code: func (b *ByteBuf) PeekByte(offset int) (byte, error) {
  	if b.Readable() < offset || offset < 0 {
  		return 0, io.ErrShortBuffer
  	}
  
  	return b.buf[b.readerIndex+offset], nil
  }
docs: // PeekByte get byte value from buf based on currently read index, after read, read index not modifed