Issue #3328234: Improve test DX *and* confidence: assert VFS state

package_manager/src/Validator/StageNotInActiveValidator.php

+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\package_manager\Validator;
+
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\package_manager\Event\PreCreateEvent;
+use Drupal\package_manager\Event\PreOperationStageEvent;
+use Drupal\package_manager\Event\StatusCheckEvent;
+use Drupal\package_manager\PathLocator;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Validates staging root is not a subdirectory of active.
+ *
+ * @internal
+ *   This is an internal part of Package Manager and may be changed or removed
+ *   at any time without warning. External code should not interact with this
+ *   class.
+ */
+class StageNotInActiveValidator implements EventSubscriberInterface {
+  use StringTranslationTrait;
+
+  /**
+   * The path locator service.
+   *
+   * @var \Drupal\package_manager\PathLocator
+   */
+  protected $pathLocator;
+
+  /**
+   * Constructs a new StageNotInActiveValidator object.
+   *
+   * @param \Drupal\package_manager\PathLocator $path_locator
+   *   The path locator service.
+   */
+  public function __construct(PathLocator $path_locator) {
+    $this->pathLocator = $path_locator;
+  }
+
+  /**
+   * Check if staging root is a subdirectory of active.
+   */
+  public function checkNotInActive(PreOperationStageEvent $event) {
+    $project_root = $this->pathLocator->getProjectRoot();
+    $staging_root = $this->pathLocator->getStagingRoot();
+    if (str_starts_with($staging_root, $project_root)) {
+      $message = $this->t("Stage directory is a subdirectory of the active directory.");
+      $event->addError([$message]);
+      $event->stopPropagation();
+    }
+
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    return [
+      PreCreateEvent::class => 'checkNotInActive',
+      StatusCheckEvent::class => 'checkNotInActive',
+    ];
+  }
+
+}