go-error-handling.go

func (b *Bus) readAndUnpack() ([]byte, error) {
	n, err := io.ReadAtLeast(b.port, b.buf[:], 2)
	got := b.buf[:n]
	if err != nil {
		return nil, newError("libzzz: cannot read 2-byte preamble [got: % 02X] - error: %w", got, err)
	}
	if got[0] != magic_number {
		return nil, newError("libzzz: bad MAGIC NUMBER in response - message starts with: [% 02X] (expected XX...)", got)
	}
	length := int(got[1])
	if length < 5 {
		return nil, newError("libzzz: response too short: len=%d < 5 [% 02X]", length, got)
	}

	// Now that we know the total length, we can read the remaining bytes of the response
	if n < length {
		n, err = io.ReadAtLeast(b.port, b.buf[n:], length-n)
		if err != nil {
			return nil, newError("libzzz: cannot read remaining bytes of a packet [prefix=% 02X][rest=% 02X] - error: %w",
				got, b.buf[len(got):][:n], err)
		}
		got = b.buf[:len(got)+n]
	}
	crc := crc(got[:length-2])
	if crc != [2]byte{got[length-2], got[length-1]} {
		return nil, newError("libzzz: bad CRC [% 02X], expected [...% 02X]", got, crc[:])
	}

	payload := append([]byte(nil), got[2:length-2]...)
	return payload, nil
}