net/http/httputil: fix unannounced trailers when body is empty

Fix unannounced trailers when body is empty and without announced trailers.

Fixes #29031

Change-Id: If49951a42fe56d4be4436a999627db4c2678659d
GitHub-Last-Rev: 3469adc8f5fd977438350274134950687853a468
GitHub-Pull-Request: golang/go#29032
Reviewed-on: https://go-review.googlesource.com/c/151898
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 10 insertions, 8 deletions

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index f0607a68ea..5d07ba3d36 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -282,14 +282,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	rw.WriteHeader(res.StatusCode)
-	if len(res.Trailer) > 0 {
-		// Force chunking if we saw a response trailer.
-		// This prevents net/http from calculating the length for short
-		// bodies and adding a Content-Length.
-		if fl, ok := rw.(http.Flusher); ok {
-			fl.Flush()
-		}
-	}
+
 	err = p.copyResponse(rw, res.Body, p.flushInterval(req, res))
 	if err != nil {
 		defer res.Body.Close()
@@ -304,6 +297,15 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 	res.Body.Close() // close now, instead of defer, to populate res.Trailer
 
+	if len(res.Trailer) > 0 {
+		// Force chunking if we saw a response trailer.
+		// This prevents net/http from calculating the length for short
+		// bodies and adding a Content-Length.
+		if fl, ok := rw.(http.Flusher); ok {
+			fl.Flush()
+		}
+	}
+
 	if len(res.Trailer) == announcedTrailers {
 		copyHeader(rw.Header(), res.Trailer)
 		return