Start plain response as soon as certain (#74)

* Start plain response as soon as it's certain

When we get the response headers it might be pretty
much already known whether we will have to serve plain
content instead of gzipping. That is either when
the content-length header is set with a small enough size,
there is already an encoding in the response or the
content-type is already set to a content we don't handle.

* Make Flush work also for non-gzipped responses

gzip.go

@@ -112,26 +112,26 @@ func (w *GzipResponseWriter) Write(b []byte) (int, error) {
  // On the first write, w.buf changes from nil to a valid slice
  w.buf = append(w.buf, b...)
 
- // If they provided a Content-Length, store that to check against minSize.
- var cl int
- if cls := w.Header().Get(contentLength); cls != "" {
- cl, _ = strconv.Atoi(cls)
- }
-
- // Only continue if they didn't already choose an encoding.
- if w.Header().Get(contentEncoding) == "" {
+ var (
+ cl, _ = strconv.Atoi(w.Header().Get(contentLength))
+ ct = w.Header().Get(contentType)
+ ce = w.Header().Get(contentEncoding)
+ )
+ // Only continue if they didn't already choose an encoding or a known unhandled content length or type.
+ if ce == "" && (cl == 0 || cl >= w.minSize) && (ct == "" || handleContentType(w.contentTypes, ct)) {
  // If the current buffer is less than minSize and a Content-Length isn't set, then wait until we have more data.
  if len(w.buf) < w.minSize && cl == 0 {
  return len(b), nil
  }
  // If the Content-Length is larger than minSize or the current buffer is larger than minSize, then continue.
  if cl >= w.minSize || len(w.buf) >= w.minSize {
  // If a Content-Type wasn't specified, infer it from the current buffer.
- if _, ok := w.Header()[contentType]; !ok {
- w.Header().Set(contentType, http.DetectContentType(w.buf))
+ if ct == "" {
+ ct = http.DetectContentType(w.buf)
+ w.Header().Set(contentType, ct)
  }
  // If the Content-Type is acceptable to GZIP, initialize the GZIP writer.
- if handleContentType(w.contentTypes, w) {
+ if handleContentType(w.contentTypes, ct) {
  if err := w.startGzip(); err != nil {
  return 0, err
  }
@@ -248,15 +248,17 @@ func (w *GzipResponseWriter) Close() error {
 // http.ResponseWriter if it is an http.Flusher. This makes GzipResponseWriter
 // an http.Flusher.
 func (w *GzipResponseWriter) Flush() {
- if w.gw == nil {
- // Only flush once startGzip has been called.
+ if w.gw == nil && !w.ignore {
+ // Only flush once startGzip or startPlain has been called.
  //
- // Flush is thus a no-op until the written body
- // exceeds minSize.
+ // Flush is thus a no-op until we're certain whether a plain
+ // or gzipped response will be served.
  return
  }
 
- w.gw.Flush()
+ if w.gw != nil {
+ w.gw.Flush()
+ }
 
  if fw, ok := w.ResponseWriter.(http.Flusher); ok {
  fw.Flush()
@@ -451,13 +453,12 @@ func acceptsGzip(r *http.Request) bool {
 }
 
 // returns true if we've been configured to compress the specific content type.
-func handleContentType(contentTypes []parsedContentType, w http.ResponseWriter) bool {
+func handleContentType(contentTypes []parsedContentType, ct string) bool {
  // If contentTypes is empty we handle all content types.
  if len(contentTypes) == 0 {
  return true
  }
 
- ct := w.Header().Get(contentType)
  mediaType, params, err := mime.ParseMediaType(ct)
  if err != nil {
  return false