net/http/httputil: flush ReverseProxy immediately if Content-Length is -1

Finish up a prior TODO by making ReverseProxy flush immediately
if Content-Length is -1, which is a case that can occur if for
example we have a streamed response, or chunked encoding, or when
the body's length wasn't known.

Fixes #41642

Change-Id: I30babaaf3e14837b99e3ecdc562a0a0e50c579bf
GitHub-Last-Rev: efc019a9fe361082d40bee77317018c3b80451a3
GitHub-Pull-Request: golang/go#41858
Reviewed-on: https://go-review.googlesource.com/c/go/+/260637
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

1 files changed, 10 insertions, 6 deletions

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index 3f48fab544..46e5f68a84 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -58,9 +58,9 @@ type ReverseProxy struct {
 	// A negative value means to flush immediately
 	// after each write to the client.
 	// The FlushInterval is ignored when ReverseProxy
-	// recognizes a response as a streaming response;
-	// for such responses, writes are flushed to the client
-	// immediately.
+	// recognizes a response as a streaming response, or
+	// if its ContentLength is -1; for such responses, writes
+	// are flushed to the client immediately.
 	FlushInterval time.Duration
 
 	// ErrorLog specifies an optional logger for errors
@@ -325,7 +325,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 	rw.WriteHeader(res.StatusCode)
 
-	err = p.copyResponse(rw, res.Body, p.flushInterval(req, res))
+	err = p.copyResponse(rw, res.Body, p.flushInterval(res))
 	if err != nil {
 		defer res.Body.Close()
 		// Since we're streaming the response, if we run into an error all we can do
@@ -397,7 +397,7 @@ func removeConnectionHeaders(h http.Header) {
 
 // flushInterval returns the p.FlushInterval value, conditionally
 // overriding its value for a specific request/response.
-func (p *ReverseProxy) flushInterval(req *http.Request, res *http.Response) time.Duration {
+func (p *ReverseProxy) flushInterval(res *http.Response) time.Duration {
 	resCT := res.Header.Get("Content-Type")
 
 	// For Server-Sent Events responses, flush immediately.
@@ -406,7 +406,11 @@ func (p *ReverseProxy) flushInterval(req *http.Request, res *http.Response) time
 		return -1 // negative means immediately
 	}
 
-	// TODO: more specific cases? e.g. res.ContentLength == -1?
+	// We might have the case of streaming for which Content-Length might be unset.
+	if res.ContentLength == -1 {
+		return -1
+	}
+
 	return p.FlushInterval
 }