*: fix transaction lazy initialization in pessimistic mode (#14446) (#…

…14474)

executor/adapter.go

@@ -433,13 +433,19 @@ func (a *ExecStmt) handleNoDelayExecutor(ctx context.Context, e Executor) (sqlex
 
 func (a *ExecStmt) handlePessimisticDML(ctx context.Context, e Executor) error {
 	sctx := a.Ctx
-	txn, err := sctx.Txn(true)
+	// Do not active the transaction here.
+	// When autocommit = 0 and transaction in pessimistic mode,
+	// statements like set xxx = xxx; should not active the transaction.
+	txn, err := sctx.Txn(false)
 	if err != nil {
 		return err
 	}
 	txnCtx := sctx.GetSessionVars().TxnCtx
 	for {
 		_, err = a.handleNoDelayExecutor(ctx, e)
+		if !txn.Valid() {
+			return err
+		}
 		if err != nil {
 			// It is possible the DML has point get plan that locks the key.
 			e, err = a.handlePessimisticLockError(ctx, err)

session/session_test.go

@@ -377,9 +377,17 @@ func (s *testSessionSuite) TestAutocommit(c *C) {
 // TestTxnLazyInitialize tests that when autocommit = 0, not all statement starts
 // a new transaction.
 func (s *testSessionSuite) TestTxnLazyInitialize(c *C) {
+	testTxnLazyInitialize(s, c, false)
+	testTxnLazyInitialize(s, c, true)
+}
+
+func testTxnLazyInitialize(s *testSessionSuite, c *C, isPessimistic bool) {
 	tk := testkit.NewTestKitWithInit(c, s.store)
 	tk.MustExec("drop table if exists t")
 	tk.MustExec("create table t (id int)")
+	if isPessimistic {
+		tk.MustExec("set tidb_txn_mode = 'pessimistic'")
+	}
 
 	tk.MustExec("set @@autocommit = 0")
 	txn, err := tk.Se.Txn(true)