Issue #3241380: Ensure DB updates are not pending

src/Validator/PendingUpdatesValidator.php

+<?php
+
+namespace Drupal\automatic_updates\Validator;
+
+use Drupal\automatic_updates\AutomaticUpdatesEvents;
+use Drupal\automatic_updates\Event\UpdateEvent;
+use Drupal\automatic_updates\Validation\ValidationResult;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\StringTranslation\TranslationInterface;
+use Drupal\Core\Update\UpdateRegistry;
+use Drupal\Core\Url;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Validates that there are no pending database updates.
+ */
+class PendingUpdatesValidator implements EventSubscriberInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The Drupal root.
+   *
+   * @var string
+   */
+  protected $appRoot;
+
+  /**
+   * The update registry service.
+   *
+   * @var \Drupal\Core\Update\UpdateRegistry
+   */
+  protected $updateRegistry;
+
+  /**
+   * Constructs an PendingUpdatesValidator object.
+   *
+   * @param string $app_root
+   *   The Drupal root.
+   * @param \Drupal\Core\Update\UpdateRegistry $update_registry
+   *   The update registry service.
+   * @param \Drupal\Core\StringTranslation\TranslationInterface $translation
+   *   The translation service.
+   */
+  public function __construct(string $app_root, UpdateRegistry $update_registry, TranslationInterface $translation) {
+    $this->appRoot = $app_root;
+    $this->updateRegistry = $update_registry;
+    $this->setStringTranslation($translation);
+  }
+
+  /**
+   * Validates that there are no pending database updates.
+   *
+   * @param \Drupal\automatic_updates\Event\UpdateEvent $event
+   *   The update event.
+   */
+  public function checkPendingUpdates(UpdateEvent $event) {
+    require_once $this->appRoot . '/core/includes/install.inc';
+    require_once $this->appRoot . '/core/includes/update.inc';
+
+    drupal_load_updates();
+    $hook_updates = update_get_update_list();
+    $post_updates = $this->updateRegistry->getPendingUpdateFunctions();
+
+    if ($hook_updates || $post_updates) {
+      $message = $this->t('Some modules have database schema updates to install. You should run the <a href=":update">database update script</a> immediately.', [
+        ':update' => Url::fromRoute('system.db_update')->toString(),
+      ]);
+      $error = ValidationResult::createError([$message]);
+      $event->addValidationResult($error);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    return [
+      AutomaticUpdatesEvents::PRE_START => 'checkPendingUpdates',
+      AutomaticUpdatesEvents::READINESS_CHECK => 'checkPendingUpdates',
+    ];
+  }
+
+}