net/http/httputil: pass through any "TE: trailers" header to backend

Fixes #21096

Change-Id: I2a4688a79bdaa25b4e8ef38e3390d93d3d0bce04
Reviewed-on: https://go-review.googlesource.com/115135
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 13 insertions, 2 deletions

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index 80ee22895a..d5d0a505f7 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -178,9 +178,20 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	// important is "Connection" because we want a persistent
 	// connection, regardless of what the client sent to us.
 	for _, h := range hopHeaders {
-		if outreq.Header.Get(h) != "" {
-			outreq.Header.Del(h)
+		hv := outreq.Header.Get(h)
+		if hv == "" {
+			continue
 		}
+		if h == "Te" && hv == "trailers" {
+			// Issue 21096: tell backend applications that
+			// care about trailer support that we support
+			// trailers. (We do, but we don't go out of
+			// our way to advertise that unless the
+			// incoming client request thought it was
+			// worth mentioning)
+			continue
+		}
+		outreq.Header.Del(h)
 	}
 
 	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {