mdl: always enable and make it read only in nextgen (pingcap#62865)

D3Hunter · web-flow · commit 2d07f5c7167e · 2025-08-07T06:11:21.000Z
ref pingcap#61702
diff --git a/pkg/ddl/ddl.go b/pkg/ddl/ddl.go
@@ -1137,7 +1137,7 @@ func (d *ddl) cleanDeadTableLock(unlockTables []model.TableLockTpInfo, se model.
 
 // SwitchMDL enables MDL or disable MDL.
 func (d *ddl) SwitchMDL(enable bool) error {
-	isEnableBefore := vardef.EnableMDL.Load()
+	isEnableBefore := vardef.IsMDLEnabled()
 	if isEnableBefore == enable {
 		return nil
 	}
@@ -1161,7 +1161,7 @@ func (d *ddl) SwitchMDL(enable bool) error {
 		return errors.New("please wait for all jobs done")
 	}
 
-	vardef.EnableMDL.Store(enable)
+	vardef.SetEnableMDL(enable)
 	err = kv.RunInNewTxn(kv.WithInternalSourceType(context.Background(), kv.InternalTxnDDL), d.store, true, func(_ context.Context, txn kv.Transaction) error {
 		m := meta.NewMutator(txn)
 		oldEnable, _, err := m.GetMetadataLock()
@@ -1185,7 +1185,7 @@ func (d *ddl) SwitchMDL(enable bool) error {
 // It should be called before any DDL that could break data consistency.
 // This provides a safe window for async commit and 1PC to commit with an old schema.
 func delayForAsyncCommit() {
-	if vardef.EnableMDL.Load() {
+	if vardef.IsMDLEnabled() {
 		// If metadata lock is enabled. The transaction of DDL must begin after
 		// pre-write of the async commit transaction, then the commit ts of DDL
 		// must be greater than the async commit transaction. In this case, the
diff --git a/pkg/ddl/job_scheduler.go b/pkg/ddl/job_scheduler.go
@@ -566,7 +566,7 @@ func (s *jobScheduler) transitOneJobStepAndWaitSync(wk *worker, jobCtx *jobConte
 	// current owner.
 	job := jobW.Job
 	if jobCtx.isUnSynced(job.ID) || (job.Started() && !jobCtx.maybeAlreadyRunOnce(job.ID)) {
-		if vardef.EnableMDL.Load() {
+		if vardef.IsMDLEnabled() {
 			version, err := s.sysTblMgr.GetMDLVer(s.schCtx, job.ID)
 			if err == nil {
 				jobCtx.logger.Info("the job have schema version un-synced",
@@ -624,7 +624,7 @@ func (s *jobScheduler) cleanMDLInfo(job *model.Job, ownerID string) {
 	defer func() {
 		metrics.DDLCleanMDLInfoHist.Observe(time.Since(start).Seconds())
 	}()
-	if !vardef.EnableMDL.Load() {
+	if !vardef.IsMDLEnabled() {
 		return
 	}
 	var sql string
diff --git a/pkg/ddl/job_worker.go b/pkg/ddl/job_worker.go
@@ -308,7 +308,7 @@ func (w *worker) updateDDLJob(jobCtx *jobContext, job *model.Job, updateRawArgs
 
