sessionctx: add cluster-wide read-only status variable

pkg/domain/infosync/info.go

@@ -83,6 +83,11 @@ const (
 // ErrPrometheusAddrIsNotSet is the error that Prometheus address is not set in PD and etcd
 var ErrPrometheusAddrIsNotSet = dbterror.ClassDomain.NewStd(errno.ErrPrometheusAddrIsNotSet)
 
+func init() {
+	vardef.SetClusterReadOnlyChecker(getClusterReadOnlyStatus)
+	vardef.SetReadOnlyStatusReporter(updateServerReadOnlyStatus)
+}
+
 // InfoSyncer stores server info to etcd when the tidb-server starts and delete when tidb-server shuts down.
 type InfoSyncer struct {
 	// `etcdClient` must be used when keyspace is not set, or when the logic to each etcd path needs to be separated by keyspace.
@@ -343,6 +348,47 @@ func GetAllServerInfo(ctx context.Context) (map[string]*serverinfo.ServerInfo, e
 	return is.svrInfoSyncer.GetAllServerInfo(ctx)
 }
 
+// UpdateServerReadOnlyStatus updates the local TiDB read-only status in the info syncer.
+func UpdateServerReadOnlyStatus(ctx context.Context) error {
+	return updateServerReadOnlyStatus(ctx)
+}
+
+func updateServerReadOnlyStatus(ctx context.Context) error {
+	is, err := getGlobalInfoSyncer()
+	if err != nil {
+		return nil
+	}
+	return is.svrInfoSyncer.UpdateServerReadOnlyStatus(
+		ctx,
+		vardef.RestrictedReadOnly.Load(),
+		vardef.VarTiDBSuperReadOnly.Load(),
+	)
+}
+
+func getClusterReadOnlyStatus(ctx context.Context) (bool, error) {
+	is, err := getGlobalInfoSyncer()
+	if err != nil {
+		return vardef.LocalTiDBReadOnlyStatus(), nil
+	}
+	allServerInfo, err := is.svrInfoSyncer.GetAllServerInfo(ctx)
+	if err != nil {
+		return false, err
+	}
+	return clusterReadOnlyStatusFromServerInfo(allServerInfo), nil
+}
+
+func clusterReadOnlyStatusFromServerInfo(allServerInfo map[string]*serverinfo.ServerInfo) bool {
+	if len(allServerInfo) == 0 {
+		return false
+	}
+	for _, info := range allServerInfo {
+		if info == nil || !info.TiDBEffectiveReadOnly {
+			return false
+		}
+	}
+	return true
+}
+
 // UpdateServerLabel updates the server label for global info syncer.
 func UpdateServerLabel(ctx context.Context, labels map[string]string) error {
 	is, err := getGlobalInfoSyncer()

pkg/domain/infosync/info_test.go

@@ -26,6 +26,7 @@ import (
 	"github.com/pingcap/tidb/pkg/keyspace"
 	"github.com/pingcap/tidb/pkg/meta/model"
 	"github.com/pingcap/tidb/pkg/parser/ast"
+	"github.com/pingcap/tidb/pkg/sessionctx/vardef"
 	"github.com/pingcap/tidb/pkg/testkit/testsetup"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/goleak"
@@ -210,3 +211,59 @@ func TestInfoSyncerMarshal(t *testing.T) {
 	require.Equal(t, info.JSONServerID, decodeInfo.JSONServerID)
 	require.Equal(t, info.Labels, decodeInfo.Labels)
 }
+
+func TestClusterReadOnlyStatus(t *testing.T) {
+	ctx := context.Background()
+	_, err := GlobalInfoSyncerInit(ctx, "test-read-only", func() uint64 { return 1 }, nil, nil, nil, nil, keyspace.CodecV1, false, nil)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		vardef.RestrictedReadOnly.Store(false)
+		vardef.VarTiDBSuperReadOnly.Store(false)
+		require.NoError(t, UpdateServerReadOnlyStatus(context.Background()))
+	})
+
+	vardef.RestrictedReadOnly.Store(false)
+	vardef.VarTiDBSuperReadOnly.Store(false)
+	require.NoError(t, UpdateServerReadOnlyStatus(ctx))
+	on, err := vardef.GetClusterReadOnlyStatus(ctx)
+	require.NoError(t, err)
+	require.False(t, on)
+
+	vardef.VarTiDBSuperReadOnly.Store(true)
+	require.NoError(t, UpdateServerReadOnlyStatus(ctx))
+	on, err = vardef.GetClusterReadOnlyStatus(ctx)
+	require.NoError(t, err)
+	require.True(t, on)
+
+	for _, tt := range []struct {
+		name   string
+		infos  map[string]*serverinfo.ServerInfo
+		expect bool
+	}{
+		{
+			name: "all effective read-only",
+			infos: map[string]*serverinfo.ServerInfo{
+				"tidb-a": {DynamicInfo: serverinfo.DynamicInfo{TiDBSuperReadOnly: true, TiDBEffectiveReadOnly: true}},
+				"tidb-b": {DynamicInfo: serverinfo.DynamicInfo{TiDBRestrictedReadOnly: true, TiDBSuperReadOnly: true, TiDBEffectiveReadOnly: true}},
+			},
+			expect: true,
+		},
+		{
+			name: "one instance is writable",
+			infos: map[string]*serverinfo.ServerInfo{
+				"tidb-a": {DynamicInfo: serverinfo.DynamicInfo{TiDBSuperReadOnly: true, TiDBEffectiveReadOnly: true}},
+				"tidb-b": {DynamicInfo: serverinfo.DynamicInfo{}},
+			},
+			expect: false,
+		},
+		{
+			name:   "no live instance",
+			infos:  map[string]*serverinfo.ServerInfo{},
+			expect: false,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.expect, clusterReadOnlyStatusFromServerInfo(tt.infos))
+		})
+	}
+}

