check that QUIC layer negotiated datagram support when handing SETTINGS

http3/server.go

@@ -301,8 +301,19 @@ func (s *Server) handleUnidirectionalStreams(sess quic.EarlySession) {
 				sess.CloseWithError(quic.ErrorCode(errorFrameError), "")
 				return
 			}
-			if _, ok := f.(*settingsFrame); !ok {
+			sf, ok := f.(*settingsFrame)
+			if !ok {
 				sess.CloseWithError(quic.ErrorCode(errorMissingSettings), "")
+				return
+			}
+			if !sf.Datagram {
+				return
+			}
+			// If datagram support was enabled on our side as well as on the client side,
+			// we can expect it to have been negotiated both on the transport and on the HTTP/3 layer.
+			// Note: ConnectionState() will block until the handshake is complete (relevant when using 0-RTT).
+			if s.EnableDatagrams && !sess.ConnectionState().SupportsDatagrams {
+				sess.CloseWithError(quic.ErrorCode(errorSettingsError), "missing QUIC Datagram support")
 			}
 		}(str)
 	}