Support overriding assets:// in core

Closes #3499242

src/FlysystemBridge.php

@@ -3,7 +3,9 @@
 namespace Drupal\flysystem;
 
 use Drupal\Core\StreamWrapper\StreamWrapperInterface;
+use Drupal\Core\StreamWrapper\StreamWrapperManager;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Url;
 use League\Flysystem\FilesystemInterface;
 use League\Flysystem\Util;
 use Codementality\FlysystemStreamWrapper\FlysystemStreamWrapper;
@@ -69,6 +71,23 @@ class FlysystemBridge extends FlysystemStreamWrapper implements StreamWrapperInt
    * {@inheritdoc}
    */
   public function getExternalUrl() {
+    // Assets is a special case as Drupal generates these on the fly, similar
+    // to how image styles have historically operated. This requires Drupal to
+    // always be inline of this request. On repeat visits, the asset will be
+    // piped through to the client, but this is not as fast as serving directly
+    // from disk or worse, from object storage over the network. Sites wishing
+    // to offload serving the generated assets should consider pairing Flysystem
+    // with CDN module and treating these assets as "far-future" URLs.
+    if (StreamWrapperManager::getScheme($this->uri) === 'assets') {
+      $target = $this->getTarget($this->uri);
+      return Url::fromRoute(
+        sprintf('system.%s_asset', current(explode(DIRECTORY_SEPARATOR, $target, 2))),
+        // @todo Use Drupal's FileSystem::basename() if we can make it static.
+        ['file_name' => basename($target)],
+      )
+        ->toString(TRUE)
+        ->getGeneratedUrl();
+    }
     return $this
       ->getFactory()
       ->getPlugin($this->getProtocol())