Golang http.Response gzip writer ERR_CONTENT_LENGTH_MISMATCH

Question

I'm trying to gzip the proxied response from httputil.ReverseProxy -> ModifyResponse. So I only have access to the http.Response object.

res.Body = ioutil.NopCloser(bytes.NewReader(minified))
res.ContentLength = int64(len(minified))
res.Header.Set("Content-Length", strconv.Itoa(len(minified)))
res.Header.Del("Content-Encoding")

This works just fine. But when I gzip the content I'll get a content-length-mismatch error.

var buf bytes.Buffer
gz := gzip.NewWriter(&buf)
gz.Write(minified)

readCloser := ioutil.NopCloser(&buf)
res.Body = readCloser

res.ContentLength = int64(buf.Len())
res.Header.Set("Content-Length", strconv.Itoa(buf.Len()))
res.Header.Set("Content-Encoding", "gzip")

Can anyone tell what i'm doing wrong? The content-length is always 10 even when the input changes.

Answer 1

You're not closing your gz writer. It's possible issue. gzip.Writer documentation says:

It is the caller's responsibility to call Close on the WriteCloser when done. Writes may be buffered and not flushed until Close.

So, try to add gz.Close() after you've completed writing data.