Optimize for a fast flush

mtglib/internal/relay/sync_pair.go

@@ -26,6 +26,7 @@ func (s *syncPair) Sync() (int64, error) {
 func (s *syncPair) Read(p []byte) (int, error) {
 	n, err := s.readBlocking(p, false)
 
+	// nothing has been delivered for readTimeout time. Let's flush.
 	if errors.Is(err, os.ErrDeadlineExceeded) {
 		if err := s.Flush(); err != nil {
 			return 0, fmt.Errorf("cannot flush writer hand-side: %w", err)
@@ -41,7 +42,17 @@ func (s *syncPair) Write(p []byte) (int, error) {
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 
-	return s.writer.Write(p) // nolint: wrapcheck
+	n, err := s.writer.Write(p) // nolint: wrapcheck
+
+	// optimization for a case when we have a small package and want to avoid a
+	// delay in readTimeout. In that case, we assume that peer has finished to
+	// sent a data it wants to send so we can flush without waiting for anything
+	// else.
+	if err == nil && n < copyBufferSize {
+		err = s.writer.Flush()
+	}
+
+	return n, err
 }
 
 func (s *syncPair) Flush() error {