How to get data from http.ResponseWriter for logging

I don't know if this question makes any sense, but I was wondering If there is any way to get the data which is written in http.ResponseWriter. I need it for logging.

I have written an API in Go.

func api1(w http.ResponseWriter, req *http.Request) {    	
	var requestData MyStruct
	err := json.NewDecoder(req.Body).Decode(&requestData)
	if err != nil {
		writeError(w, "JSON request is not in correct format")
		return
	}
	log.Println(" Request Data :", req.Body) // I am logging req
	result, err := compute()                 // getting result from a function

	if err != nil {
		errRes := ErrorResponse{"ERROR", err}
		response, er = json.Marshal(errRes) // getting error response
	} else {
		response, er = json.Marshal(result)
	}
	if er != nil {
		http.Error(w, er.Error(), 500) // writing error
		return
	}
	io.WriteString(w, string(response)) // writing response
}

The aim is to create a single log with request and response data. The response can be either an error response or processed response.

I was thinking if I could get data which is written on http.ResponseWriter then, I can create single meaningful log.

Is this possible? If not, please suggest how can I achieve this.

You could use io.MultiWriter - it creates a writer that duplicates its writes to all the provided writers. So to log the reponse

func api1(w http.ResponseWriter, req *http.Request) {
    var log bytes.Buffer
    rsp := io.MultiWriter(w, &log)
    // from this point on use rsp instead of w, ie
    err := json.NewDecoder(req.Body).Decode(&requestData)
    if err != nil {
        writeError(rsp, "JSON request is not in correct format")
        return
    }
    ...
}

Now you have duplicate of the info written into rsp in both w and log and you can save the content of the log buffer onto disc of show it on console etc.

You can use io.TeeReader to create a Reader that writes to given Writer what it reads from given reader - this would allow you to save copy of the req.Body into log, ie

func api1(w http.ResponseWriter, req *http.Request) {
    var log bytes.Buffer
    tee := io.TeeReader(req.Body, &log)
    err := json.NewDecoder(tee).Decode(&requestData)
    ...
}

Now since json decoder reads form tee the content of the req.Body is also copied into the log buffer.

Adding to accepted answer.

I love using io.MultiWriter, however it only writes the response body in this context.

If you want the headers, use the .Headers().Write function like this...

func (w http.ResponseWriter, req *http.Request) {
    var log bytes.Buffer
    rsp := io.MultiWriter(w, &log)
    // from this point on use rsp instead of w, ie

    // get the response headers
    w.Header().Write(&log)

    err := json.NewDecoder(req.Body).Decode(&requestData)
    if err != nil {
        writeError(rsp, "JSON request is not in correct format")
        return
    }
    ...
}