json.Decoding a TCP connection and logging the original data at the same time in Go

Question

I'm using a json.Decoder to decode a net.Conn into a custom struct. I would like to also log the original JSON string sent on the net.Conn without modifying it.
I am looking for an efficient way to achieve this. I currently have the following code:
reader := bufio.NewReader(conn)   // conn is a net.Conn
dec := json.NewDecoder(reader)

for {

// How can I also log the original data before decoding it into the struct?

var dst customStruct
    if err = dec.Decode(&dst); err == io.EOF {
        break
    } else if err != nil {
        log.Errorf("failed to decode message: %s", err)
        break
    }

// do something with dst...

}

Flimcy · Answer

Use iotest.NewReadLogger to log the data as the data is read:
prefix := fmt.Sprintf("%p:", conn)
reader := bufio.NewReader(iotest.NewReadLogger(prefix, conn)) 
dec := json.NewDecoder(reader)

for {

var dst customStruct
    if err = dec.Decode(&dst); err == io.EOF {
        break
    } else if err != nil {
        log.Errorf("failed to decode message: %s", err)
        break
    }

// do something with dst...

}

If the read logger output does not match your taste, then copy the implementation and adjust as needed. The implementation is very simple.

poWar · Answer

You can use ioutil.ReadAll() and then print and decode from the bytes. But since you are reading all of the bytes without using a buffer, you should ideally put this behind a debug flag check.
bs,err := ioutil.ReadAll()
if err != nil {
    panic(err)
}

log.Printf("output: %s", string(bs))

var dst customStruct
if err := json.Unmarshal(bs, &dst); err != nil {
    panic(err)
}

json.Decoding a TCP connection and logging the original data at the same time in Go

2 Answers

Add your own answers!

Ask a Question