async def read_chunk(self, size: int = chunk_size) -> bytes:
        """Reads body part content chunk of the specified size.

        size: chunk size
        """
        if self._at_eof:
            return b""
        if self._length:
            chunk = await self._read_chunk_from_length(size)
        else:
            chunk = await self._read_chunk_from_stream(size)

        self._read_bytes += len(chunk)
        if self._read_bytes == self._length:
            self._at_eof = True
        if self._at_eof:
            clrf = await self._content.readline()
            assert (
                b"\r\n" == clrf
            ), "reader did not read all the data or it is malformed"
        return chunk
Reads body part content chunk of the specified size.

        size: chunk size
        
lib/aiohttp/multipart.py