[go-nuts] How many bytes binary.Write write before an error?

[pw2191195]

Hi all, I'm doing a simple message serialization, message defines like 
below:
type Envelope struct {
    Magic uint32
    DataSize uint32
}

type Message struct {
    Envelope
    Data []byte
}

and I want to implement the io.WriterTo interface for Message
func (m *Message) WriteTo(w io.Writer) (n int64, err error) {
    err = binary.Write(w, binary.BigEndian, &m.Envelope)
    if err != nil {
        // here, how should I return a proper n? Since w maybe *net.TCPConn 
and binary.Write maybe write 1 bytes and peer has dropped the connection.
        return 0, fmt.Errorf("Write message envelope error: %s", err)
    }

    // multiple lines skipped
}

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