In HTTP response for a chunked data how to set Content-Length

Question

We have written a service which sends some encoded data as chunked to a proxy service which need Content-Length header to be set so that it can send proper response to end point. Even if I set the Content-Length header still it get stripped as part of the response to the client. Below is the code which set the header

func HTTPSuccessResponse(rw http.ResponseWriter, bufferLen int, media []byte) {
        rw.WriteHeader(http.StatusOK)

        rw.Header().Set("Content-Type", "opus/ogg; audio/ogg; codec=opus")
        length := strconv.Itoa(len(media));
        rw.Header().Set("Content-Length", length)
        rw.Write(media)
}

Below is the response I get when I try to the request using curl

bash-4.2# curl -v -X GET -k -H  -i 'http://127.0.0.1:8090/preview'
* About to connect() to 127.0.0.1 port 8090 (#0)
*   Trying 127.0.0.1...
* Connected to 127.0.0.1 (127.0.0.1) port 8090 (#0)
> GET /preview HTTP/1.1
> User-Agent: curl/7.29.0
> Host: 127.0.0.1:8090
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Tue, 14 May 2019 13:08:20 GMT
< Content-Type: text/plain; charset=utf-8
< Transfer-Encoding: chunked
<

I am using Gorrila Mux library for setting up HTTP server. Any thoughts how to make the header as part of the response.

Answer 1

Remove the WriteHeader call at the top. You can only write headers to a response once. After you call WriteHeader you can't set any more headers.

Per the ResponseWriter documentation:

    // Changing the header map after a call to WriteHeader (or
    // Write) has no effect unless the modified headers are
    // trailers.

So you can't call it first; but you also don't need to call it at all - from the same docs:

    // If WriteHeader is not called explicitly, the first call to Write
    // will trigger an implicit WriteHeader(http.StatusOK).
    // Thus explicit calls to WriteHeader are mainly used to
    // send error codes.