var (
		cntNewline int64
		buf        bytes.Buffer
		blk        block
		totalSize  int
	)

	// feedTokens copies data in blocks from r into buf until there are
	// at least cnt newlines in buf. It will not read more blocks than needed.
	feedTokens := func(n int64) error {
		for cntNewline < n {
			totalSize += len(blk)
			if totalSize > maxSpecialFileSize {
				return errSparseTooLong