Method writeCompressedMessage

lib/protocol/protocol.go:829–871

Write msg out compressed, given its uncompressed marshaled payload. The first return value indicates whether compression succeeded. If not, the caller should retry without compression.

(msg proto.Message, marshaled []byte)

// The first return value indicates whether compression succeeded.
// If not, the caller should retry without compression.
func (c *rawConnection) writeCompressedMessage(msg proto.Message, marshaled []byte) (ok bool, err error) {
	hdr := &bep.Header{
		Type:        typeOf(msg),
		Compression: bep.MessageCompression_MESSAGE_COMPRESSION_LZ4,
	}
	hdrSize := proto.Size(hdr)
	if hdrSize > 1<<16-1 {
		panic("impossibly large header")
	}

	cOverhead := 2 + hdrSize + 4

	metricDeviceSentUncompressedBytes.WithLabelValues(c.idString).Add(float64(cOverhead + len(marshaled)))

	// The compressed size may be at most n-n/32 = .96875*n bytes,
	// I.e., if we can't save at least 3.125% bandwidth, we forgo compression.
	// This number is arbitrary but cheap to compute.
	maxCompressed := cOverhead + len(marshaled) - len(marshaled)/32
	buf := BufferPool.Get(maxCompressed)
	defer BufferPool.Put(buf)

	compressedSize, err := lz4Compress(marshaled, buf[cOverhead:])
	totSize := compressedSize + cOverhead
	if err != nil {
		return false, nil
	}

	// Header length
	binary.BigEndian.PutUint16(buf, uint16(hdrSize))
	// Header
	if _, err := protoutil.MarshalTo(buf[2:], hdr); err != nil {
		return true, fmt.Errorf("marshalling header: %w", err)
	}
	// Message length
	binary.BigEndian.PutUint32(buf[2+hdrSize:], uint32(compressedSize))

	n, err := c.cw.Write(buf[:totSize])
	l.Debugf("wrote %d bytes on the wire (2 bytes length, %d bytes header, 4 bytes message length, %d bytes message (%d uncompressed)), err=%v", n, hdrSize, compressedSize, len(marshaled), err)
	if err != nil {
		return true, fmt.Errorf("writing message: %w", err)
	}
	return true, nil
}

func typeOf(msg proto.Message) bep.MessageType {
	switch msg.(type) {