Override sessionStart()
Just noticed this https://github.com/civicrm/civicrm-core/commit/671f655b395151fb9899ef9d76a02cecfcd0498d#diff-dba87af7e0599e4604e302ebad9b8180. Should be something like this because we can't rely on drupal_session_start
being there:
/**
* Start a new session.
*/
public function sessionStart() {
if (function_exists('backdrop_session_start')) {
// https://issues.civicrm.org/jira/browse/CRM-14356
if (!(isset($GLOBALS['lazy_session']) && $GLOBALS['lazy_session'] == TRUE)) {
backdrop_session_start();
}
$_SESSION = [];
}
else {
session_start();
}
}