Slowly responding Go net/http.Server endpoint for testing proxy behavior during shutdown, or connection scalability.

server.go

func slow(w http.ResponseWriter, r *http.Request) {
	// The slow return of this function is to take 5 minutes.
	// We shall return ~1MB total. and use american english dictionary for fun.
	f, err := os.Open("/usr/share/dict/words")
	if err != nil {
		log.Print("couldn't open /usr/share/dict/words")
		return
	}
	defer f.Close()
	t := 2 * time.Minute //default to 1m
	d := r.Form.Get("duration")
	if d != `` {
		if t2, err := time.ParseDuration(d); err == nil {
			t = t2
		} else {
			log.Print("couldn't parse query parameter", d, err)
		}
	}

	delay := 2 * time.Second
	st, err := f.Stat()
	if err != nil {
		log.Print("couldn't stat /usr/share/dict/words")
		return
	}
	src, dst := f, w
	sz := int(st.Size())
	chunk := sz / int(t/delay)
	log.Printf("/slow writing %d every %s for %s", chunk, delay, t)
	w.Header().Set("content-length", strconv.Itoa(sz))
	buf := make([]byte, chunk)
	// lifted from io:
	for {
		nr, er := src.Read(buf)
		if nr > 0 {
			nw, ew := dst.Write(buf[0:nr])
			if nw < 0 || nr < nw {
				nw = 0
				if ew == nil {
					ew = errInvalidWrite
				}
			}

			time.Sleep(delay)
			if ew != nil {
				err = ew
				break
			}
			if nr != nw {
				err = io.ErrShortWrite
				break
			}
		}
		if er != nil {
			if er != io.EOF {
				err = er
			}
			break
		}
	}
	if err != nil {
		log.Printf("/slow error writing %s", err)
	}
}