session, ddl: rollback txn for prepare statement (#39433)

close #39432

ddl/metadatalocktest/mdl_test.go

@@ -1105,3 +1105,16 @@ func TestMDLRenameTable(t *testing.T) {
 	tk.MustGetErrCode("select * from test2.t1;", mysql.ErrNoSuchTable)
 	tk.MustExec("commit")
 }
+
+func TestMDLPrepareFail(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+
+	tk := testkit.NewTestKit(t, store)
+	tk2 := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("create table t(a int);")
+	_, _, _, err := tk.Session().PrepareStmt("select b from t")
+	require.Error(t, err)
+
+	tk2.MustExec("alter table test.t add column c int")
+}

session/session.go

@@ -2450,7 +2450,6 @@ func (s *session) PrepareStmt(sql string) (stmtID uint32, paramCount int, fields
 	}
 
 	ctx := context.Background()
-	inTxn := s.GetSessionVars().InTxn()
 	// NewPrepareExec may need startTS to build the executor, for example prepare statement has subquery in int.
 	// So we have to call PrepareTxnCtx here.
 	if err = s.PrepareTxnCtx(ctx); err != nil {
@@ -2467,13 +2466,12 @@ func (s *session) PrepareStmt(sql string) (stmtID uint32, paramCount int, fields
 	}
 	prepareExec := executor.NewPrepareExec(s, sql)
 	err = prepareExec.Next(ctx, nil)
+	// Rollback even if err is nil.
+	s.rollbackOnError(ctx)
+
 	if err != nil {
 		return
 	}
-	if !inTxn {
-		// We could start a transaction to build the prepare executor before, we should rollback it here.
-		s.RollbackTxn(ctx)
-	}
 	return prepareExec.ID, prepareExec.ParamCount, prepareExec.Fields, nil
 }