Recursively get cached placeholders.

Closes #3513928

core/lib/Drupal/Core/Render/Placeholder/CachedStrategy.php

@@ -21,7 +21,41 @@ public function __construct(
    * {@inheritdoc}
    */
   public function processPlaceholders(array $placeholders) {
-    return $this->renderCache->getMultiple($placeholders);
+    $return = $this->renderCache->getMultiple($placeholders);
+    if ($return) {
+      $return = $this->processNestedPlaceholders($return);
+    }
+
+    return $return;
+  }
+
+  /**
+   * Fetch any nested placeholders from cache.
+   *
+   * Placeholders returned from cache may have placeholders in #attached, which
+   * can themselves be fetched from the cache. By recursively processing the
+   * placeholders here, we're able to use multiple cache get to fetch the cache
+   * items at each level of recursion.
+   */
+  private function processNestedPlaceholders(array $placeholders): array {
+    $sets = [];
+    foreach ($placeholders as $key => $placeholder) {
+      if (!empty($placeholder['#attached']['placeholders'])) {
+        $sets[] = $placeholder['#attached']['placeholders'];
+      }
+    }
+    if ($sets) {
+      $cached = $this->renderCache->getMultiple(...array_merge($sets));
+      if ($cached) {
+        $cached = $this->processNestedPlaceholders($cached);
+        foreach ($placeholders as $key => $placeholder) {
+          if (!empty($placeholder['#attached']['placeholders'])) {
+            $placeholders[$key]['#attached']['placeholders'] = array_replace($placeholder['#attached']['placeholders'], $cached);
+          }
+        }
+      }
+    }
+    return $placeholders;
   }
 
 }