http2: support SETTINGS_NO_RFC7540_PRIORITIES in SETTINGS frame

To make sure that clients do not unnecessarily send RFC 7540 priority
signals when it would be treated as a no-op, this change makes it so
that our server always sends SETTINGS_NO_RFC7540_PRIORITIES in our
SETTINGS frame when our write scheduler is set to anything other than
the RFC 7540 write scheduler.

For golang/go#75500

Change-Id: I7a54251022087319999deda7efb663f8b251aa95
Reviewed-on: https://go-review.googlesource.com/c/net/+/729141
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>

http2/http2.go

@@ -169,6 +169,7 @@ const (
 	SettingMaxFrameSize          SettingID = 0x5
 	SettingMaxHeaderListSize     SettingID = 0x6
 	SettingEnableConnectProtocol SettingID = 0x8
+	SettingNoRFC7540Priorities   SettingID = 0x9
 )
 
 var settingName = map[SettingID]string{
@@ -179,6 +180,7 @@ var settingName = map[SettingID]string{
 	SettingMaxFrameSize:          "MAX_FRAME_SIZE",
 	SettingMaxHeaderListSize:     "MAX_HEADER_LIST_SIZE",
 	SettingEnableConnectProtocol: "ENABLE_CONNECT_PROTOCOL",
+	SettingNoRFC7540Priorities:   "NO_RFC7540_PRIORITIES",
 }
 
 func (s SettingID) String() string {

http2/server.go

@@ -955,6 +955,9 @@ func (sc *serverConn) serve(conf http2Config) {
 	if !disableExtendedConnectProtocol {
 		settings = append(settings, Setting{SettingEnableConnectProtocol, 1})
 	}
+	if sc.writeSchedIgnoresRFC7540() {
+		settings = append(settings, Setting{SettingNoRFC7540Priorities, 1})
+	}
 	sc.writeFrame(FrameWriteRequest{
 		write: settings,
 	})
@@ -1822,6 +1825,10 @@ func (sc *serverConn) processSetting(s Setting) error {
 	case SettingEnableConnectProtocol:
 		// Receipt of this parameter by a server does not
 		// have any impact
+	case SettingNoRFC7540Priorities:
+		if s.Val > 1 {
+			return ConnectionError(ErrCodeProtocol)
+		}
 	default:
 		// Unknown setting: "An endpoint that receives a SETTINGS
 		// frame with any unknown or unsupported identifier MUST

http2/server_test.go

@@ -5152,6 +5152,68 @@ func testServerSendDataAfterRequestBodyClose(t testing.TB) {
 	st.wantIdle()
 }
 
+func TestServerSettingNoRFC7540Priorities(t *testing.T) {
+	synctestTest(t, testServerSettingNoRFC7540Priorities)
+}
+func testServerSettingNoRFC7540Priorities(t testing.TB) {
+	tests := []struct {
+		ws                   func() WriteScheduler
+		wantNoRFC7540Setting bool
+	}{
+		{
+			ws: func() WriteScheduler {
+				return newPriorityWriteSchedulerRFC7540(nil)
+			},
+			wantNoRFC7540Setting: false,
+		},
+		{
+			ws:                   newPriorityWriteSchedulerRFC9218,
+			wantNoRFC7540Setting: true,
+		},
+		{
+			ws:                   NewRandomWriteScheduler,
+			wantNoRFC7540Setting: true,
+		},
+		{
+			ws:                   newRoundRobinWriteScheduler,
+			wantNoRFC7540Setting: true,
+		},
+	}
+	for _, tt := range tests {
+		st := newServerTester(t, nil, func(s *Server) {
+			s.NewWriteScheduler = tt.ws
+		})
+		defer st.Close()
+
+		var gotNoRFC7540Setting bool
+		st.greetAndCheckSettings(func(s Setting) error {
+			if s.ID != SettingNoRFC7540Priorities {
+				return nil
+			}
+			gotNoRFC7540Setting = s.Val == 1
+			return nil
+		})
+		if tt.wantNoRFC7540Setting != gotNoRFC7540Setting {
+			t.Errorf("want SETTINGS_NO_RFC7540_PRIORITIES to be %v, got %v", tt.wantNoRFC7540Setting, gotNoRFC7540Setting)
+		}
+	}
+}
+
+func TestServerSettingNoRFC7540PrioritiesInvalid(t *testing.T) {
+	synctestTest(t, testServerSettingNoRFC7540PrioritiesInvalid)
+}
+func testServerSettingNoRFC7540PrioritiesInvalid(t testing.TB) {
+	st := newServerTester(t, nil)
+	defer st.Close()
+
+	st.writePreface()
+	st.writeSettings(Setting{ID: SettingNoRFC7540Priorities, Val: 2})
+	synctest.Wait()
+	st.readFrame() // SETTINGS frame
+	st.readFrame() // WINDOW_UPDATE frame
+	st.wantGoAway(0, ErrCodeProtocol)
+}
+
 // This test documents current behavior, rather than ideal behavior that we
 // would necessarily like to see. Refer to go.dev/issues/75936 for details.
 func TestServerRFC7540PrioritySmallPayload(t *testing.T) {