How to use the waitress.receiver.FixedStreamReceiver function in waitress

def _makeOne(self, cl, buf):
        from waitress.receiver import FixedStreamReceiver

        return FixedStreamReceiver(cl, buf)

buf = OverflowableBuffer(self.adj.inbuf_overflow)
                self.body_rcv = ChunkedReceiver(buf)
            expect = headers.get('EXPECT', '').lower()
            self.expect_continue = expect == '100-continue'
            if connection.lower() == 'close':
                self.connection_close = True

        if not self.chunked:
            try:
                cl = int(headers.get('CONTENT_LENGTH', 0))
            except ValueError:
                cl = 0
            self.content_length = cl
            if cl > 0:
                buf = OverflowableBuffer(self.adj.inbuf_overflow)
                self.body_rcv = FixedStreamReceiver(cl, buf)

expect = headers.get("EXPECT", "").lower()
            self.expect_continue = expect == "100-continue"
            if connection.lower() == "close":
                self.connection_close = True

        if not self.chunked:
            try:
                cl = int(headers.get("CONTENT_LENGTH", 0))
            except ValueError:
                raise ParsingError("Content-Length is invalid")

            self.content_length = cl
            if cl > 0:
                buf = OverflowableBuffer(self.adj.inbuf_overflow)
                self.body_rcv = FixedStreamReceiver(cl, buf)