net/http: introduce DialerAndTLSConfigSupportsHTTP2 in Transport

Even when a custom TLS config or custom dialer is specified,
enables HTTP/2 if DialerAndTLSConfigSupportsHTTP2 is true.
By this change, avoid automatically enabling HTTP/2 if DialContext is set.

This change also ensures that DefaultTransport still automatically
enable HTTP/2 as discussed in #14391.

Updates #14391
Fixes #27011

Change-Id: Icc46416810bee61dbd65ebc96468335030b80573
Reviewed-on: https://go-review.googlesource.com/c/go/+/130256Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport.go

@@ -46,6 +46,7 @@ var DefaultTransport RoundTripper = &Transport{
 		KeepAlive: 30 * time.Second,
 		DualStack: true,
 	}).DialContext,
+	DialerAndTLSConfigSupportsHTTP2: true,
 	MaxIdleConns:                    100,
 	IdleConnTimeout:                 90 * time.Second,
 	TLSHandshakeTimeout:             10 * time.Second,
@@ -257,6 +258,12 @@ type Transport struct {
 	// h2transport (via onceSetNextProtoDefaults)
 	nextProtoOnce sync.Once
 	h2transport   h2Transport // non-nil if http2 wired up
+
+	// DialerAndTLSConfigSupportsHTTP2 controls whether HTTP/2 is enabled when a non-zero
+	// TLSClientConfig or Dial, DialTLS or DialContext func is provided. By default, use of any those fields conservatively
+	// disables HTTP/2. To use a customer dialer or TLS config and still attempt HTTP/2
+	// upgrades, set this to true.
+	DialerAndTLSConfigSupportsHTTP2 bool
 }
 
 // h2Transport is the interface we expect to be able to call from
@@ -296,12 +303,13 @@ func (t *Transport) onceSetNextProtoDefaults() {
 		// Transport.
 		return
 	}
-	if t.TLSClientConfig != nil || t.Dial != nil || t.DialTLS != nil {
+	if !t.DialerAndTLSConfigSupportsHTTP2 && (t.TLSClientConfig != nil || t.Dial != nil || t.DialTLS != nil || t.DialContext != nil) {
 		// Be conservative and don't automatically enable
 		// http2 if they've specified a custom TLS config or
 		// custom dialers. Let them opt-in themselves via
 		// http2.ConfigureTransport so we don't surprise them
 		// by modifying their tls.Config. Issue 14275.
+		// However, if DialerAndTLSConfigSupportsHTTP2 is true, it overrides the above checks.
 		return
 	}
 	t2, err := http2configureTransport(t)

src/net/http/transport_test.go

@@ -3593,6 +3593,13 @@ func TestTransportAutomaticHTTP2(t *testing.T) {
 	testTransportAutoHTTP(t, &Transport{}, true)
 }
 
+func TestTransportAutomaticHTTP2_DialerAndTLSConfigSupportsHTTP2AndTLSConfig(t *testing.T) {
+	testTransportAutoHTTP(t, &Transport{
+		DialerAndTLSConfigSupportsHTTP2: true,
+		TLSClientConfig:                 new(tls.Config),
+	}, true)
+}
+
 // golang.org/issue/14391: also check DefaultTransport
 func TestTransportAutomaticHTTP2_DefaultTransport(t *testing.T) {
 	testTransportAutoHTTP(t, DefaultTransport.(*Transport), true)
@@ -3623,6 +3630,13 @@ func TestTransportAutomaticHTTP2_Dial(t *testing.T) {
 	}, false)
 }
 
+func TestTransportAutomaticHTTP2_DialContext(t *testing.T) {
+	var d net.Dialer
+	testTransportAutoHTTP(t, &Transport{
+		DialContext: d.DialContext,
+	}, false)
+}
+
 func TestTransportAutomaticHTTP2_DialTLS(t *testing.T) {
 	testTransportAutoHTTP(t, &Transport{
 		DialTLS: func(network, addr string) (net.Conn, error) {