From 7ef5efd7eecf137917b3213bb800b4a07fd8da54 Mon Sep 17 00:00:00 2001
From: ekexium <eke@fastmail.com>
Date: Wed, 12 Nov 2025 15:08:59 +0800
Subject: [PATCH] txn: force locking temp index keys in dml in next-gen
 (#64398)

ref pingcap/tidb#64241
---
 pkg/ddl/tests/indexmerge/BUILD.bazel   |  2 +-
 pkg/ddl/tests/indexmerge/merge_test.go | 57 ++++++++++++++++++++++++++
 pkg/session/test/tidb_test.go          | 17 ++++++++
 pkg/session/txn.go                     |  6 +++
 4 files changed, 81 insertions(+), 1 deletion(-)

diff --git a/pkg/ddl/tests/indexmerge/BUILD.bazel b/pkg/ddl/tests/indexmerge/BUILD.bazel
index eec2b6cf0a..d8678c48a5 100644
--- a/pkg/ddl/tests/indexmerge/BUILD.bazel
+++ b/pkg/ddl/tests/indexmerge/BUILD.bazel
@@ -9,7 +9,7 @@ go_test(
     ],
     flaky = True,
     race = "on",
-    shard_count = 23,
+    shard_count = 24,
     deps = [
         "//pkg/config",
         "//pkg/config/kerneltype",
diff --git a/pkg/ddl/tests/indexmerge/merge_test.go b/pkg/ddl/tests/indexmerge/merge_test.go
index e9c9a40b8c..daacbbe07d 100644
--- a/pkg/ddl/tests/indexmerge/merge_test.go
+++ b/pkg/ddl/tests/indexmerge/merge_test.go
@@ -17,6 +17,7 @@ package indexmergetest
 import (
 	"strconv"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -476,6 +477,62 @@ func TestAddIndexMergeConflictWithPessimistic(t *testing.T) {
 	tk.MustQuery("select * from t;").Check(testkit.Rows("1 2"))
 }
 
+func TestNextGenPessimisticTxnNotFailWithTempIndex(t *testing.T) {
+	if !kerneltype.IsNextGen() {
+		t.Skip("only meaningful when next-gen kernel enforces temp index locking")
+	}
+	store, dom := testkit.CreateMockStoreAndDomain(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t_pess")
+	tk.MustExec("create table t_pess (id int primary key, b int);")
+	tk.MustExec("insert into t_pess values (1, 1);")
+
+	origLit := ingest.LitInitialized
+	ingest.LitInitialized = false
+	t.Cleanup(func() { ingest.LitInitialized = origLit })
+
+	txnReady := make(chan struct{})
+	txnDone := make(chan error, 1)
+	var startTxnOnce sync.Once
+	var txnStarted atomic.Bool
+	testfailpoint.EnableCall(t, "github.com/pingcap/tidb/pkg/ddl/afterWaitSchemaSynced", func(job *model.Job) {
+		if t.Failed() || job.Type != model.ActionAddIndex || job.SchemaState != model.StateWriteReorganization {
+			return
+		}
+		idx := testutil.FindIdxInfo(dom, "test", "t_pess", "idx_b")
+		if idx == nil || idx.BackfillState != model.BackfillStateRunning {
+			return
+		}
+		triggered := false
+		startTxnOnce.Do(func() {
+			triggered = true
+			txnStarted.Store(true)
+			// let the user transaction acquire pessimistic lock first
+			go func() {
+				tk2 := testkit.NewTestKit(t, store)
+				tk2.MustExec("use test")
+				tk2.MustExec("begin pessimistic")
+				tk2.MustExec("update t_pess set b = b + 10 where id = 1")
+				txnReady <- struct{}{}
+				time.Sleep(200 * time.Millisecond)
+				txnDone <- tk2.ExecToErr("commit")
+			}()
+		})
+		// Wait for the txn to hold the pessimistic lock
+		if triggered {
+			<-txnReady
+		}
+	})
+
+	// Assert: both user transaction and DDL will succeed, and data are consistent
+	tk.MustExec("alter table t_pess add index idx_b(b);")
+	require.True(t, txnStarted.Load())
+	require.NoError(t, <-txnDone)
+	tk.MustExec("admin check table t_pess")
+	tk.MustQuery("select * from t_pess").Check(testkit.Rows("1 11"))
+}
+
 func TestAddIndexMergeInsertOnMerging(t *testing.T) {
 	store := testkit.CreateMockStore(t)
 