pkg/domain/serverinfo/info.go

@@ -88,13 +88,19 @@ func (i *StaticInfo) IsAssumed() bool {
 // To update the dynamic server information, use `InfoSyncer.cloneDynamicServerInfo` to obtain a copy of the dynamic server info.
 // After making modifications, use `InfoSyncer.setDynamicServerInfo` to update the dynamic server information.
 type DynamicInfo struct {
-	Labels map[string]string `json:"labels"`
+	Labels                 map[string]string `json:"labels"`
+	TiDBRestrictedReadOnly bool              `json:"tidb_restricted_read_only,omitempty"`
+	TiDBSuperReadOnly      bool              `json:"tidb_super_read_only,omitempty"`
+	TiDBEffectiveReadOnly  bool              `json:"tidb_effective_read_only,omitempty"`
 }
 
 // Clone the DynamicInfo.
 func (d *DynamicInfo) Clone() *DynamicInfo {
 	return &DynamicInfo{
-		Labels: maps.Clone(d.Labels),
+		Labels:                 maps.Clone(d.Labels),
+		TiDBRestrictedReadOnly: d.TiDBRestrictedReadOnly,
+		TiDBSuperReadOnly:      d.TiDBSuperReadOnly,
+		TiDBEffectiveReadOnly:  d.TiDBEffectiveReadOnly,
 	}
 }
 

pkg/domain/serverinfo/syncer.go

@@ -180,6 +180,39 @@ func (s *Syncer) UpdateServerLabel(ctx context.Context, labels map[string]string
 	return nil
 }
 
+// UpdateServerReadOnlyStatus updates the local server read-only status in etcd.
+func (s *Syncer) UpdateServerReadOnlyStatus(ctx context.Context, restricted, super bool) error {
+	dynamicInfo := s.cloneDynamicServerInfo()
+	effective := restricted || super
+	if dynamicInfo.TiDBRestrictedReadOnly == restricted &&
+		dynamicInfo.TiDBSuperReadOnly == super &&
+		dynamicInfo.TiDBEffectiveReadOnly == effective {
+		return nil
+	}
+
+	dynamicInfo.TiDBRestrictedReadOnly = restricted
+	dynamicInfo.TiDBSuperReadOnly = super
+	dynamicInfo.TiDBEffectiveReadOnly = effective
+	if s.etcdCli == nil || s.session == nil {
+		s.setDynamicServerInfo(dynamicInfo)
+		return nil
+	}
+
+	info := s.GetLocalServerInfo().Clone()
+	info.DynamicInfo = *dynamicInfo
+	infoBuf, err := info.Marshal()
+	if err != nil {
+		return errors.Trace(err)
+	}
+	str := string(hack.String(infoBuf))
+	err = util.PutKVToEtcd(ctx, s.etcdCli, KeyOpDefaultRetryCnt, s.serverInfoPath, str, clientv3.WithLease(s.session.Lease()))
+	if err != nil {
+		return err
+	}
+	s.setDynamicServerInfo(dynamicInfo)
+	return nil
+}
+
 // cloneDynamicServerInfo returns a clone of the dynamic server info.
 func (s *Syncer) cloneDynamicServerInfo() *DynamicInfo {
 	return s.info.Load().DynamicInfo.Clone()
@@ -205,7 +238,7 @@ func (s *Syncer) GetAllServerInfo(ctx context.Context) (map[string]*ServerInfo,
 	allInfo := make(map[string]*ServerInfo)
 	if s.etcdCli == nil {
 		info := s.info.Load()
-		allInfo[info.ID] = getServerInfo(info.ID, info.ServerIDGetter, "")
+		allInfo[info.ID] = info
 		return allInfo, nil
 	}
 	allInfo, err := getInfo(ctx, s.etcdCli, ServerInformationPath, KeyOpDefaultRetryCnt, KeyOpDefaultTimeout, clientv3.WithPrefix())

