Issue #3248929: List update that will be applied on the UpdateReady form

package_manager/tests/modules/package_manager_test_fixture/src/EventSubscriber/FixtureStager.php

+<?php
+
+namespace Drupal\package_manager_test_fixture\EventSubscriber;
+
+use Drupal\Core\State\StateInterface;
+use Drupal\package_manager\Event\PostRequireEvent;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\Filesystem\Filesystem;
+
+/**
+ * Defines an event subscriber which copies certain files into the staging area.
+ *
+ * This is most useful in conjunction with package_manager_bypass, which quietly
+ * turns all Composer Stager operations into no-ops. In such cases, no staging
+ * area will be physically created, but if a test needs to simulate certain
+ * conditions in a staging area without actually staging the active code base,
+ * this event subscriber is the way to do it.
+ */
+class FixtureStager implements EventSubscriberInterface {
+
+  /**
+   * The state service.
+   *
+   * @var \Drupal\Core\State\StateInterface
+   */
+  protected $state;
+
+  /**
+   * The Symfony file system service.
+   *
+   * @var \Symfony\Component\Filesystem\Filesystem
+   */
+  protected $fileSystem;
+
+  /**
+   * Constructs a FixtureStager.
+   *
+   * @param \Drupal\Core\State\StateInterface $state
+   *   The state service.
+   * @param \Symfony\Component\Filesystem\Filesystem $file_system
+   *   The Symfony file system service.
+   */
+  public function __construct(StateInterface $state, Filesystem $file_system) {
+    $this->state = $state;
+    $this->fileSystem = $file_system;
+  }
+
+  /**
+   * Copies files from a fixture into the staging area.
+   *
+   * Tests which use this functionality are responsible for cleaning up the
+   * staging area.
+   *
+   * @param \Drupal\package_manager\Event\PostRequireEvent $event
+   *   The event object.
+   *
+   * @see \Drupal\Tests\automatic_updates\Functional\AutomaticUpdatesFunctionalTestBase::tearDown()
+   */
+  public function copyFilesFromFixture(PostRequireEvent $event): void {
+    $fixturePath = $this->state->get(static::class);
+    if ($fixturePath && is_dir($fixturePath)) {
+      $this->fileSystem->mirror($fixturePath, $event->getStage()->getStageDirectory(), NULL, [
+        'override' => TRUE,
+        'delete' => TRUE,
+      ]);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    return [
+      PostRequireEvent::class => 'copyFilesFromFixture',
+    ];
+  }
+
+  /**
+   * Sets the path of the fixture to copy into the staging area.
+   *
+   * @param string $path
+   *   The path of the fixture to copy into the staging area.
+   */
+  public static function setFixturePath(string $path): void {
+    \Drupal::state()->set(static::class, $path);
+  }
+
+}