Race condition in drush_mkdir()

We are seeing a relatively high number of transients errors with packaging on Drupal.org. A typical session is:

ERROR: /usr/bin/php /var/www/drupal.org/tools/drush5/drush.php --no-cache --include=/var/www/drupal.org/tools/drupalorg_drush make --drupal-org=contrib --drupal-org-build-root=/var/tmp/branch.20167/commerce_kickstart --drupal-org-log-errors-to-file --drupal-org-log-package-metadata=metadata-contrib.json drupal-org.make . failed with status 1

     >> Project omega contains 5 themes: omega, alpha, starterkit_omega_html5, starterkit_omega_xhtml, starterkit_alpha_xhtml.
     >> Project custom_breadcrumbs contains 7 modules: custom_breadcrumbs_taxonomy, custom_breadcrumbsapi, custom_breadcrumbs_panels, custom_breadcrumbs_paths, custom_breadcrumbs_identifiers, custom_breadcrumbs_views, custom_breadcrumbs.
     >> [... a lot of similar success messages ...]

    ERROR: >> Unable to create /tmp/drush_tmp_1339675650_4fd9d402e4f3e.
    ERROR: >> Project cs_adaptive_image (7.x-1.0-alpha2) could not be downloaded to /tmp/make_tmp_1339675649_4fd9d401ded3d/__build__/modules/contrib/cs_adaptive_image.
    ERROR: Build for commerce_kickstart-7.x-2.x-dev-no-core failed.

Looking into it, there is an obvious race condition in drush_mkdir(), which currently read this way:

  if (!is_dir($path)) {
    if (drush_mkdir(dirname($path))) {
      if (@mkdir($path)) {
        return TRUE;
      }
      else {
        // Error.
      }
    }
  }

Drush Make working concurrently, it can happen that a process created the directory while another process was between !is_dir() and @mkdir.

I don't know if this is the root cause of our troubles, but it seems important to fix nonetheless.