Re-added and simplified file_transfer() by reusing the logic of StreamedResponse.

core/includes/file.inc

@@ -1967,6 +1967,28 @@ function file_unmanaged_save_data($data, $destination = NULL, $replace = FILE_EX
   return file_unmanaged_move($temp_name, $destination, $replace);
 }
 
+/**
+ * Transfers a file to the client using HTTP.
+ *
+ * Pipes a file through Drupal to the client.
+ *
+ * @param $uri
+ *   String specifying the file URI to transfer.
+ * @param $headers
+ *   An array of HTTP headers to send along with file.
+ */
+function file_transfer($uri, $headers) {
+  return new StreamedResponse(function() use ($uri) {
+    // Transfer file in 1024 byte chunks to save memory usage.
+    if (file_exists($uri) && $fd = fopen($uri, 'rb')) {
+      while (!feof($fd)) {
+        print fread($fd, 1024);
+      }
+      fclose($fd);
+    }
+  }, 200, $headers);
+}
+
 /**
  * Page callback: Handles private file transfers.
  *
@@ -2003,16 +2025,7 @@ function file_download() {
       }
     }
     if (count($headers)) {
-      return new StreamedResponse(function() use ($uri) {
-        $scheme = file_uri_scheme($uri);
-        // Transfer file in 1024 byte chunks to save memory usage.
-        if ($scheme && file_stream_wrapper_valid_scheme($scheme) && $fd = fopen($uri, 'rb')) {
-          while (!feof($fd)) {
-            print fread($fd, 1024);
-          }
-          fclose($fd);
-        }
-      }, 200, $headers);
+      return file_transfer($uri, $headers);
     }
     throw new AccessDeniedHttpException();
   }

core/modules/image/image.module

@@ -734,16 +734,7 @@ function image_style_deliver($style, $scheme) {
       'Content-Type' => $image->info['mime_type'],
       'Content-Length' => $image->info['file_size'],
     );
-    return new StreamedResponse(function() use ($uri) {
-      $scheme = file_uri_scheme($uri);
-      // Transfer file in 1024 byte chunks to save memory usage.
-      if ($scheme && file_stream_wrapper_valid_scheme($scheme) && $fd = fopen($uri, 'rb')) {
-        while (!feof($fd)) {
-          print fread($fd, 1024);
-        }
-        fclose($fd);
-      }
-    }, 200, $headers);
+    return file_transfer($uri, $headers);
   }
   else {
     watchdog('image', 'Unable to generate the derived image located at %path.', array('%path' => $derivative_uri));

core/modules/update/tests/modules/update_test/update_test.module

@@ -115,19 +115,12 @@ function update_test_mock_page($project_name) {
 
   $path = drupal_get_path('module', 'update_test');
   $file = "$path/$project_name.$availability_scenario.xml";
+  $headers = array('Content-Type' => 'text/xml; charset=utf-8');
   if (!is_file($file)) {
     // Return an empty response.
-    return new Response('', 200, array('Content-Type' =>  'text/xml; charset=utf-8'));
+    return new Response('', 200, $headers);
   }
-  return new StreamedResponse(function() use ($file) {
-    // Transfer file in 1024 byte chunks to save memory usage.
-    if ($fd = fopen($file, 'rb')) {
-      while (!feof($fd)) {
-        print fread($fd, 1024);
-      }
-      fclose($fd);
-    }
-  }, 200, array('Content-Type' =>  'text/xml; charset=utf-8'));
+  return file_transfer($file, $headers);
 }
 
 /**