// hence n may be less than len(p).
// To read exactly len(p) bytes, use io.ReadFull(b, p).
// If the underlying [Reader] can return a non-zero count with io.EOF,
// then this Read method can do so as well; see the [io.Reader] docs.
func (b *Reader) Read(p []byte) (n int, err error) {
	n = len(p)
	if n == 0 {
		if b.Buffered() > 0 {
			return 0, nil
		}
		return 0, b.readErr()
	}
	if b.r == b.w {