wal/encoder.go
func newFileEncoder(f *os.File, prevCrc uint32) (*encoder, error) {
	offset, err := f.Seek(0, io.SeekCurrent)
	if err != nil {
		return nil, err
	}
	return newEncoder(f, prevCrc, int(offset)), nil
}
func newFileEncoder(f *os.File, prevCrc uint32) (*encoder, error) {
	offset, err := f.Seek(0, io.SeekCurrent)
	if err != nil {
		return nil, err
	}
	return newEncoder(f, prevCrc, int(offset)), nil
}
// newFileEncoder creates a new encoder with current file offset for the page writer.