http3: don't automatically set RoundTripper.QuicConfig.EnableDatagrams (

quic-go#4340)

If the user provides a quic.Config, we shouldn't modify it. Instead, we
should return an error if the user enables HTTP Datagrams but fails to
enable datagrams on the QUIC layer.

http3/client.go

@@ -73,6 +73,10 @@ var _ roundTripCloser = &client{}
 func newClient(hostname string, tlsConf *tls.Config, opts *roundTripperOpts, conf *quic.Config, dialer dialFunc) (roundTripCloser, error) {
 	if conf == nil {
 		conf = defaultQuicConfig.Clone()
+		conf.EnableDatagrams = opts.EnableDatagram
+	}
+	if opts.EnableDatagram && !conf.EnableDatagrams {
+		return nil, errors.New("HTTP Datagrams enabled, but QUIC Datagrams disabled")
 	}
 	if len(conf.Versions) == 0 {
 		conf = conf.Clone()
@@ -84,7 +88,6 @@ func newClient(hostname string, tlsConf *tls.Config, opts *roundTripperOpts, con
 	if conf.MaxIncomingStreams == 0 {
 		conf.MaxIncomingStreams = -1 // don't allow any bidirectional streams
 	}
-	conf.EnableDatagrams = opts.EnableDatagram
 	logger := utils.DefaultLogger.WithPrefix("h3 client")
 
 	if tlsConf == nil {

http3/roundtrip.go

@@ -50,9 +50,8 @@ type RoundTripper struct {
 	// If nil, reasonable default values will be used.
 	QuicConfig *quic.Config
 
-	// Enable support for HTTP/3 datagrams.
-	// If set to true, QuicConfig.EnableDatagram will be set.
-	// See https://datatracker.ietf.org/doc/html/rfc9297.
+	// Enable support for HTTP/3 datagrams (RFC 9297).
+	// If a QuicConfig is set, datagram support also needs to be enabled on the QUIC layer by setting EnableDatagrams.
 	EnableDatagrams bool
 
 	// Additional HTTP/3 settings.

http3/roundtrip_test.go

@@ -99,6 +99,19 @@ var _ = Describe("RoundTripper", func() {
 			Expect(receivedConfig.HandshakeIdleTimeout).To(Equal(config.HandshakeIdleTimeout))
 		})
 
+		It("requires quic.Config.EnableDatagram if HTTP datagrams are enabled", func() {
+			rt.QuicConfig = &quic.Config{EnableDatagrams: false}
+			rt.Dial = func(_ context.Context, _ string, _ *tls.Config, config *quic.Config) (quic.EarlyConnection, error) {
+				return nil, errors.New("handshake error")
+			}
+			rt.EnableDatagrams = true
+			_, err := rt.RoundTrip(req)
+			Expect(err).To(MatchError("HTTP Datagrams enabled, but QUIC Datagrams disabled"))
+			rt.QuicConfig.EnableDatagrams = true
+			_, err = rt.RoundTrip(req)
+			Expect(err).To(MatchError("handshake error"))
+		})
+
 		It("uses the custom dialer, if provided", func() {
 			var dialed bool
 			dialer := func(_ context.Context, _ string, tlsCfgP *tls.Config, cfg *quic.Config) (quic.EarlyConnection, error) {