tokens = need + hdr

		} else { // part of header can be accommodated
			r.opts.HeaderSize -= b - 1
			need = 1 // to ensure we read at least one byte for every Read
			tokens = b
		}
	} else { // all tokens go towards payload
		need = int(math.Min(float64(b), float64(need)))
		tokens = need
	}

	err = r.throttle.WaitN(r.ctx, tokens)
	if err != nil {
		return

	dst = append(dst, in...)
	return dst, err
}

// csvSplitSize is the size of each block.
// Blocks will read this much and find the first following newline.
// 128KB appears to be a very reasonable default.
const csvSplitSize = 128 << 10

// startReaders will read the header if needed and spin up a parser
// and a number of workers based on GOMAXPROCS.