Can't determine logged-in cid during a REST call

When a user is logged in normally, you can get the active Civi contact ID with:

$session = CRM_Core_Session::singleton();
$cid = $session->get('userID');

and the active Drupal one with:

global $user;
$uid = $user->uid;

During a REST call, for example in a custom API or a hook, the first is not available- the session variable has not been set. The Drupal uid is available, even though the Civi cid must have been known to get it, because API keys are attached to a Civi contact.

This was demonstrated in custom code from FF and a fix found with help from Tim Otten. PR to follow.