def advance(self, size: int) -> None:
        """
        Advance the current buffer position by ``size`` bytes.
        """
        assert 0 < size <= self._size
        self._size -= size
        pos = self._first_pos

        buffers = self._buffers
        while buffers and size > 0:
            is_large, b = buffers[0]
            b_remain = len(b) - size - pos
            if b_remain <= 0:
                buffers.popleft()
                size -= len(b) - pos
                pos = 0
            elif is_large:
                pos += size
                size = 0
            else:
                # Amortized O(1) shrink for Python 2
                pos += size
                if len(b) <= 2 * pos:
                    del typing.cast(bytearray, b)[:pos]
                    pos = 0
                size = 0

        assert size == 0
        self._first_pos = pos
Advance the current buffer position by ``size`` bytes.