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
	}

	n, err = r.r.Read(buf[:need])
	if err != nil {
		r.lastErr = err
		return
	}
	r.m.updateMeasurement(r.opts.BucketOptions, uint64(tokens))
	return
}