'Get the {blobname -> dbfile} mapping index for the given dir
and lang.
\'dfb\' stands for \'dbfile_from_blobname\'.
This must be called with the lock held.'

def dfb_from_dir(self, dir, sublang, default=None):

    blob_index = self.load_index(dir, 'blob_index', default=default)
    try:
        return blob_index[sublang]
    except KeyError as ex:
        if (default is not None):
            return default
        raise
