Convert from and to messagepack

Question

So I'm trying to get the string representation of a JSON message in Golang. I just want to receive the messagepack encoded JSON, modify some values and send it back.

I haven't found an easy way to do it. Most of the times, I can't know in advance what is the structure of the JSON (apart from having JSON structure), so want I want to do is to receive the binary message. Decode it as a JSON, print it as a string to the standard output, modify the content, convert it to MessagePack again and send it back.

I've been looking at these two packages, but I don't know how to properly use them for a simple task like that:

• https://godoc.org/github.com/vmihailenco/msgpack
• https://godoc.org/github.com/ugorji/go/codec

So I will receive something like this:

DF 00 00 00 01 A7 6D 65 73 73 61 67 65 A3 48 69 21

I want to print this:

{"message": "Hi!"}

Modify the "Hi!":

{"message": "Hello Sir!"}

Send it as messagepack:

DF 00 00 00 01 A7 6D 65 73 73 61 67 65 AA 48 65 6C 6C 6F 20 53 69 72 21

Current Python code I'm trying to port to Golang:

def decode_msgpack(jsonData):
    packedStuff = 0
    for key in jsonData.keys():
        if type(jsonData[key]) is bytes:
            packedStuff += 1
            try:
                jsonData[key] = umsgpack.unpackb(jsonData[key])
            except umsgpack.InvalidStringException:
                try:
                    jsonData[key] = umsgpack.unpackb(jsonData[key], allow_invalid_utf8=True)
                except umsgpack.InsufficientDataException:
                    print("[!] InsufficientDataException")
                    jsonData[key] = base64.b64encode(jsonData[key]).decode('utf-8')
                else:
                    jsonData[key] = base64.b64encode(jsonData[key]).decode('utf-8')

    if packedStuff > 0:
        return decode_msgpack(jsonData)
    else:
        return jsonData

Answer 1

Using the codec library and assuming that {"message": "Hi"} is a map, the code would look something like this.

package main

import (
        "fmt"

        "github.com/ugorji/go/codec"
)

func main() {
        var data []byte
        original := map[string]string{"message": "Hi!"}
        enc := codec.NewEncoderBytes(&data, new(codec.MsgpackHandle))
        if err := enc.Encode(&original); err != nil {
                panic(err)
        }
        fmt.Printf("Encoded: ")
        for _, b := range data {
                fmt.Printf("%X ", b)
        }
        fmt.Printf("\n")
        decoded := make(map[string]string)
        dec := codec.NewDecoderBytes(data, new(codec.MsgpackHandle))
        if err := dec.Decode(&decoded); err != nil {
                panic(err)
        }
        fmt.Printf("Decoded: %v\n", decoded)
        decoded["message"] = "Hello Sir!"
        /* reinitialize the encoder */
        enc = codec.NewEncoderBytes(&data, new(codec.MsgpackHandle))
        if err := enc.Encode(&decoded); err != nil {
                panic(err)
        }
        fmt.Printf("Encoded: ")
        for _, b := range data {
                fmt.Printf("%X ", b)
        }
        fmt.Printf("\n")
}

That said, if you get {"message": "Hi"} as a JSON string, you can use codec to decode the JSON into a map, update the map and then re-encode it as msgpack.

Answer 2

The best way is to first decode it, make your changes via Go structs and then re-encode it.

data := []byte(`{"message": "Hi!"}`)
var p map[string]interface{}

// Decode into Struct
if err := json.Unmarshal(data, &p); err != nil {
    // TODO: handle err
}

// Modify contents
p["message"] = "Hello Sir!"

// Encode from struct
newData, err := json.Marshal(p)
if err != nil {
    // TODO: Handle err
}

fmt.Println(string(newData))