On Drupal, global $base_url not set during a REST call

May be related to CRM-17413 in that it concerns the REST bootstrap procedure.

I have a custom API function that does some Drupal-specific stuff, including the use of the url() function. I noticed that this was returning URLs without a base, even with the "absolute" option. (It works when called from drush.) I then found this was because the global $base_url was not set (or was empty).

Further debugging:
This variable is normally initialised in drupal_settings_initialize() in Drupal's includes/bootstrap.inc. This contains a line

if (isset($base_url)) {

which, if I changed the isset to !empty, caused a base path to be set. Something similar is achieved by inserting

if (empty($GLOBALS['base_url'])) {
  unset($GLOBALS['base_url']);
}

before the drupal_bootstrap line in CRM_Utils_System_Drupal.

However, this still isn't right: a spurious /sites/all/modules/civicrm/extern is inserted that doesn't appear with drush. This is fixed with

unset($_SERVER['SCRIPT_NAME']);

(before CRM_Utils_System::loadBootStrap in CRM_Utils_REST) as the path from that variable is also used by drupal_settings_initialize to construct the base_url. I then get the paths I want out of url().

These changes feel like a bit of a hack though.