planner: "for update" should not work in a single autocommit statement

cmd/explaintest/r/explain_easy.result

@@ -458,7 +458,7 @@ Projection_3	10000.00	root	or(NULL, gt(test.t.a, 1))
   └─TableScan_4	10000.00	cop	table:t, range:[-inf,+inf], keep order:false, stats:pseudo
 explain select * from t where a = 1 for update;
 id	count	task	operator info
-Point_Get_1	1.00	root	table:t, handle:1
+Point_Get_1	1.00	root	table:t, handle:1, lock
 drop table if exists ta, tb;
 create table ta (a varchar(20));
 create table tb (a varchar(20));

planner/core/point_get_plan.go

@@ -95,6 +95,9 @@ func (p *PointGetPlan) ExplainInfo() string {
 			fmt.Fprintf(buffer, ", handle:%d", p.Handle)
 		}
 	}
+	if p.Lock {
+		fmt.Fprintf(buffer, ", lock")
+	}
 	return buffer.String()
 }
 
@@ -148,8 +151,14 @@ func TryFastPlan(ctx sessionctx.Context, node ast.Node) Plan {
 				return tableDual.Init(ctx, &property.StatsInfo{})
 			}
 			if x.LockTp == ast.SelectLockForUpdate {
-				fp.Lock = true
-				fp.IsForUpdate = true
+				// Locking of rows for update using SELECT FOR UPDATE only applies when autocommit
+				// is disabled (either by beginning transaction with START TRANSACTION or by setting
+				// autocommit to 0. If autocommit is enabled, the rows matching the specification are not locked.
+				// See https://dev.mysql.com/doc/refman/5.7/en/innodb-locking-reads.html
+				if ctx.GetSessionVars().InTxn() {
+					fp.Lock = true
+					fp.IsForUpdate = true
+				}
 			}
 			return fp
 		}

planner/core/point_get_plan_test.go

@@ -14,9 +14,13 @@
 package core_test
 
 import (
+	"fmt"
 	"math"
+	"strings"
 
 	. "github.com/pingcap/check"
+	"github.com/pingcap/tidb/domain"
+	"github.com/pingcap/tidb/kv"
 	"github.com/pingcap/tidb/metrics"
 	"github.com/pingcap/tidb/planner/core"
 	"github.com/pingcap/tidb/util/testkit"
@@ -27,20 +31,31 @@ import (
 var _ = Suite(&testPointGetSuite{})
 
 type testPointGetSuite struct {
+	store kv.Storage
+	dom   *domain.Domain
 }
 
-func (s *testPointGetSuite) TestPointGetPlanCache(c *C) {
-	defer testleak.AfterTest(c)()
+func (s *testPointGetSuite) SetUpSuite(c *C) {
+	testleak.BeforeTest()
 	store, dom, err := newStoreWithBootstrap()
 	c.Assert(err, IsNil)
-	tk := testkit.NewTestKit(c, store)
+	s.store = store
+	s.dom = dom
+}
+
+func (s *testPointGetSuite) TearDownSuite(c *C) {
+	s.dom.Close()
+	s.store.Close()
+	testleak.AfterTest(c)()
+}
+
+func (s *testPointGetSuite) TestPointGetPlanCache(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
 	orgEnable := core.PreparedPlanCacheEnabled()
 	orgCapacity := core.PreparedPlanCacheCapacity
 	orgMemGuardRatio := core.PreparedPlanCacheMemoryGuardRatio
 	orgMaxMemory := core.PreparedPlanCacheMaxMemory
 	defer func() {
-		dom.Close()
-		store.Close()
 		core.SetPreparedPlanCache(orgEnable)
 		core.PreparedPlanCacheCapacity = orgCapacity
 		core.PreparedPlanCacheMemoryGuardRatio = orgMemGuardRatio
@@ -152,3 +167,29 @@ func (s *testPointGetSuite) TestPointGetPlanCache(c *C) {
 	hit = pb.GetCounter().GetValue()
 	c.Check(hit, Equals, float64(2))
 }
+
+func (s *testPointGetSuite) TestPointGetForUpdate(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("create table fu (id int primary key)")
+	tk.MustExec("insert into fu values (6)")
+	tk.MustQuery("select * from fu where id = 6 for update").Check(testkit.Rows("6"))
+
+	// In autocommit mode, outside a transaction, "for update" doesn't take effect.
+	res := tk.MustQuery("explain select * from fu where id = 6 for update")
+	// Point_Get_1	1.00	root	table:fu, handle:6
+	opInfo := res.Rows()[0][3]
+	selectLock := strings.Contains(fmt.Sprintf("%s", opInfo), "lock")
+	c.Assert(selectLock, IsFalse)
+
+	tk.MustExec("begin")
+	tk.MustQuery("select * from fu where id = 6 for update").Check(testkit.Rows("6"))
+	res = tk.MustQuery("explain select * from fu where id = 6 for update")
+	// Point_Get_1	1.00	root	table:fu, handle:6
+	opInfo = res.Rows()[0][3]
+	selectLock = strings.Contains(fmt.Sprintf("%s", opInfo), "lock")
+	c.Assert(selectLock, IsTrue)
+
+	tk.MustExec("rollback")
+
+}