 // registerMDLInfo registers metadata lock info.
 func (w *worker) registerMDLInfo(job *model.Job, ver int64) error {
-	if !vardef.EnableMDL.Load() {
+	if !vardef.IsMDLEnabled() {
 		return nil
 	}
 	if ver == 0 {
@@ -1124,7 +1124,7 @@ func updateGlobalVersionAndWaitSynced(
 	err = jobCtx.schemaVerSyncer.OwnerUpdateGlobalVersion(ctx, latestSchemaVersion)
 	if err != nil {
 		logutil.DDLLogger().Info("update latest schema version failed", zap.Int64("ver", latestSchemaVersion), zap.Error(err))
-		if vardef.EnableMDL.Load() {
+		if vardef.IsMDLEnabled() {
 			return err
 		}
 		if terror.ErrorEqual(err, context.DeadlineExceeded) {
diff --git a/pkg/ddl/schemaver/mem_syncer.go b/pkg/ddl/schemaver/mem_syncer.go
@@ -68,7 +68,7 @@ func (s *MemSyncer) UpdateSelfVersion(_ context.Context, jobID int64, version in
 			failpoint.Return(errors.New("mock update mdl to etcd error"))
 		}
 	})
-	if vardef.EnableMDL.Load() {
+	if vardef.IsMDLEnabled() {
 		s.mdlSchemaVersions.Store(jobID, version)
 	} else {
 		atomic.StoreInt64(&s.selfSchemaVersion, version)
@@ -117,7 +117,7 @@ func (s *MemSyncer) WaitVersionSynced(ctx context.Context, jobID int64, latestVe
 		case <-ctx.Done():
 			return errors.Trace(ctx.Err())
 		case <-ticker.C:
-			if vardef.EnableMDL.Load() {
+			if vardef.IsMDLEnabled() {
 				ver, ok := s.mdlSchemaVersions.Load(jobID)
 				if ok && ver.(int64) >= latestVer {
 					return nil
diff --git a/pkg/ddl/schemaver/syncer.go b/pkg/ddl/schemaver/syncer.go
@@ -282,7 +282,7 @@ func (s *etcdSyncer) UpdateSelfVersion(ctx context.Context, jobID int64, version
 	ver := strconv.FormatInt(version, 10)
 	var err error
 	var path string
-	if vardef.EnableMDL.Load() {
+	if vardef.IsMDLEnabled() {
 		// If jobID is 0, it doesn't need to put into etcd `DDLAllSchemaVersionsByJob` key.
 		if jobID == 0 {
 			return nil
@@ -325,7 +325,7 @@ func (s *etcdSyncer) removeSelfVersionPath() error {
 // WaitVersionSynced implements Syncer.WaitVersionSynced interface.
 func (s *etcdSyncer) WaitVersionSynced(ctx context.Context, jobID int64, latestVer int64, checkAssumedSvr bool) error {
 	startTime := time.Now()
-	if !vardef.EnableMDL.Load() {
+	if !vardef.IsMDLEnabled() {
 		time.Sleep(CheckVersFirstWaitTime)
 	}
 	notMatchVerCnt := 0
@@ -348,7 +348,7 @@ func (s *etcdSyncer) WaitVersionSynced(ctx context.Context, jobID int64, latestV
 			return errors.Trace(err)
 		}
 
-		if vardef.EnableMDL.Load() {
+		if vardef.IsMDLEnabled() {
 			serverInfos, err := infosync.GetServersForISSync(ctx, checkAssumedSvr)
 			if err != nil {
 				return err
@@ -375,7 +375,7 @@ func (s *etcdSyncer) WaitVersionSynced(ctx context.Context, jobID int64, latestV
 		}
 
 		// Check all schema versions.
-		if vardef.EnableMDL.Load() {
+		if vardef.IsMDLEnabled() {
 			notifyCh := make(chan struct{})
 			var unmatchedNodeInfo atomic.Pointer[string]
 			matchFn := func(nodeVersions map[string]int64) bool {
diff --git a/pkg/ddl/schemaver/syncer_nokit_test.go b/pkg/ddl/schemaver/syncer_nokit_test.go
@@ -174,7 +174,7 @@ func TestSyncJobSchemaVerLoop(t *testing.T) {
 	require.NoError(t, err)
 
 	// job 4 is matched using WaitVersionSynced
-	vardef.EnableMDL.Store(true)
+	vardef.SetEnableMDL(true)
 	serverInfos := map[string]*serverinfo.ServerInfo{"aa": {StaticInfo: serverinfo.StaticInfo{ID: "aa", IP: "test", Port: 4000}}}
 	bytes, err := json.Marshal(serverInfos)
 	require.NoError(t, err)
diff --git a/pkg/ddl/schemaver/syncer_test.go b/pkg/ddl/schemaver/syncer_test.go
@@ -39,7 +39,7 @@ import (
 const minInterval = 10 * time.Nanosecond // It's used to test timeout.
 
