net/http/httputil: fix missing previous headers in response when switching protocol in ReverseProxy

When using switching protocol, previous headers set before the reverse proxy are lost.

Fixes #29407

Change-Id: Ia2b9784022d9bccef8625519ccbabbe8a276dfc0
GitHub-Last-Rev: 79bb493dcbb9b76d9d2ff9cd0854b29d634f8b73
GitHub-Pull-Request: golang/go#29408
Reviewed-on: https://go-review.googlesource.com/c/155741Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -497,6 +497,9 @@ func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.R
 		p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch protocol %q when %q was requested", resUpType, reqUpType))
 		return
 	}
+
+	copyHeader(res.Header, rw.Header())
+
 	hj, ok := rw.(http.Hijacker)
 	if !ok {
 		p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))

src/net/http/httputil/reverseproxy_test.go

@@ -1013,7 +1013,12 @@ func TestReverseProxyWebSocket(t *testing.T) {
 	rproxy := NewSingleHostReverseProxy(backURL)
 	rproxy.ErrorLog = log.New(ioutil.Discard, "", 0) // quiet for tests
 
-	frontendProxy := httptest.NewServer(rproxy)
+	handler := http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		rw.Header().Set("X-Header", "X-Value")
+		rproxy.ServeHTTP(rw, req)
+	})
+
+	frontendProxy := httptest.NewServer(handler)
 	defer frontendProxy.Close()
 
 	req, _ := http.NewRequest("GET", frontendProxy.URL, nil)
@@ -1028,6 +1033,13 @@ func TestReverseProxyWebSocket(t *testing.T) {
 	if res.StatusCode != 101 {
 		t.Fatalf("status = %v; want 101", res.Status)
 	}
+
+	got := res.Header.Get("X-Header")
+	want := "X-Value"
+	if got != want {
+		t.Errorf("Header(XHeader) = %q; want %q", got, want)
+	}
+
 	if upgradeType(res.Header) != "websocket" {
 		t.Fatalf("not websocket upgrade; got %#v", res.Header)
 	}
@@ -1042,8 +1054,8 @@ func TestReverseProxyWebSocket(t *testing.T) {
 	if !bs.Scan() {
 		t.Fatalf("Scan: %v", bs.Err())
 	}
-	got := bs.Text()
-	want := `backend got "Hello"`
+	got = bs.Text()
+	want = `backend got "Hello"`
 	if got != want {
 		t.Errorf("got %#q, want %#q", got, want)
 	}