net/http: further restrict when Transport's automatic HTTP/2 happens

Make the temporary, conservative restrictions from rev 79d9f48c in Go
1.6 permanent, and also don't do automatic TLS if the user configured
a Dial or DialTLS hook. (Go 1.7 has Transport.Dialer instead, for
tweaking dialing parameters)

Fixes #14275

Change-Id: I5550d5c1e3a293e103eb4251a3685dc204a23941
Reviewed-on: https://go-review.googlesource.com/23222Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport.go

@@ -195,13 +195,12 @@ func (t *Transport) onceSetNextProtoDefaults() {
 		// Transport.
 		return
 	}
-	if t.TLSClientConfig != nil {
-		// Be conservative for now (for Go 1.6) at least and
-		// don't automatically enable http2 if they've
-		// specified a custom TLS config. Let them opt-in
-		// themselves via http2.ConfigureTransport so we don't
-		// surprise them by modifying their tls.Config.
-		// Issue 14275.
+	if t.TLSClientConfig != nil || t.Dial != nil || t.DialTLS != 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.
 		return
 	}
 	if t.ExpectContinueTimeout != 0 && t != DefaultTransport {

src/net/http/transport_test.go

@@ -2986,6 +2986,21 @@ func TestTransportAutomaticHTTP2_ExpectContinueTimeout(t *testing.T) {
 	}, false)
 }
 
+func TestTransportAutomaticHTTP2_Dial(t *testing.T) {
+	var d net.Dialer
+	testTransportAutoHTTP(t, &Transport{
+		Dial: d.Dial,
+	}, false)
+}
+
+func TestTransportAutomaticHTTP2_DialTLS(t *testing.T) {
+	testTransportAutoHTTP(t, &Transport{
+		DialTLS: func(network, addr string) (net.Conn, error) {
+			panic("unused")
+		},
+	}, false)
+}
+
 func testTransportAutoHTTP(t *testing.T, tr *Transport, wantH2 bool) {
 	_, err := tr.RoundTrip(new(Request))
 	if err == nil {