http3: add a RoundTripOpt to check the server's SETTINGS frame (#4355)

For some requests, the client is required to check the server's HTTP/3
SETTINGS. For example, a client is only allowed to send HTTP/3 datagrams
if the server explicitly enabled support.

SETTINGS are sent asynchronously on a control stream (usually the first
unidirectional stream). This means that the SETTINGS might not be
available at the beginning of the connection. This is not expected to be
the common case, since the server can send the SETTINGS in 0.5-RTT data,
but we have to be able to deal with arbitrary delays.

For WebTransport, there are even more SETTINGS values that the client
needs to check. By making CheckSettings a callback on the RoundTripOpt,
this entire validation logic can live at the WebTransport layer.

http3/client.go

@@ -58,6 +58,9 @@ type client struct {
 	dialer       dialFunc
 	handshakeErr error
 
+	receivedSettings chan struct{} // closed once the server's SETTINGS frame was processed
+	settings         *Settings     // set once receivedSettings is closed
+
 	requestWriter *requestWriter
 
 	decoder *qpack.Decoder
@@ -107,14 +110,15 @@ func newClient(hostname string, tlsConf *tls.Config, opts *roundTripperOpts, con
 	tlsConf.NextProtos = []string{versionToALPN(conf.Versions[0])}
 
 	return &client{
-		hostname:      authorityAddr("https", hostname),
-		tlsConf:       tlsConf,
-		requestWriter: newRequestWriter(logger),
-		decoder:       qpack.NewDecoder(func(hf qpack.HeaderField) {}),
-		config:        conf,
-		opts:          opts,
-		dialer:        dialer,
-		logger:        logger,
+		hostname:         authorityAddr("https", hostname),
+		tlsConf:          tlsConf,
+		requestWriter:    newRequestWriter(logger),
+		receivedSettings: make(chan struct{}),
+		decoder:          qpack.NewDecoder(func(hf qpack.HeaderField) {}),
+		config:           conf,
+		opts:             opts,
+		dialer:           dialer,
+		logger:           logger,
 	}, nil
 }
 
@@ -234,6 +238,12 @@ func (c *client) handleUnidirectionalStreams(conn quic.EarlyConnection) {
 				conn.CloseWithError(quic.ApplicationErrorCode(ErrCodeMissingSettings), "")
 				return
 			}
+			c.settings = &Settings{
+				EnableDatagram:        sf.Datagram,
+				EnableExtendedConnect: sf.ExtendedConnect,
+				Other:                 sf.Other,
+			}
+			close(c.receivedSettings)
 			if !sf.Datagram {
 				return
 			}
@@ -299,6 +309,18 @@ func (c *client) roundTripOpt(req *http.Request, opt RoundTripOpt) (*http.Respon
 		}
 	}
 
+	if opt.CheckSettings != nil {
+		// wait for the server's SETTINGS frame to arrive
+		select {
+		case <-c.receivedSettings:
+		case <-conn.Context().Done():
+			return nil, context.Cause(conn.Context())
+		}
+		if err := opt.CheckSettings(*c.settings); err != nil {
+			return nil, err
+		}
+	}
+
 	str, err := conn.OpenStreamSync(req.Context())
 	if err != nil {
 		return nil, err