def _mark_in_progress(self, node_id):
        """Mark the node as 'in progress'.

        Callers must hold the lock.

        :param str node_id: The node ID to mark as in progress.
        """
        self.queued.remove(node_id)
        self.in_progress.add(node_id)
Mark the node as 'in progress'.

        Callers must hold the lock.

        :param str node_id: The node ID to mark as in progress.