http3: don't write response headers if the handler panicked (#3950)

http3/server.go

@@ -594,7 +594,6 @@ func (s *Server) handleRequest(conn quic.Connection, str quic.Stream, decoder *q
 	ctx = context.WithValue(ctx, http.LocalAddrContextKey, conn.LocalAddr())
 	req = req.WithContext(ctx)
 	r := newResponseWriter(str, conn, s.logger)
-	defer r.Flush()
 	handler := s.Handler
 	if handler == nil {
 		handler = http.DefaultServeMux
@@ -622,10 +621,10 @@ func (s *Server) handleRequest(conn quic.Connection, str quic.Stream, decoder *q
 		return requestError{err: errHijacked}
 	}
 
-	if panicked {
-		r.WriteHeader(http.StatusInternalServerError)
-	} else {
+	// only write response when there is no panic
+	if !panicked {
 		r.WriteHeader(http.StatusOK)
+		r.Flush()
 	}
 	// If the EOF was read by the handler, CancelRead() is a no-op.
 	str.CancelRead(quic.StreamErrorCode(ErrCodeNoError))