class BufferOverflow(Exception):
    def __init__(self):
        super().__init__("Buffer overflow")

class LineBuffer():
    __slots__ = 'buf', 'offset',

    BUFFER_SIZE = 4096

    def __init__(self, size: int=BUFFER_SIZE):
        self.buf:    bytearray = bytearray(size)
        self.offset: int = 0

    def readline(self, sock: socket.socket) -> bytes:
        while True:
            #
            # Check and see if there is already a line in the
            # internal buffer.
            #
            offset = self.buf.find(b'\n', self.offset, self.count)

            if offset < 0:
                #
                # There is no line in the buffer.  Check and see if
                # there is more room in the buffer to continue
                # reading.
                #
                if offset == self.BUFFER_SIZE - 1:
                    #
                    # There is no room left in the buffer; the line
                    # must be too long.  Panic.
                    #
                    raise BufferOverflow()

                #
                # Read from the socket to attempt to fill the buffer
                # more.
                #
                data = sock.recv(self.BUFFER_SIZE - self.offset)

                #
                # Check to see if recv() returned any data.  If not,
                # the socket is closed.  Return an empty string.
                #
                if data == b'':
                    return ''

                count = len(data)

                #
                # Now, fill the buffer.  The next iteration of the
                # loop will be used to check for a newline.
                #
                self.buf[self.offset:self.offset+count] = data
            else:
                #
                # There is a line in the buffer.  Prepare to return
                # it.
                #
                ret = self.buf[self.offset:offset]

                #
                # Advance the offset to one character beyond the
                # newline.
                #
                self.offset = offset + 1

                return str(ret, 'ascii')