Core\Url $url
+   *   The url object to run the subrequest on.
+   * @param callable $extract
+   *   The callback to run within the sub-request.
+   *
+   * @return \Closure
+   *   The callback to invoke to load the result for this buffer item.
+   */
+  public function add(Url $url, callable $extract) {
+    $item = new \ArrayObject([
+      'url' => $url,
+      'extract' => $extract,
+    ]);
+
+    return $this->createBufferResolver($item);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getBufferId($item) {
+    /** @var \Drupal\Core\GeneratedUrl $url */
+    $url = $item['url']->toString(TRUE);
+
+    return hash('sha256', json_encode([
+      'url' => $url->getGeneratedUrl(),
+      'tags' => $url->getCacheTags(),
+      'contexts' => $url->getCacheContexts(),
+      'age' => $url->getCacheMaxAge(),
+    ]));
+  }
+
+  /**
+   * Create a sub-request for the given url.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $current
+   *   The current main request.
+   * @param array $buffer
+   *   The buffer.
+   * @param string $url
+   *   The url to run the subrequest on.
+   *
+   * @return \Symfony\Component\HttpFoundation\Request
+   *   The request object.
+   */
+  protected function createRequest(Request $current, array $buffer, string $url) {
+    $request = Request::create(
+      $url,
+      'GET',
+      $current->query->all(),
+      $current->cookies->all(),
+      $current->files->all(),
+      $current->server->all()
+    );
+
+    $request->attributes->set('_graphql_subrequest', function () use ($buffer) {
+      return array_map(function ($item) {
+        return $item['extract']($item['url']);
+      }, $buffer);
+    });
+
+    if ($session = $current->getSession()) {
+      $request->setSession($session);
+    }
+
+    return $request;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function resolveBufferArray(array $buffer) {
+    /** @var \Drupal\Core\GeneratedUrl $url */
+    $url = reset($buffer)['url']->toString(TRUE);
+
+    $current = $this->requestStack->getCurrentRequest();
+    $target = $url->getGeneratedUrl();
+    $request = $this->createRequest($current, $buffer, $target);
+
+    /** @var \Drupal\graphql\SubRequestResponse $response */
+    $response = $this->httpKernel->handle($request, HttpKernelInterface::SUB_REQUEST);
+    while ($response instanceof LocalRedirectResponse) {
+      $target = $response->getTargetUrl();
+      $request = $this->createRequest($current, $buffer, $target);
+      $response = $this->httpKernel->handle($request, HttpKernelInterface::SUB_REQUEST);
+    }
+
+    if (!($response instanceof SubRequestResponse)) {
+      return array_fill_keys(array_keys($buffer), NULL);
+    }
+
+    // @todo Remove the request stack manipulation once the core issue described at
+    // https://www.drupal.org/node/2613044 is resolved.
+    while ($this->requestStack->getCurrentRequest() !== $current) {
+      $this->requestStack->pop();
+    }
+
+    if ($url instanceof CacheableDependencyInterface) {
+      $response->addCacheableDependency($url);
+    }
+
+    return $response->getResult();
+  }
+
+}