def readLine(self):
        """ read a line
        Maintains its own buffer, callers of the transport should not mix
        calls to readBytes and readLine.
        """
        if self.buf is None:
            self.buf = []

        # Buffer may already have a line if we've received unilateral
        # response(s) from the server
        if len(self.buf) == 1 and b"\n" in self.buf[0]:
            (line, b) = self.buf[0].split(b"\n", 1)
            self.buf = [b]
            return line

        while True:
            b = self.readBytes(4096)
            if b"\n" in b:
                result = b"".join(self.buf)
                (line, b) = b.split(b"\n", 1)
                self.buf = [b]
                return result + line
            self.buf.append(b)
read a line
        Maintains its own buffer, callers of the transport should not mix
        calls to readBytes and readLine.