[net] http2: support SETTINGS_NO_RFC7540_PRIORITIES in SETTINGS frame

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Nicholas Husin (Gerrit)

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Dec 10, 2025, 3:43:11 PM (11 days ago) Dec 10

to Damien Neil, goph...@pubsubhelper.golang.org, golang-co...@googlegroups.com

Attention needed from Damien Neil

Nicholas Husin has uploaded the change for review

Nicholas Husin would like Damien Neil to review this change.

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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

Change diff

diff --git a/http2/http2.go b/http2/http2.go
index 105fe12..6320f4e 100644
--- a/http2/http2.go
+++ b/http2/http2.go
@@ -169,6 +169,7 @@
 	SettingMaxFrameSize          SettingID = 0x5
 	SettingMaxHeaderListSize     SettingID = 0x6
 	SettingEnableConnectProtocol SettingID = 0x8
+	SettingNoRFC7540Priorities   SettingID = 0x9
 )
 
 var settingName = map[SettingID]string{
@@ -179,6 +180,7 @@
 	SettingMaxFrameSize:          "MAX_FRAME_SIZE",
 	SettingMaxHeaderListSize:     "MAX_HEADER_LIST_SIZE",
 	SettingEnableConnectProtocol: "ENABLE_CONNECT_PROTOCOL",
+	SettingNoRFC7540Priorities:   "NO_RFC7540_PRIORITIES",
 }
 
 func (s SettingID) String() string {
diff --git a/http2/server.go b/http2/server.go
index d1fa038..25331bf 100644
--- a/http2/server.go
+++ b/http2/server.go
@@ -942,6 +942,9 @@
 	if !disableExtendedConnectProtocol {
 		settings = append(settings, Setting{SettingEnableConnectProtocol, 1})
 	}
+	if _, ok := sc.writeSched.(*priorityWriteSchedulerRFC7540); !ok {
+		settings = append(settings, Setting{SettingNoRFC7540Priorities, 1})
+	}
 	sc.writeFrame(FrameWriteRequest{
 		write: settings,
 	})
@@ -1809,6 +1812,10 @@
 	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
diff --git a/http2/server_test.go b/http2/server_test.go
index 5e1848f..e21f721 100644
--- a/http2/server_test.go
+++ b/http2/server_test.go
@@ -5152,6 +5152,68 @@
 	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) {

Change information

Files:

M http2/http2.go
M http2/server.go
M http2/server_test.go

Change size: M

Delta: 3 files changed, 71 insertions(+), 0 deletions(-)

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Damien Neil (Gerrit)

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Dec 19, 2025, 6:09:38 PM (2 days ago) Dec 19

to Nicholas Husin, goph...@pubsubhelper.golang.org, Go LUCI, golang-co...@googlegroups.com

Attention needed from Nicholas Husin

Damien Neil added 1 comment

File http2/server.go

Line 946, Patchset 1 (Latest): settings = append(settings, Setting{SettingNoRFC7540Priorities, 1})

Damien Neil . unresolved

User-provided write schedulers might handle RFC 7540 priorities.

I think this should be flipped: Send SettingNoRFC7540Priorities only if the write scheduler is one we know doesn't use that priority scheme.