Added test in which an entity gets resaved in an implementation of hook_entity_update().

core/modules/editor/tests/modules/editor_test.module

@@ -5,9 +5,45 @@
  * Helper module for the Text Editor tests.
  */
 
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\node\NodeInterface;
 use Drupal\filter\FilterFormatInterface;
 use Drupal\file\FileInterface;
 
+/**
+ * Implements hook_entity_update().
+ *
+ * @see \Drupal\Tests\editor\Kernel\EntityUpdateTest
+ */
+function editor_test_entity_update(EntityInterface $entity) {
+  // Only act on nodes.
+  if (!$entity instanceof NodeInterface) {
+    return;
+  }
+
+  // Avoid infinite loop by only going through our post save logic once.
+  if (!empty($entity->editor_test_updating)) {
+    return;
+  }
+
+  // Set flag for whether or not the entity needs to be resaved.
+  $needs_update = FALSE;
+
+  // Perform our post save logic.
+  if ($entity->title->value == 'test updated') {
+    // Change the node title.
+    $entity->title->value = 'test updated 2';
+    $needs_update = TRUE;
+  }
+
+  if ($needs_update) {
+    // Set flag on entity that our logic was already executed.
+    $entity->editor_test_updating = TRUE;
+    // And resave entity.
+    $entity->save();
+  }
+}
+
 /**
  * Implements hook_editor_js_settings_alter().
  */

core/modules/editor/tests/src/Kernel/EntityUpdateTest.php

+<?php
+
+namespace Drupal\Tests\editor\Kernel;
+
+use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
+use Drupal\node\Entity\NodeType;
+use Drupal\node\Entity\Node;
+
+/**
+ * Tests updating an entity.
+ *
+ * @group editor
+ */
+class EntityUpdateTest extends EntityKernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['editor', 'editor_test', 'node'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->installSchema('node', ['node_access']);
+    $this->installConfig(['node']);
+
+    // Create a node type for testing.
+    $type = NodeType::create(['type' => 'page', 'name' => 'page']);
+    $type->save();
+
+    // Set editor_test module weight to be lower than editor module's weight so
+    // that editor_test_entity_update() is called before editor_entity_update().
+    $extension_config = \Drupal::configFactory()->get('core.extension');
+    $editor_module_weight = $extension_config->get('module.editor');
+    module_set_weight('editor_test', $editor_module_weight - 1);
+  }
+
+  /**
+   * Tests updating an existing entity.
+   *
+   * @see editor_test_entity_update()
+   */
+  public function testEntityUpdate() {
+    // Create a node.
+    $node = Node::create([
+      'type' => 'page',
+      'title' => 'test',
+    ]);
+    $node->save();
+
+    // Update the node.
+    // What happens is the following:
+    // 1. \Drupal\Core\Entity\EntityStorageBase::doPostSave() gets called.
+    // 2. editor_test_entity_update() gets called.
+    // 3. A resave of the updated entity gets triggered (second save call).
+    // 4. \Drupal\Core\Entity\EntityStorageBase::doPostSave() gets called.
+    // 5. editor_test_entity_update() gets called.
+    // 6. editor_entity_update() gets called (caused by the second save call).
+    // 7. editor_entity_update() gets called (caused by the first save call).
+    $node->title->value = 'test updated';
+    $node->save();
+  }
+
+}