'Get the set of user ids that are currently syncing on this HS.
Returns:
set(str): A set of user_id strings.'

def get_currently_syncing_users(self):

    syncing_user_ids = {user_id for (user_id, count) in self.user_to_num_current_syncs.items() if count}
    for user_ids in self.external_process_to_current_syncs.values():
        syncing_user_ids.update(user_ids)
    return syncing_user_ids
