Issue #3383449 by catch, Wim Leers, Kingdutch: Add Fibers support to Drupal\Core\Render\Renderer

core/lib/Drupal/Core/Render/Renderer.php

@@ -162,22 +162,43 @@ public function renderPlain(&$elements) {
   }
 
   /**
-   * {@inheritdoc}
+   * Renders a placeholder into markup.
+   *
+   * @param array $placeholder_element
+   *   The placeholder element by reference.
+   *
+   * @return \Drupal\Component\Render\MarkupInterface|string
+   *   The rendered HTML.
    */
-  public function renderPlaceholder($placeholder, array $elements) {
-    // Get the render array for the given placeholder
-    $placeholder_elements = $elements['#attached']['placeholders'][$placeholder];
-
+  protected function doRenderPlaceholder(array &$placeholder_element): MarkupInterface|string {
     // Prevent the render array from being auto-placeholdered again.
-    $placeholder_elements['#create_placeholder'] = FALSE;
+    $placeholder_element['#create_placeholder'] = FALSE;
 
     // Render the placeholder into markup.
-    $markup = $this->renderPlain($placeholder_elements);
+    $markup = $this->renderPlain($placeholder_element);
+    return $markup;
+  }
 
+  /**
+   * Replaces a placeholder with its markup.
+   *
+   * @param string $placeholder
+   *   The placeholder HTML.
+   * @param \Drupal\Component\Render\MarkupInterface|string $markup
+   *   The markup to replace the placeholder with.
+   * @param array $elements
+   *   The render array that the placeholder is from.
+   * @param array $placeholder_element
+   *   The placeholder element render array.
+   *
+   * @return \Drupal\Component\Render\MarkupInterface|string
+   *   The rendered HTML.
+   */
+  protected function doReplacePlaceholder(string $placeholder, string|MarkupInterface $markup, array $elements, array $placeholder_element): array {
     // Replace the placeholder with its rendered markup, and merge its
     // bubbleable metadata with the main elements'.
     $elements['#markup'] = Markup::create(str_replace($placeholder, $markup, $elements['#markup']));
-    $elements = $this->mergeBubbleableMetadata($elements, $placeholder_elements);
+    $elements = $this->mergeBubbleableMetadata($elements, $placeholder_element);
 
     // Remove the placeholder that we've just rendered.
     unset($elements['#attached']['placeholders'][$placeholder]);
@@ -185,6 +206,16 @@ public function renderPlaceholder($placeholder, array $elements) {
     return $elements;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function renderPlaceholder($placeholder, array $elements) {
+    // Get the render array for the given placeholder
+    $placeholder_element = $elements['#attached']['placeholders'][$placeholder];
+    $markup = $this->doRenderPlaceholder($placeholder_element);
+    return $this->doReplacePlaceholder($placeholder, $markup, $elements, $placeholder_element);
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -665,13 +696,47 @@ protected function replacePlaceholders(array &$elements) {
 
     // First render all placeholders except 'status messages' placeholders.
     $message_placeholders = [];
+    $fibers = [];
     foreach ($elements['#attached']['placeholders'] as $placeholder => $placeholder_element) {
       if (isset($placeholder_element['#lazy_builder']) && $placeholder_element['#lazy_builder'][0] === 'Drupal\Core\Render\Element\StatusMessages::renderMessages') {
         $message_placeholders[] = $placeholder;
       }
       else {
-        $elements = $this->renderPlaceholder($placeholder, $elements);
+        // Get the render array for the given placeholder
+        $fibers[$placeholder] = new \Fiber(function () use ($placeholder_element) {
+          return [$this->doRenderPlaceholder($placeholder_element), $placeholder_element];
+        });
+      }
+    }
+    while (count($fibers) > 0) {
+      $iterations = 0;
+      foreach ($fibers as $placeholder => $fiber) {
+        if (!$fiber->isStarted()) {
+          $fiber->start();
+        }
+        elseif ($fiber->isSuspended()) {
+          $fiber->resume();
+        }
+        // If the Fiber hasn't terminated by this point, move onto the next
+        // placeholder, we'll resume this fiber again when we get back here.
+        if (!$fiber->isTerminated()) {
+          // If we've gone through the placeholders once already, and they're
+          // still not finished, then start to allow code higher up the stack to
+          // get on with something else.
+          if ($iterations) {
+            $fiber = \Fiber::getCurrent();
+            if ($fiber !== NULL) {
+              $fiber->suspend();
+            }
+          }
+          continue;
+        }
+        [$markup, $placeholder_element] = $fiber->getReturn();
+
+        $elements = $this->doReplacePlaceholder($placeholder, $markup, $elements, $placeholder_element);
+        unset($fibers[$placeholder]);
       }
+      $iterations++;
     }
 
     // Then render 'status messages' placeholders.

core/tests/Drupal/Tests/Core/Render/RendererTestBase.php

@@ -314,6 +314,11 @@ public static function callback($animal, $use_animal_as_array_key = FALSE) {
    *   A renderable array.
    */
   public static function callbackPerUser($animal) {
+    // As well as adding the user cache context, additionally suspend the
+    // current Fiber if there is one.
+    if ($fiber = \Fiber::getCurrent()) {
+      $fiber->suspend();
+    }
     $build = static::callback($animal);
     $build['#cache']['contexts'][] = 'user';
     return $build;