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

Fixes #21096

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

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 {

src/net/http/httputil/reverseproxy_test.go

@@ -49,6 +49,9 @@ func TestReverseProxy(t *testing.T) {
 		if c := r.Header.Get("Connection"); c != "" {
 			t.Errorf("handler got Connection header value %q", c)
 		}
+		if c := r.Header.Get("Te"); c != "trailers" {
+			t.Errorf("handler got Te header value %q; want 'trailers'", c)
+		}
 		if c := r.Header.Get("Upgrade"); c != "" {
 			t.Errorf("handler got Upgrade header value %q", c)
 		}
@@ -85,6 +88,7 @@ func TestReverseProxy(t *testing.T) {
 	getReq, _ := http.NewRequest("GET", frontend.URL, nil)
 	getReq.Host = "some-name"
 	getReq.Header.Set("Connection", "close")
+	getReq.Header.Set("Te", "trailers")
 	getReq.Header.Set("Proxy-Connection", "should be deleted")
 	getReq.Header.Set("Upgrade", "foo")
 	getReq.Close = true