refactor HTTP/3 stream handling to use a dedicated stream

Reading from and writing onto this stream applies HTTP/3 DATA framing.

http3/client.go

@@ -298,15 +298,59 @@ func (c *client) RoundTripOpt(req *http.Request, opt RoundTripOpt) (*http.Respon
 	return rsp, rerr.err
 }
 
+func (c *client) sendRequestBody(str Stream, body io.ReadCloser) error {
+	defer body.Close()
+	b := make([]byte, bodyCopyBufferSize)
+	for {
+		n, rerr := body.Read(b)
+		if n == 0 {
+			if rerr == nil {
+				continue
+			}
+			if rerr == io.EOF {
+				break
+			}
+		}
+		if _, err := str.Write(b[:n]); err != nil {
+			return err
+		}
+		if rerr != nil {
+			if rerr == io.EOF {
+				break
+			}
+			str.CancelWrite(quic.StreamErrorCode(errorRequestCanceled))
+			return rerr
+		}
+	}
+	return nil
+}
+
 func (c *client) doRequest(req *http.Request, str quic.Stream, opt RoundTripOpt, reqDone chan struct{}) (*http.Response, requestError) {
 	var requestGzip bool
 	if !c.opts.DisableCompression && req.Method != "HEAD" && req.Header.Get("Accept-Encoding") == "" && req.Header.Get("Range") == "" {
 		requestGzip = true
 	}
-	if err := c.requestWriter.WriteRequest(str, req, opt.DontCloseRequestStream, requestGzip); err != nil {
+	if err := c.requestWriter.WriteRequestHeader(str, req, requestGzip); err != nil {
 		return nil, newStreamError(errorInternalError, err)
 	}
 
+	if req.Body == nil && !opt.DontCloseRequestStream {
+		str.Close()
+	}
+
+	hstr := newStream(str, func() { c.conn.CloseWithError(quic.ApplicationErrorCode(errorFrameUnexpected), "") })
+	if req.Body != nil {
+		// send the request body asynchronously
+		go func() {
+			if err := c.sendRequestBody(hstr, req.Body); err != nil {
+				c.logger.Errorf("Error writing request: %s", err)
+			}
+			if !opt.DontCloseRequestStream {
+				hstr.Close()
+			}
+		}()
+	}
+
 	frame, err := parseNextFrame(str, nil)
 	if err != nil {
 		return nil, newStreamError(errorFrameError, err)
@@ -348,9 +392,7 @@ func (c *client) doRequest(req *http.Request, str quic.Stream, opt RoundTripOpt,
 			res.Header.Add(hf.Name, hf.Value)
 		}
 	}
-	respBody := newResponseBody(str, c.conn, reqDone, func() {
-		c.conn.CloseWithError(quic.ApplicationErrorCode(errorFrameUnexpected), "")
-	})
+	respBody := newResponseBody(hstr, c.conn, reqDone)
 
 	// Rules for when to set Content-Length are defined in https://tools.ietf.org/html/rfc7230#section-3.3.2.
 	_, hasTransferEncoding := res.Header["Transfer-Encoding"]