net/http: enable HTTP/2 on all Transports, not just the DefaultTransport

This mirrors the same behavior and API from the server code to the
client side: if TLSNextProto is nil, HTTP/2 is on by default for
both. If it's non-nil, the user was trying to do something fancy and
step out of their way.

Updates #6891

Change-Id: Ia31808b71f336a8d5b44b985591d72113429e1d4
Reviewed-on: https://go-review.googlesource.com/17300Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport.go

@@ -40,15 +40,6 @@ var DefaultTransport RoundTripper = &Transport{
 	ExpectContinueTimeout: 1 * time.Second,
 }
 
-func init() {
-	if !strings.Contains(os.Getenv("GODEBUG"), "h2client=0") {
-		err := http2ConfigureTransport(DefaultTransport.(*Transport))
-		if err != nil {
-			panic(err)
-		}
-	}
-}
-
 // DefaultMaxIdleConnsPerHost is the default value of Transport's
 // MaxIdleConnsPerHost.
 const DefaultMaxIdleConnsPerHost = 2
@@ -138,12 +129,30 @@ type Transport struct {
 	// called with the request's authority (such as "example.com"
 	// or "example.com:1234") and the TLS connection. The function
 	// must return a RoundTripper that then handles the request.
+	// If TLSNextProto is nil, HTTP/2 support is enabled automatically.
 	TLSNextProto map[string]func(authority string, c *tls.Conn) RoundTripper
 
+	nextProtoOnce sync.Once // guards initialization of TLSNextProto (onceSetNextProtoDefaults)
+
 	// TODO: tunable on global max cached connections
 	// TODO: tunable on timeout on cached connections
 }
 
+// onceSetNextProtoDefaults initializes TLSNextProto.
+// It must be called via t.nextProtoOnce.Do.
+func (t *Transport) onceSetNextProtoDefaults() {
+	if strings.Contains(os.Getenv("GODEBUG"), "h2client=0") {
+		return
+	}
+	if t.TLSNextProto != nil {
+		return
+	}
+	err := http2ConfigureTransport(t)
+	if err != nil {
+		log.Printf("Error enabling Transport HTTP/2 support: %v", err)
+	}
+}
+
 // ProxyFromEnvironment returns the URL of the proxy to use for a
 // given request, as indicated by the environment variables
 // HTTP_PROXY, HTTPS_PROXY and NO_PROXY (or the lowercase versions
@@ -216,6 +225,7 @@ func (tr *transportRequest) extraHeaders() Header {
 // For higher-level HTTP client support (such as handling of cookies
 // and redirects), see Get, Post, and the Client type.
 func (t *Transport) RoundTrip(req *Request) (*Response, error) {
+	t.nextProtoOnce.Do(t.onceSetNextProtoDefaults)
 	if req.URL == nil {
 		req.closeBody()
 		return nil, errors.New("http: nil Request.URL")

src/net/http/transport_test.go

@@ -2921,6 +2921,27 @@ func TestTransportPrefersResponseOverWriteError(t *testing.T) {
 	}
 }
 
+func TestTransportAutomaticHTTP2(t *testing.T) {
+	tr := &Transport{}
+	_, err := tr.RoundTrip(new(Request))
+	if err == nil {
+		t.Error("expected error from RoundTrip")
+	}
+	if tr.TLSNextProto["h2"] == nil {
+		t.Errorf("HTTP/2 not registered.")
+	}
+
+	// Now with TLSNextProto set:
+	tr = &Transport{TLSNextProto: make(map[string]func(string, *tls.Conn) RoundTripper)}
+	_, err = tr.RoundTrip(new(Request))
+	if err == nil {
+		t.Error("expected error from RoundTrip")
+	}
+	if tr.TLSNextProto["h2"] != nil {
+		t.Errorf("HTTP/2 registered, despite non-nil TLSNextProto field")
+	}
+}
+
 func wantBody(res *Response, err error, want string) error {
 	if err != nil {
 		return err