rewrite content length within a middleware - go

Question

below code rewrites the http body response of some queries.
However, it fails to update the "content length" header field, it always remains the same original value.
How can i update the content length header field of the http response ?
type writeReplacer struct {
http.ResponseWriter
search []byte
replace func(*http.Request) string
buf []byte
r *http.Request
dir string
}
func (w *writeReplacer) Write(in []byte) (int, error) {
if w.buf == nil {
w.buf = []byte{}
}
w.buf = append(w.buf, in...)
n := len(in)
if index := bytes.LastIndex(w.buf, w.search); index > -1 {
var r []byte
if w.dir == "before" {
g := []byte(w.replace(w.r))
n += len(g)
r = append(g, w.buf[index:]...)
w.buf = append(w.buf[:index], r...)
} else {
g := []byte(w.replace(w.r))
n += len(g)
r = append(r, w.buf[:index+len(w.search)]...)
r = append(r, g...)
r = append(r, w.buf[index:]...)
w.buf = r
}
}
return n, nil
}
func (w *writeReplacer) Flush() {
w.ResponseWriter.Header().Set("Content-Length", fmt.Sprint(len(w.buf)))
w.ResponseWriter.Write(w.buf[:])
w.buf = w.buf[:0]
}
func InsertAfter(h http.Handler, path string, search []byte, replace func(*http.Request) string) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
if r.URL.Path == path {
w.Header().Del("Content-length")
w = &writeReplacer{ResponseWriter: w, search: search, replace: replace, r: r, dir: "after"}
defer w.(http.Flusher).Flush()
}
h.ServeHTTP(w, r)
})
}
func InsertBefore(h http.Handler, path string, search []byte, replace func(*http.Request) string) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
if r.URL.Path == path {
w.Header().Del("Content-length")
w = &writeReplacer{ResponseWriter: w, search: search, replace: replace, r: r, dir: "before"}
defer w.(http.Flusher).Flush()
}
h.ServeHTTP(w, r)
})
}
I receive two errors message, the first one from nginx
2019/03/08 05:58:37 [error] 31194#0: *19
upstream prematurely closed connection while reading upstream,
client: 82.21.18.16, server: buycoffee.online, request:
"GET / HTTP/1.1", upstream: "http://127.0.0.1:8081/", host: "buycoffee.online"
the second one from curl
curl: (18) transfer closed with 6237 bytes remaining to read

Answer 1

as mkopriva suggested the trick was to rewrite ResponseWriter.WriteHeader.
I believe it has to do with the fact that write might call writeheader at first call, and from there i was not putting the instructions at the right place.
in order to prevent further difficulties i prefer to use chunked transfer.
the code change is:
func (w *writeReplacer) WriteHeader(statusCode int) {
w.Header().Del("Content-length")
w.Header().Set("Transfer-Encoding", "chunked")
w.ResponseWriter.WriteHeader(statusCode)
}