pkg/sessionctx/vardef/readonly.go

@@ -0,0 +1,64 @@
+// Copyright 2026 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package vardef
+
+import (
+	"context"
+	"sync/atomic"
+)
+
+// ClusterReadOnlyChecker returns whether all live TiDB instances are effectively read-only.
+type ClusterReadOnlyChecker func(context.Context) (bool, error)
+
+// ReadOnlyStatusReporter publishes the local TiDB read-only status.
+type ReadOnlyStatusReporter func(context.Context) error
+
+var (
+	clusterReadOnlyChecker atomic.Value
+	readOnlyStatusReporter atomic.Value
+)
+
+// LocalTiDBReadOnlyStatus returns the local TiDB effective read-only status.
+func LocalTiDBReadOnlyStatus() bool {
+	return RestrictedReadOnly.Load() || VarTiDBSuperReadOnly.Load()
+}
+
+// SetClusterReadOnlyChecker registers the checker used by the derived tidb_is_read_only variable.
+func SetClusterReadOnlyChecker(checker ClusterReadOnlyChecker) {
+	clusterReadOnlyChecker.Store(checker)
+}
+
+// GetClusterReadOnlyStatus returns whether the whole TiDB cluster is effectively read-only.
+func GetClusterReadOnlyStatus(ctx context.Context) (bool, error) {
+	checker, ok := clusterReadOnlyChecker.Load().(ClusterReadOnlyChecker)
+	if !ok || checker == nil {
+		return LocalTiDBReadOnlyStatus(), nil
+	}
+	return checker(ctx)
+}
+
+// SetReadOnlyStatusReporter registers a reporter for the local TiDB read-only status.
+func SetReadOnlyStatusReporter(reporter ReadOnlyStatusReporter) {
+	readOnlyStatusReporter.Store(reporter)
+}
+
+// ReportReadOnlyStatus publishes the local TiDB read-only status when a reporter is available.
+func ReportReadOnlyStatus(ctx context.Context) error {
+	reporter, ok := readOnlyStatusReporter.Load().(ReadOnlyStatusReporter)
+	if !ok || reporter == nil {
+		return nil
+	}
+	return reporter(ctx)
+}

pkg/sessionctx/vardef/tidb_vars.go

@@ -780,6 +780,9 @@ const (
 	// TiDBSuperReadOnly is tidb's variant of mysql's super_read_only, which has some differences from mysql's super_read_only.
 	TiDBSuperReadOnly = "tidb_super_read_only"
 
+	// TiDBIsReadOnly is a read-only derived status that indicates whether all TiDB instances are effectively read-only.
+	TiDBIsReadOnly = "tidb_is_read_only"
+
 	// TiDBShardAllocateStep indicates the max size of continuous rowid shard in one transaction.
 	TiDBShardAllocateStep = "tidb_shard_allocate_step"
 	// TiDBEnableTelemetry indicates that whether usage data report to PingCAP is enabled.

