net/http: be more conservative about enabling http2 on Transports

For now, don't enable http2 when Transport.TLSConfig != nil.
See background in #14275.

Also don't enable http2 when ExpectContinueTimeout is specified for
now, in case somebody depends on that functionality. (It is not yet
implemented in http2, and was only just added to net/http too in Go
1.6, so nobody would be setting it yet).

Updates #14275
Updates #13851

Change-Id: I192d555f5fb0a567bd89b6ad87175bbdd7891ae3
Reviewed-on: https://go-review.googlesource.com/19424Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport.go

@@ -163,6 +163,22 @@ func (t *Transport) onceSetNextProtoDefaults() {
 		return
 	}
 	if t.TLSNextProto != nil {
+		// This is the documented way to disable http2 on a
+		// 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.
+		return
+	}
+	if t.ExpectContinueTimeout != 0 {
+		// Unsupported in http2, so disable http2 for now.
+		// Issue 13851.
 		return
 	}
 	t2, err := http2configureTransport(t)

src/net/http/transport_test.go

@@ -2885,23 +2885,34 @@ 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.")
-	}
+	testTransportAutoHTTP(t, &Transport{}, true)
+}
 
-	// Now with TLSNextProto set:
-	tr = &Transport{TLSNextProto: make(map[string]func(string, *tls.Conn) RoundTripper)}
-	_, err = tr.RoundTrip(new(Request))
+func TestTransportAutomaticHTTP2_TLSNextProto(t *testing.T) {
+	testTransportAutoHTTP(t, &Transport{
+		TLSNextProto: make(map[string]func(string, *tls.Conn) RoundTripper),
+	}, false)
+}
+
+func TestTransportAutomaticHTTP2_TLSConfig(t *testing.T) {
+	testTransportAutoHTTP(t, &Transport{
+		TLSClientConfig: new(tls.Config),
+	}, false)
+}
+
+func TestTransportAutomaticHTTP2_ExpectContinueTimeout(t *testing.T) {
+	testTransportAutoHTTP(t, &Transport{
+		ExpectContinueTimeout: 1 * time.Second,
+	}, false)
+}
+
+func testTransportAutoHTTP(t *testing.T, tr *Transport, wantH2 bool) {
+	_, 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")
+	if reg := tr.TLSNextProto["h2"] != nil; reg != wantH2 {
+		t.Errorf("HTTP/2 registered = %v; want %v", reg, wantH2)
 	}
 }