session,executor: fix point get under @@tidb_snapshot (#22460)

executor/adapter.go

@@ -706,30 +706,30 @@ func (a *ExecStmt) buildExecutor() (Executor, error) {
 	ctx := a.Ctx
 	stmtCtx := ctx.GetSessionVars().StmtCtx
 	if _, ok := a.Plan.(*plannercore.Execute); !ok {
-		// Do not sync transaction for Execute statement, because the real optimization work is done in
-		// "ExecuteExec.Build".
-		useMaxTS, err := plannercore.IsPointGetWithPKOrUniqueKeyByAutoCommit(ctx, a.Plan)
-		if err != nil {
-			return nil, err
-		}
-		if useMaxTS {
-			logutil.BgLogger().Debug("init txnStartTS with MaxUint64", zap.Uint64("conn", ctx.GetSessionVars().ConnectionID), zap.String("text", a.Text))
-			err = ctx.InitTxnWithStartTS(math.MaxUint64)
-		} else if ctx.GetSessionVars().SnapshotTS != 0 {
-			if _, ok := a.Plan.(*plannercore.CheckTable); ok {
-				err = ctx.InitTxnWithStartTS(ctx.GetSessionVars().SnapshotTS)
+		if snapshotTS := ctx.GetSessionVars().SnapshotTS; snapshotTS != 0 {
+			if err := ctx.InitTxnWithStartTS(snapshotTS); err != nil {
+				return nil, err
 			}
-		}
-		if err != nil {
-			return nil, err
-		}
-
-		if stmtPri := stmtCtx.Priority; stmtPri == mysql.NoPriority {
-			switch {
-			case useMaxTS:
-				stmtCtx.Priority = kv.PriorityHigh
-			case a.LowerPriority:
-				stmtCtx.Priority = kv.PriorityLow
+		} else {
+			// Do not sync transaction for Execute statement, because the real optimization work is done in
+			// "ExecuteExec.Build".
+			useMaxTS, err := plannercore.IsPointGetWithPKOrUniqueKeyByAutoCommit(ctx, a.Plan)
+			if err != nil {
+				return nil, err
+			}
+			if useMaxTS {
+				logutil.BgLogger().Debug("init txnStartTS with MaxUint64", zap.Uint64("conn", ctx.GetSessionVars().ConnectionID), zap.String("text", a.Text))
+				if err := ctx.InitTxnWithStartTS(math.MaxUint64); err != nil {
+					return nil, err
+				}
+			}
+			if stmtPri := stmtCtx.Priority; stmtPri == mysql.NoPriority {
+				switch {
+				case useMaxTS:
+					stmtCtx.Priority = kv.PriorityHigh
+				case a.LowerPriority:
+					stmtCtx.Priority = kv.PriorityLow
+				}
 			}
 		}
 	}

executor/point_get_test.go

@@ -545,6 +545,44 @@ func (s *testPointGetSuite) TestReturnValues(c *C) {
 	tk.MustExec("rollback")
 }
 
+func (s *testPointGetSuite) TestWithTiDBSnapshot(c *C) {
+	// Fix issue https://github.com/pingcap/tidb/issues/22436
+	// Point get should not use math.MaxUint64 when variable @@tidb_snapshot is set.
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists xx")
+	tk.MustExec(`create table xx (id int key)`)
+	tk.MustExec(`insert into xx values (1), (7)`)
+
+	// Unrelated code, to make this test pass in the unit test.
+	// The `tikv_gc_safe_point` global variable must be there, otherwise the 'set @@tidb_snapshot' operation fails.
+	timeSafe := time.Now().Add(-48 * 60 * 60 * time.Second).Format("20060102-15:04:05 -0700 MST")
+	safePointSQL := `INSERT HIGH_PRIORITY INTO mysql.tidb VALUES ('tikv_gc_safe_point', '%[1]s', '')
+			       ON DUPLICATE KEY
+			       UPDATE variable_value = '%[1]s'`
+	tk.MustExec(fmt.Sprintf(safePointSQL, timeSafe))
+
+	// Record the current tso.
+	tk.MustExec("begin")
+	tso := tk.Se.GetSessionVars().TxnCtx.StartTS
+	tk.MustExec("rollback")
+	c.Assert(tso > 0, IsTrue)
+
+	// Insert data.
+	tk.MustExec("insert into xx values (8)")
+
+	// Change the snapshot before the tso, the inserted data should not be seen.
+	tk.MustExec(fmt.Sprintf("set @@tidb_snapshot = '%d'", tso))
+	tk.MustQuery("select * from xx where id = 8").Check(testkit.Rows())
+
+	tk.MustQuery("select * from xx").Check(testkit.Rows("1", "7"))
+
+	// Check the query inside a transaction.
+	tk.MustExec("begin")
+	tk.MustQuery("select * from xx where id = 8").Check(testkit.Rows())
+	tk.MustExec("rollback")
+}
+
 func (s *testPointGetSuite) TestPointGetLockExistKey(c *C) {
 	var wg sync.WaitGroup
 	errCh := make(chan error)

session/session.go

@@ -2192,6 +2192,11 @@ func (s *session) PrepareTxnCtx(ctx context.Context) {
 
 // PrepareTSFuture uses to try to get ts future.
 func (s *session) PrepareTSFuture(ctx context.Context) {
+	if s.sessionVars.SnapshotTS != 0 {
+		// Do nothing when @@tidb_snapshot is set.
+		// In case the latest tso is misused.
+		return
+	}
 	if !s.txn.validOrPending() {
 		// Prepare the transaction future if the transaction is invalid (at the beginning of the transaction).
 		txnFuture := s.getTxnFuture(ctx)