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\PathLocator;
+use Drupal\automatic_updates\Validation\ValidationResult;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Checks that the file system is writable.
+ */
+class WritableFileSystemValidator implements EventSubscriberInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The path locator service.
+   *
+   * @var \Drupal\automatic_updates\PathLocator
+   */
+  protected $pathLocator;
+
+  /**
+   * The Drupal root.
+   *
+   * @var string
+   */
+  protected $appRoot;
+
+  /**
+   * Constructs a WritableFileSystemValidator object.
+   *
+   * @param \Drupal\automatic_updates\PathLocator $path_locator
+   *   The path locator service.
+   * @param string $app_root
+   *   The Drupal root.
+   */
+  public function __construct(PathLocator $path_locator, string $app_root) {
+    $this->pathLocator = $path_locator;
+    $this->appRoot = $app_root;
+  }
+
+  /**
+   * Checks that the file system is writable.
+   *
+   * @param \Drupal\automatic_updates\Event\UpdateEvent $event
+   *   The event object.
+   *
+   * @todo It might make sense to use a more sophisticated method of testing
+   *   writability than is_writable(), since it's not clear if that can return
+   *   false negatives/positives due to things like SELinux, exotic file
+   *   systems, and so forth.
+   */
+  public function checkPermissions(UpdateEvent $event): void {
+    $messages = [];
+
+    if (!is_writable($this->appRoot)) {
+      $messages[] = $this->t('The Drupal directory "@dir" is not writable.', [
+        '@dir' => $this->appRoot,
+      ]);
+    }
+
+    $dir = $this->pathLocator->getVendorDirectory();
+    if (!is_writable($dir)) {
+      $messages[] = $this->t('The vendor directory "@dir" is not writable.', ['@dir' => $dir]);
+    }
+
+    if ($messages) {
+      $result = ValidationResult::createError($messages, $this->t('The file system is not writable.'));
+      $event->addValidationResult($result);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    return [
+      AutomaticUpdatesEvents::READINESS_CHECK => 'checkPermissions',
+      AutomaticUpdatesEvents::PRE_START => 'checkPermissions',
+    ];
+  }
+
+}