async def METHOD_NAME(self):
    self._block_count = await self.raw_decoder.read_long()
    if self.codec == "null":
        # Skip a long; we don't need to use the length.
        await self.raw_decoder.skip_long()
        self._datum_decoder = self._raw_decoder
    else:
        raise DataFileException("Unknown codec: %r" % self.codec)