pkg/sessionctx/variable/sysvar.go

@@ -81,6 +81,12 @@
 	return func(sv *SysVar) { sv.MinValue = minVal }
 }
 
+func reportTiDBReadOnlyStatus(ctx context.Context) {
+	if err := vardef.ReportReadOnlyStatus(ctx); err != nil {
+		logutil.BgLogger().Warn("update TiDB read-only status failed", zap.Error(err))
+	}
+}
+
 // newExecConcurrencySysVar creates a session/global SysVar for executor concurrency settings.
 func newExecConcurrencySysVar(name string, defValue int, setter concurrencySetter, opts ...execConcurrencySysVarOption) *SysVar {
 	sv := &SysVar{
@@ -971,7 +977,7 @@
 			tikvstore.TxnCommitBatchSize.Store(uint64(TidbOptInt64(val, int64(tikvstore.DefTxnCommitBatchSize))))
 			return nil
 		}},
-	{Scope: vardef.ScopeGlobal, Name: vardef.TiDBRestrictedReadOnly, Value: BoolToOnOff(vardef.DefTiDBRestrictedReadOnly), Type: vardef.TypeBool, SetGlobal: func(_ context.Context, s *SessionVars, val string) error {
+	{Scope: vardef.ScopeGlobal, Name: vardef.TiDBRestrictedReadOnly, Value: BoolToOnOff(vardef.DefTiDBRestrictedReadOnly), Type: vardef.TypeBool, SetGlobal: func(ctx context.Context, s *SessionVars, val string) error {
 		on := TiDBOptOn(val)
 		// For user initiated SET GLOBAL, also change the value of TiDBSuperReadOnly
 		if on && s.StmtCtx.StmtType == "Set" {
@@ -985,6 +991,7 @@
 			}
 		}
 		vardef.RestrictedReadOnly.Store(on)
+		reportTiDBReadOnlyStatus(ctx)
 		return nil
 	}},
 	{Scope: vardef.ScopeGlobal, Name: vardef.TiDBSuperReadOnly, Value: BoolToOnOff(vardef.DefTiDBSuperReadOnly), Type: vardef.TypeBool, Validation: func(s *SessionVars, normalizedValue string, _ string, _ vardef.ScopeFlag) (string, error) {
@@ -999,10 +1006,18 @@
 			}
 		}
 		return normalizedValue, nil
-	}, SetGlobal: func(_ context.Context, s *SessionVars, val string) error {
+	}, SetGlobal: func(ctx context.Context, s *SessionVars, val string) error {
 		vardef.VarTiDBSuperReadOnly.Store(TiDBOptOn(val))
+		reportTiDBReadOnlyStatus(ctx)
 		return nil
 	}},
+	{Scope: vardef.ScopeGlobal, Name: vardef.TiDBIsReadOnly, Value: vardef.Off, Type: vardef.TypeBool, ReadOnly: true, GetGlobal: func(ctx context.Context, s *SessionVars) (string, error) {
+		on, err := vardef.GetClusterReadOnlyStatus(ctx)
+		if err != nil {
+			return vardef.Off, err
+		}
+		return BoolToOnOff(on), nil
+	}},
 	{Scope: vardef.ScopeGlobal, Name: vardef.TiDBEnableGOGCTuner, Value: BoolToOnOff(vardef.DefTiDBEnableGOGCTuner), Type: vardef.TypeBool, SetGlobal: func(_ context.Context, s *SessionVars, val string) error {
 		on := TiDBOptOn(val)
 		gctuner.EnableGOGCTuner.Store(on)

pkg/sessionctx/variable/sysvar_test.go

@@ -445,6 +445,40 @@ func TestReadOnlyNoop(t *testing.T) {
 	}
 }
 
+func TestTiDBIsReadOnly(t *testing.T) {
+	vars := NewSessionVars(nil)
+	sv := GetSysVar(vardef.TiDBIsReadOnly)
+	require.NotNil(t, sv)
+	require.True(t, sv.ReadOnly)
+	require.True(t, sv.HasGlobalScope())
+
+	t.Cleanup(func() {
+		vardef.RestrictedReadOnly.Store(false)
+		vardef.VarTiDBSuperReadOnly.Store(false)
+	})
+
+	_, err := sv.Validate(vars, vardef.On, vardef.ScopeGlobal)
+	require.Error(t, err)
+	require.Equal(t, "[variable:1238]Variable 'tidb_is_read_only' is a read only variable", err.Error())
+
+	vardef.RestrictedReadOnly.Store(false)
+	vardef.VarTiDBSuperReadOnly.Store(false)
+	val, err := sv.GetGlobalFromHook(context.Background(), vars)
+	require.NoError(t, err)
+	require.Equal(t, vardef.Off, val)
+
+	vardef.VarTiDBSuperReadOnly.Store(true)
+	val, err = sv.GetGlobalFromHook(context.Background(), vars)
+	require.NoError(t, err)
+	require.Equal(t, vardef.On, val)
+
+	vardef.VarTiDBSuperReadOnly.Store(false)
+	vardef.RestrictedReadOnly.Store(true)
+	val, err = sv.GetGlobalFromHook(context.Background(), vars)
+	require.NoError(t, err)
+	require.Equal(t, vardef.On, val)
+}
+
 func TestSkipInit(t *testing.T) {
 	sv := SysVar{Scope: vardef.ScopeGlobal, Name: "skipinit1", Value: vardef.On, Type: vardef.TypeBool}
 	require.True(t, sv.SkipInit())