Get the set of block IDs that were backed up

def get_backup_blocks(cls, impl, working_dir):
        """
        Get the set of block IDs that were backed up
        """
        ret = []
        backup_dir = config.get_backups_directory(impl, working_dir)
        if not os.path.exists(backup_dir):
            return []

        for name in os.listdir( backup_dir ):
            if ".bak." not in name:
                continue 

            suffix = name.split(".bak.")[-1]
            try:
                block_id = int(suffix)
            except:
                continue 

            # must exist...
            backup_paths = cls.get_backup_paths(block_id, impl, working_dir)
            for p in backup_paths:
                if not os.path.exists(p):
                    # doesn't exist
                    block_id = None
                    continue

            if block_id is not None:
                # have backup at this block 
                ret.append(block_id)

        return ret