nbuf      [chunk]byte     // next internal buffer
	fillReq   chan struct{}
	fillReady chan int64
	readerErr error // underlying reader error, if any
}

func newScanner(r io.Reader) *scanner {
	sr := &scanner{
		end:       maxInt,
		fillReq:   make(chan struct{}),
		fillReady: make(chan int64),
	}

	go func() {
		var rpos int64 // total bytes read into buffer

		defer func() {