// concurrently, check for it before we
				// write the filler byte.
				select {
				case err := <-doneCh:
					if err != nil {
						write([]byte{1})
						write([]byte(err.Error()))
					} else {
						write([]byte{0})
					}
					return
				default:
				}

				// Response not ready, write a filler byte.
				write([]byte{32})
				if canWrite {
					w.(http.Flusher).Flush()