diff --git a/pkg/session/test/tidb_test.go b/pkg/session/test/tidb_test.go
index 741351eede..242e4b6501 100644
--- a/pkg/session/test/tidb_test.go
+++ b/pkg/session/test/tidb_test.go
@@ -18,6 +18,7 @@ import (
 	"bytes"
 	"testing"
 
+	"github.com/pingcap/tidb/pkg/config/kerneltype"
 	"github.com/pingcap/tidb/pkg/domain"
 	"github.com/pingcap/tidb/pkg/kv"
 	"github.com/pingcap/tidb/pkg/planner/util/coretestsdk"
@@ -44,6 +45,8 @@ func TestKeysNeedLock(t *testing.T) {
 	rowKey := tablecodec.EncodeRowKeyWithHandle(1, kv.IntHandle(1))
 	uniqueIndexKey := tablecodec.EncodeIndexSeekKey(1, 1, []byte{1})
 	nonUniqueIndexKey := tablecodec.EncodeIndexSeekKey(1, 2, []byte{1})
+	tempIndexKey := tablecodec.EncodeIndexSeekKey(1, 3, []byte{1})
+	tablecodec.IndexKey2TempIndexKey(tempIndexKey)
 	uniqueValue := make([]byte, 8)
 	uniqueUntouched := append(uniqueValue, '1')
 	nonUniqueVal := []byte{'0'}
@@ -79,4 +82,18 @@ func TestKeysNeedLock(t *testing.T) {
 			require.True(t, session.KeyNeedToLock(test.key, test.val, flag))
 		}
 	}
+
+	tempIdxValue := (&tablecodec.TempIndexValueElem{
+		Value:  nonUniqueVal,
+		KeyVer: tablecodec.TempIndexKeyTypeBackfill,
+	}).Encode(nil)
+	need := session.KeyNeedToLock(tempIndexKey, tempIdxValue, 0)
+	if kerneltype.IsNextGen() {
+		require.True(t, need)
+	} else {
+		require.False(t, need)
+	}
+
+	flag := kv.KeyFlags(1)
+	require.True(t, session.KeyNeedToLock(tempIndexKey, tempIdxValue, flag))
 }
diff --git a/pkg/session/txn.go b/pkg/session/txn.go
index 6453e3a92d..2295510b2c 100644
--- a/pkg/session/txn.go
+++ b/pkg/session/txn.go
@@ -26,6 +26,7 @@ import (
 	"github.com/pingcap/errors"
 	"github.com/pingcap/failpoint"
 	"github.com/pingcap/tidb/pkg/config"
+	"github.com/pingcap/tidb/pkg/config/kerneltype"
 	"github.com/pingcap/tidb/pkg/kv"
 	"github.com/pingcap/tidb/pkg/meta/model"
 	"github.com/pingcap/tidb/pkg/parser/terror"
@@ -652,6 +653,11 @@ func KeyNeedToLock(k, v []byte, flags kv.KeyFlags) bool {
 	}
 
 	if tablecodec.IsTempIndexKey(k) {
+		// We force DMLs to lock all temporary index keys in next-gen, because
+		// next-gen enforces conflict check on all keys, including non-unique index keys.
+		if kerneltype.IsNextGen() {
+			return true
+		}
 		tmpVal, err := tablecodec.DecodeTempIndexValue(v)
 		if err != nil {
 			logutil.BgLogger().Warn("decode temp index value failed", zap.Error(err))