net/http/httptest: add EnableHTTP2 to Server

Adds a knob EnableHTTP2, that enables an unstarted
Server and its respective client to speak HTTP/2,
but only after StartTLS has been invoked.

Fixes #34939

Change-Id: I287c568b8708a4d3c03e7d9eca7c323b8f4c65b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/201557
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httptest/example_test.go

@@ -55,6 +55,28 @@ func ExampleServer() {
 	// Output: Hello, client
 }
 
+func ExampleServer_hTTP2() {
+	ts := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintf(w, "Hello, %s", r.Proto)
+	}))
+	ts.EnableHTTP2 = true
+	ts.StartTLS()
+	defer ts.Close()
+
+	res, err := ts.Client().Get(ts.URL)
+	if err != nil {
+		log.Fatal(err)
+	}
+	greeting, err := ioutil.ReadAll(res.Body)
+	res.Body.Close()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%s", greeting)
+
+	// Output: Hello, HTTP/2.0
+}
+
 func ExampleNewTLSServer() {
 	ts := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		fmt.Fprintln(w, "Hello, client")

src/net/http/httptest/server.go

@@ -27,6 +27,11 @@ type Server struct {
 	URL      string // base URL of form http://ipaddr:port with no trailing slash
 	Listener net.Listener
 
+	// EnableHTTP2 controls whether HTTP/2 is enabled
+	// on the server. It must be set between calling
+	// NewUnstartedServer and calling Server.StartTLS.
+	EnableHTTP2 bool
+
 	// TLS is the optional TLS configuration, populated with a new config
 	// after TLS is started. If set on an unstarted server before StartTLS
 	// is called, existing fields are copied into the new config.
@@ -151,7 +156,11 @@ func (s *Server) StartTLS() {
 		s.TLS = new(tls.Config)
 	}
 	if s.TLS.NextProtos == nil {
-		s.TLS.NextProtos = []string{"http/1.1"}
+		nextProtos := []string{"http/1.1"}
+		if s.EnableHTTP2 {
+			nextProtos = []string{"h2"}
+		}
+		s.TLS.NextProtos = nextProtos
 	}
 	if len(s.TLS.Certificates) == 0 {
 		s.TLS.Certificates = []tls.Certificate{cert}
@@ -166,6 +175,7 @@ func (s *Server) StartTLS() {
 		TLSClientConfig: &tls.Config{
 			RootCAs: certpool,
 		},
+		ForceAttemptHTTP2: s.EnableHTTP2,
 	}
 	s.Listener = tls.NewListener(s.Listener, s.TLS)
 	s.URL = "https://" + s.Listener.Addr().String()

src/net/http/httptest/server_test.go

@@ -202,3 +202,39 @@ func TestServerZeroValueClose(t *testing.T) {
 
 	ts.Close() // tests that it doesn't panic
 }
+
+func TestTLSServerWithHTTP2(t *testing.T) {
+	modes := []struct {
+		name      string
+		wantProto string
+	}{
+		{"http1", "HTTP/1.1"},
+		{"http2", "HTTP/2.0"},
+	}
+
+	for _, tt := range modes {
+		t.Run(tt.name, func(t *testing.T) {
+			cst := NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.Header().Set("X-Proto", r.Proto)
+			}))
+
+			switch tt.name {
+			case "http2":
+				cst.EnableHTTP2 = true
+				cst.StartTLS()
+			default:
+				cst.Start()
+			}
+
+			defer cst.Close()
+
+			res, err := cst.Client().Get(cst.URL)
+			if err != nil {
+				t.Fatalf("Failed to make request: %v", err)
+			}
+			if g, w := res.Header.Get("X-Proto"), tt.wantProto; g != w {
+				t.Fatalf("X-Proto header mismatch:\n\tgot:  %q\n\twant: %q", g, w)
+			}
+		})
+	}
+}