 func TestSyncerSimple(t *testing.T) {
-	vardef.EnableMDL.Store(false)
+	vardef.SetEnableMDL(false)
 	if runtime.GOOS == "windows" {
 		t.Skip("integration.NewClusterV3 will create file contains a colon which is not allowed on Windows")
 	}
diff --git a/pkg/ddl/serverstate/syncer_test.go b/pkg/ddl/serverstate/syncer_test.go
@@ -43,7 +43,7 @@ func checkRespKV(t *testing.T, kvCount int, key, val string, kvs ...*mvccpb.KeyV
 }
 
 func TestStateSyncerSimple(t *testing.T) {
-	vardef.EnableMDL.Store(false)
+	vardef.SetEnableMDL(false)
 	if runtime.GOOS == "windows" {
 		t.Skip("integration.NewClusterV3 will create file contains a colon which is not allowed on Windows")
 	}
diff --git a/pkg/ddl/tests/metadatalock/BUILD.bazel b/pkg/ddl/tests/metadatalock/BUILD.bazel
@@ -8,9 +8,10 @@ go_test(
         "mdl_test.go",
     ],
     flaky = True,
-    shard_count = 37,
+    shard_count = 38,
     deps = [
         "//pkg/config",
+        "//pkg/config/kerneltype",
         "//pkg/ddl",
         "//pkg/ddl/ingest/testutil",
         "//pkg/errno",
diff --git a/pkg/ddl/tests/metadatalock/mdl_test.go b/pkg/ddl/tests/metadatalock/mdl_test.go
@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"github.com/pingcap/failpoint"
+	"github.com/pingcap/tidb/pkg/config/kerneltype"
 	"github.com/pingcap/tidb/pkg/ddl"
 	ingesttestutil "github.com/pingcap/tidb/pkg/ddl/ingest/testutil"
 	mysql "github.com/pingcap/tidb/pkg/errno"
@@ -1159,6 +1160,9 @@ func TestMDLEnable2Disable(t *testing.T) {
 }
 
 func TestSwitchMDL(t *testing.T) {
+	if kerneltype.IsNextGen() {
+		t.Skip("MDL is always enabled and read only in nextgen")
+	}
 	store, dom := testkit.CreateMockStoreAndDomain(t)
 	sv := server.CreateMockServer(t, store)
 
@@ -1179,6 +1183,19 @@ func TestSwitchMDL(t *testing.T) {
 	tk.MustQuery("show global variables like 'tidb_enable_metadata_lock'").Check(testkit.Rows("tidb_enable_metadata_lock OFF"))
 }
 
+func TestSetMDLInNextGen(t *testing.T) {
+	if kerneltype.IsClassic() {
+		t.Skip("only run in nextgen")
+	}
+	store, _ := testkit.CreateMockStoreAndDomain(t)
+	tk := testkit.NewTestKit(t, store)
+
+	require.ErrorContains(t, tk.ExecToErr("set global tidb_enable_metadata_lock=0"),
+		"setting tidb_enable_metadata_lock is not supported in the next generation of TiDB")
+	require.ErrorContains(t, tk.ExecToErr("set global tidb_enable_metadata_lock=1"),
+		"setting tidb_enable_metadata_lock is not supported in the next generation of TiDB")
+}
+
 func TestMDLViewItself(t *testing.T) {
 	store, dom := testkit.CreateMockStoreAndDomain(t)
 	sv := server.CreateMockServer(t, store)
diff --git a/pkg/infoschema/issyncer/syncer.go b/pkg/infoschema/issyncer/syncer.go
@@ -226,7 +226,7 @@ func (s *Syncer) MDLCheckLoop(ctx context.Context) {
 			return
 		}
 
-		if !vardef.EnableMDL.Load() {
+		if !vardef.IsMDLEnabled() {
 			continue
 		}
 
diff --git a/pkg/infoschema/isvalidator/validator.go b/pkg/infoschema/isvalidator/validator.go
@@ -233,7 +233,7 @@ func (v *validator) Check(txnTS uint64, schemaVer int64, relatedPhysicalTableIDs
 		// is true, we need to check by schema delta.
 		// When switch MDL from on to off, the needCheckSchemaByDelta is false,
 		// and EnableMDL is also false, so we still need to check by schema delta.
-		if needCheckSchemaByDelta || !vardef.EnableMDL.Load() {
+		if needCheckSchemaByDelta || !vardef.IsMDLEnabled() {
 			changed := v.isRelatedTablesChanged(schemaVer, relatedPhysicalTableIDs)
 			if changed {
 				return nil, validatorapi.ResultFail
diff --git a/pkg/planner/core/logical_plans_test.go b/pkg/planner/core/logical_plans_test.go
@@ -507,7 +507,7 @@ func TestDupRandJoinCondsPushDown(t *testing.T) {
 }
 
