Initial proof of concept for delayed multiple loading of entities.

Closes #1237636

core/lib/Drupal/Core/Entity/EntityStorageBase.php

@@ -89,6 +89,11 @@ abstract class EntityStorageBase extends EntityHandlerBase implements EntityStor
    */
   protected $memoryCacheTag;
 
+  /**
+   * Entity IDs awaiting loading.
+   */
+  protected array $entityIdsToLoad = [];
+
   /**
    * Constructs an EntityStorageBase instance.
    *
@@ -285,6 +290,34 @@ public function loadMultiple(?array $ids = NULL) {
       $ids = array_keys(array_diff_key($flipped_ids, $entities));
     }
 
+    if ($ids && $this->entityType->isStaticallyCacheable()) {
+      if (\Fiber::getCurrent() !== NULL) {
+        // Before suspending the fiber, add the IDs passed in to the full list
+        // of entities to load, so that another call can load everything at
+        // once.
+        $this->entityIdsToLoad = array_merge($this->entityIdsToLoad, $ids);
+        \Fiber::suspend();
+        // Another call to this method may have reset the entityIdsToLoad
+        // property after loading entities. Combine it with the passed in IDs
+        // again in case this has happened.
+        $this->entityIdsToLoad = array_merge($this->entityIdsToLoad, $ids);
+        $entities += $this->getFromStaticCache($this->entityIdsToLoad);
+        if ($entities) {
+          // Remove any entities found in the static cache from the IDs to load.
+          $ids = array_keys(array_diff_key(array_flip($this->entityIdsToLoad), $entities));
+        }
+        else {
+          // If nothing was found in the static cache, load every entity ID
+          // requested so far.
+          $ids = array_unique($this->entityIdsToLoad);
+        }
+        // Now that we've reached this point, unset the list of entity IDs to
+        // load so that further calls start with a blank slate (apart from the
+        // entity static cache itself).
+        $this->entityIdsToLoad = [];
+      }
+    }
+
     // Try to gather any remaining entities from a 'preload' method. This method
     // can invoke a hook to be used by modules that need, for example, to swap
     // the default revision of an entity with a different one. Even though the