Issue #3016814 by jhedstrom, peximo, alexpott, jungle, scuba_fly, mr.baileys,...

    $url = file_create_url($file->getFileUri());

    // Set file_test access header to allow the download.

    file_test_reset();

    file_test_set_return('download', ['x-foo' => 'Bar']);

    $this->drupalGet($url);

    $this->assertEqual($this->drupalGetHeader('x-foo'), 'Bar', 'Found header set by file_test module on private download.');

    $this->assertNull($this->drupalGetHeader('x-drupal-cache'), 'Page cache is disabled on private file download.');

    $this->assertResponse(200, 'Correctly allowed access to a file when file_test provides headers.');

    // Ensure hook_file_download is fired correctly.

    $this->assertEquals($file->getFileUri(), \Drupal::state()->get('file_test.results')['download'][0][0]);

    // Test that the file transferred correctly.

    $this->assertSame($contents, $this->getSession()->getPage()->getContent(), 'Contents of the file are correct.');

    $this->assertSame(403, $response->getStatusCode(), 'Correctly denied access to a file when file_test sets the header to -1.');

    // Try non-existent file.

    file_test_reset();

    $url = file_create_url('private://' . $this->randomMachineName());

    $response = $http_client->head($url, ['http_errors' => FALSE]);

    $this->assertSame(404, $response->getStatusCode(), 'Correctly returned 404 response for a non-existent file.');

    // Assert that hook_file_download is not called.

    $this->assertEquals([], \Drupal::state()->get('file_test.results')['download']);

    // Try requesting the private file url without a file specified.

    file_test_reset();

    $this->drupalGet('/system/files');

    $this->assertSession()->statusCodeEquals(404);

    // Assert that hook_file_download is not called.

    $this->assertEquals([], \Drupal::state()->get('file_test.results')['download']);

  }

  /**

    // Merge remaining path arguments into relative file path.

    $uri = $scheme . '://' . $target;

    if ($this->streamWrapperManager->isValidScheme($scheme) && file_exists($uri)) {

    if ($this->streamWrapperManager->isValidScheme($scheme) && is_file($uri)) {

      // Let other modules provide headers and controls access to the file.

      $headers = $this->moduleHandler()->invokeAll('file_download', [$uri]);