 func TestTablePartition(t *testing.T) {
-	vardef.EnableMDL.Store(false)
+	vardef.SetEnableMDL(false)
 	definitions := []model.PartitionDefinition{
 		{
 			ID:       41,
@@ -1127,7 +1127,7 @@ func TestAggPrune(t *testing.T) {
 }
 
 func TestVisitInfo(t *testing.T) {
-	vardef.EnableMDL.Store(false)
+	vardef.SetEnableMDL(false)
 	tests := []struct {
 		sql string
 		ans []visitInfo
diff --git a/pkg/planner/memo/group_test.go b/pkg/planner/memo/group_test.go
@@ -98,7 +98,7 @@ func TestGroupExists(t *testing.T) {
 }
 
 func TestGroupFingerPrint(t *testing.T) {
-	vardef.EnableMDL.Store(false)
+	vardef.SetEnableMDL(false)
 	p := parser.New()
 	stmt1, err := p.ParseOneStmt("select * from t where a > 1 and a < 100", "", "")
 	require.NoError(t, err)
@@ -230,7 +230,7 @@ func TestFirstElemAfterDelete(t *testing.T) {
 }
 
 func TestBuildKeyInfo(t *testing.T) {
-	vardef.EnableMDL.Store(false)
+	vardef.SetEnableMDL(false)
 	p := parser.New()
 	ctx := plannercore.MockContext()
 	defer func() {
diff --git a/pkg/session/session.go b/pkg/session/session.go
@@ -3417,7 +3417,7 @@ func InitMDLVariableForBootstrap(store kv.Storage) error {
 	if err != nil {
 		return err
 	}
-	vardef.EnableMDL.Store(true)
+	vardef.SetEnableMDL(true)
 	return nil
 }
 
@@ -3461,9 +3461,9 @@ func InitMDLVariableForUpgrade(store kv.Storage) (bool, error) {
 		return nil
 	})
 	if isNull || !enable {
-		vardef.EnableMDL.Store(false)
+		vardef.SetEnableMDL(false)
 	} else {
-		vardef.EnableMDL.Store(true)
+		vardef.SetEnableMDL(true)
 	}
 	return isNull, err
 }
@@ -3490,7 +3490,7 @@ func InitMDLVariable(store kv.Storage) error {
 		}
 		return nil
 	})
-	vardef.EnableMDL.Store(enable)
+	vardef.SetEnableMDL(enable)
 	return err
 }
 
diff --git a/pkg/sessionctx/vardef/BUILD.bazel b/pkg/sessionctx/vardef/BUILD.bazel
@@ -1,4 +1,4 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 
 go_library(
     name = "vardef",
@@ -11,6 +11,7 @@ go_library(
     visibility = ["//visibility:public"],
     deps = [
         "//pkg/config",
+        "//pkg/config/kerneltype",
         "//pkg/executor/join/joinversion",
         "//pkg/parser/mysql",
         "//pkg/util/memory",
@@ -20,3 +21,15 @@ go_library(
         "@org_uber_go_atomic//:atomic",
     ],
 )
+
+go_test(
+    name = "vardef_test",
+    timeout = "short",
+    srcs = ["tidb_vars_test.go"],
+    embed = [":vardef"],
+    flaky = True,
+    deps = [
+        "//pkg/config/kerneltype",
+        "@com_github_stretchr_testify//require",
+    ],
+)
diff --git a/pkg/sessionctx/vardef/tidb_vars.go b/pkg/sessionctx/vardef/tidb_vars.go
@@ -23,6 +23,7 @@ import (
 	"time"
 
