Marshalling binary data to JSON results in corruption

encoding.go

package main

import (
	"bytes"
	"compress/gzip"
	"encoding/json"
	"io/ioutil"
)

const ec2_userdata string = `#cloud-config
bootcmd:
  - echo "hello world"
`

func check(e error) {
	if e != nil {
		panic(e)
	}
}

func main() {
	var buffer bytes.Buffer
	gzipWriter := gzip.NewWriter(&buffer)
	gzipWriter.Write([]byte(ec2_userdata))
	gzipWriter.Close()

	mapD := map[string]string{"gzip": buffer.String()}
	mapB, _ := json.Marshal(mapD)

	var err error
	var raw []byte

	err = ioutil.WriteFile("output.json", mapB, 0644)
	check(err)

	raw, err = ioutil.ReadFile("output.json")
	check(err)

	var unmarshalled map[string]interface{}
	err = json.Unmarshal([]byte(raw), &unmarshalled)
	check(err)

	gzip_in := unmarshalled["gzip"].(string)
	err = ioutil.WriteFile("unmarshalled.gz", []byte(gzip_in), 0644)
	check(err)
}

Try to gunzip the resulting unmarshalled.gz file, it won't work.

$ gunzip unmarshalled.gz
gunzip: unmarshalled.gz: not in gzip format