Issue #3521812: Use public stream wrapper for generating the redirect URL

Closes #3521812

src/EventSubscriber/StageFileProxyConfigSubscriber.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\stage_file_proxy\EventSubscriber;
+
+use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
+use Drupal\Core\Config\ConfigCrudEvent;
+use Drupal\Core\Config\ConfigEvents;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Stage file proxy subscriber for configuration changes.
+ */
+class StageFileProxyConfigSubscriber implements EventSubscriberInterface {
+
+  /**
+   * Constructs a StageFileProxyConfigSubscriber object.
+   */
+  public function __construct(
+    protected CacheTagsInvalidatorInterface $cacheTagsInvalidator,
+  ) {
+  }
+
+  /**
+   * Invalidates 4xx-response cache tag if the origin is added.
+   *
+   * @param \Drupal\Core\Config\ConfigCrudEvent $event
+   *   The configuration event.
+   */
+  public function onConfigSave(ConfigCrudEvent $event): void {
+    $saved_config = $event->getConfig();
+    if ($saved_config->getName() === 'stage_file_proxy.settings') {
+      if ($event->isChanged('origin') && empty($event->getConfig()->getOriginal('origin'))) {
+        // The default behavior is to do nothing if the origin is undefined,
+        // so once a new origin is added, any previous 4xx responses in the
+        // cache should be invalidated so that stage_file_proxy may intercept
+        // it.
+        $this->cacheTagsInvalidator->invalidateTags(['4xx-response']);
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    $events = [];
+    $events[ConfigEvents::SAVE][] = ['onConfigSave'];
+
+    return $events;
+  }
+
+}