 	"github.com/pingcap/tidb/pkg/config"
+	"github.com/pingcap/tidb/pkg/config/kerneltype"
 	"github.com/pingcap/tidb/pkg/executor/join/joinversion"
 	"github.com/pingcap/tidb/pkg/parser/mysql"
 	"github.com/pingcap/tidb/pkg/util/memory"
@@ -1732,7 +1733,7 @@ var (
 	EnableDistTask                      = atomic.NewBool(DefTiDBEnableDistTask)
 	EnableFastCreateTable               = atomic.NewBool(DefTiDBEnableFastCreateTable)
 	EnableNoopVariables                 = atomic.NewBool(DefTiDBEnableNoopVariables)
-	EnableMDL                           = atomic.NewBool(false)
+	enableMDL                           = atomic.NewBool(false)
 	AutoAnalyzePartitionBatchSize       = atomic.NewInt64(DefTiDBAutoAnalyzePartitionBatchSize)
 	AutoAnalyzeConcurrency              = atomic.NewInt32(DefTiDBAutoAnalyzeConcurrency)
 	// TODO: set value by session variable
@@ -2133,3 +2134,22 @@ func SetMaxDeltaSchemaCount(cnt int64) {
 func GetMaxDeltaSchemaCount() int64 {
 	return goatomic.LoadInt64(&MaxDeltaSchemaCount)
 }
+
+// IsMDLEnabled returns if MDL is enabled.
+func IsMDLEnabled() bool {
+	if kerneltype.IsNextGen() {
+		// MDL is very useful to avoid the 'Information schema is changed' error,
+		// in next-gen TiDB, MDL is always enabled, as we don't have the compatibility
+		// debts.
+		// some tests might call SetEnableMDL(false) to disable MDL, but it is not
+		// expected in nextgen, we use this branch to ensure MDL is always enabled,
+		// even in test.
+		return true
+	}
+	return enableMDL.Load()
+}
+
+// SetEnableMDL sets the MDL enable status.
+func SetEnableMDL(enabled bool) {
+	enableMDL.Store(enabled)
+}
diff --git a/pkg/sessionctx/vardef/tidb_vars_test.go b/pkg/sessionctx/vardef/tidb_vars_test.go
@@ -0,0 +1,38 @@
+// Copyright 2025 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 (
+	"testing"
+
+	"github.com/pingcap/tidb/pkg/config/kerneltype"
+	"github.com/stretchr/testify/require"
+)
+
+func TestIsMDLEnabledInNextGen(t *testing.T) {
+	if kerneltype.IsClassic() {
+		t.Skip("only run in next-gen")
+	}
+	bak := enableMDL.Load()
+	t.Cleanup(func() {
+		SetEnableMDL(bak)
+	})
+
+	// IsMDLEnabled must return true regardless of the value of enableMDL.
+	SetEnableMDL(false)
+	require.True(t, IsMDLEnabled())
+	SetEnableMDL(true)
+	require.True(t, IsMDLEnabled())
+}
diff --git a/pkg/sessionctx/variable/sysvar.go b/pkg/sessionctx/variable/sysvar.go
diff --git a/pkg/sessiontxn/isolation/base.go b/pkg/sessiontxn/isolation/base.go
diff --git a/pkg/table/tables/bench_test.go b/pkg/table/tables/bench_test.go

Original file line number	Diff line number	Diff line change
`@@ -39,7 +39,7 @@ import (`
`39`	`39`	`const minInterval = 10 * time.Nanosecond // It's used to test timeout.`
`40`	`40`
`41`	`41`	`func TestSyncerSimple(t *testing.T) {`
`42`		`- vardef.EnableMDL.Store(false)`
	`42`	`+ vardef.SetEnableMDL(false)`
`43`	`43`	`if runtime.GOOS == "windows" {`
`44`	`44`	`t.Skip("integration.NewClusterV3 will create file contains a colon which is not allowed on Windows")`
`45`	`45`	`}`
Original file line number	Diff line number	Diff line change
`@@ -43,7 +43,7 @@ func checkRespKV(t testing.T, kvCount int, key, val string, kvs ...mvccpb.KeyV`
`43`	`43`	`}`
`44`	`44`
`45`	`45`	`func TestStateSyncerSimple(t *testing.T) {`
`46`		`- vardef.EnableMDL.Store(false)`
	`46`	`+ vardef.SetEnableMDL(false)`
`47`	`47`	`if runtime.GOOS == "windows" {`
`48`	`48`	`t.Skip("integration.NewClusterV3 will create file contains a colon which is not allowed on Windows")`
`49`	`49`	`}`