Issue #3231996: Ensure file system is writeable

src/Validator/WritableFileSystemValidator.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 Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+class WritableFileSystemValidator implements EventSubscriberInterface {
+
+  public function checkFileSystemWritable(UpdateEvent $event){
+    $result = ValidationResult::createError([t('file system not writable')]);
+    $event->addValidationResult($result);
+    \Drupal::messenger()->addMessage("GOT HERE");
+  }
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    return [
+      AutomaticUpdatesEvents::READINESS_CHECK => 'checkFileSystemWritable',
+    ];
+  }
+
+}