Flush stream response to make it stream (#1364)

* update flush policy

* update response real time flush

* add comments

pkg/object/httpserver/mux.go

@@ -249,7 +249,7 @@ func (mi *muxInstance) sendResponse(ctx *context.Context, stdw http.ResponseWrit
 	stdw.WriteHeader(resp.StatusCode())
 
 	var writer io.Writer
-	if responseNeedFlush(resp) {
+	if responseIsRealTime(resp) {
 		writer = NewResponseFlushWriter(stdw)
 	} else {
 		writer = stdw
@@ -287,15 +287,25 @@ func NewResponseFlushWriter(w http.ResponseWriter) *ResponseFlushWriter {
 	}
 }
 
-func responseNeedFlush(resp *httpprot.Response) bool {
+// responseIsRealTime returns whether the response needs to be flushed immediately.
+// The response needs to be flushed immediately if the response has no content length (chunked),
+// or the response is a Server-Sent Events response.
+func responseIsRealTime(resp *httpprot.Response) bool {
+	// Based on https://en.wikipedia.org/wiki/Chunked_transfer_encoding
+	// If the Transfer-Encoding header field is present in a response and its value is "chunked",
+	// then the body of response is considered as a stream of chunks.
+	if len(resp.TransferEncoding) > 0 && resp.TransferEncoding[0] == "chunked" && resp.ContentLength <= 0 {
+		return true
+	}
+
 	resCTHeader := resp.Std().Header.Get("Content-Type")
 	resCT, _, err := mime.ParseMediaType(resCTHeader)
-
 	// For Server-Sent Events responses, flush immediately.
 	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
 	if err == nil && resCT == "text/event-stream" {
 		return true
 	}
+
 	return false
 }