net/http/httputil: remove custom hop-by-hop headers from response in ReverseProxy

Hop-by-hop headers (explicitly mentioned in RFC 2616) were already
removed from the response. This removes the custom hop-by-hop
headers listed in the "Connection" header of the response.

Updates #16875

Change-Id: I6b8f261d38b8d72040722f3ded29755ef0303427
Reviewed-on: https://go-review.googlesource.com/28810Reviewed-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

@@ -156,7 +156,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	// copied above) so we only copy it if necessary.
 	copiedHeaders := false
 
-	// Remove headers with the same name as the connection-tokens.
+	// Remove hop-by-hop headers listed in the "Connection" header.
 	// See RFC 2616, section 14.10.
 	if c := outreq.Header.Get("Connection"); c != "" {
 		for _, f := range strings.Split(c, ",") {
@@ -202,6 +202,16 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		return
 	}
 
+	// Remove hop-by-hop headers listed in the
+	// "Connection" header of the response.
+	if c := res.Header.Get("Connection"); c != "" {
+		for _, f := range strings.Split(c, ",") {
+			if f = strings.TrimSpace(f); f != "" {
+				res.Header.Del(f)
+			}
+		}
+	}
+
 	for _, h := range hopHeaders {
 		res.Header.Del(h)
 	}

src/net/http/httputil/reverseproxy_test.go

@@ -148,6 +148,9 @@ func TestReverseProxyStripHeadersPresentInConnection(t *testing.T) {
 		if c := r.Header.Get("Upgrade"); c != "" {
 			t.Errorf("handler got header %q = %q; want empty", "Upgrade", c)
 		}
+		w.Header().Set("Connection", "Upgrade, "+fakeConnectionToken)
+		w.Header().Set("Upgrade", "should be deleted")
+		w.Header().Set(fakeConnectionToken, "should be deleted")
 		io.WriteString(w, backendResponse)
 	}))
 	defer backend.Close()
@@ -180,6 +183,12 @@ func TestReverseProxyStripHeadersPresentInConnection(t *testing.T) {
 	if got, want := string(bodyBytes), backendResponse; got != want {
 		t.Errorf("got body %q; want %q", got, want)
 	}
+	if c := res.Header.Get("Upgrade"); c != "" {
+		t.Errorf("handler got header %q = %q; want empty", "Upgrade", c)
+	}
+	if c := res.Header.Get(fakeConnectionToken); c != "" {
+		t.Errorf("handler got header %q = %q; want empty", fakeConnectionToken, c)
+	}
 }
 
 func TestXForwardedFor(t *testing.T) {