From 38792204f961622ecfacd6c6aee711bbebf2d07b Mon Sep 17 00:00:00 2001
From: Zijie Lu <wslzj40@gmail.com>
Date: Thu, 24 Dec 2020 16:09:30 +0800
Subject: [PATCH] executor, session, sessionctx: make last_plan_from_cache and
 last_plan_from_binding read-only (#21953)

---
 executor/set.go                      |  8 --------
 session/session_test.go              |  9 +++++++++
 sessionctx/variable/sysvar.go        |  4 ++--
 sessionctx/variable/varsutil_test.go | 12 ++----------
 4 files changed, 13 insertions(+), 20 deletions(-)

diff --git a/executor/set.go b/executor/set.go
index f1944bc1e69db..503871fbe4a55 100644
--- a/executor/set.go
+++ b/executor/set.go
@@ -180,14 +180,6 @@ func (e *SetExecutor) setSysVariable(name string, v *expression.VarAssignment) e
 		if name == variable.TxnIsolationOneShot && sessionVars.InTxn() {
 			return errors.Trace(ErrCantChangeTxCharacteristics)
 		}
-		if name == variable.TiDBFoundInPlanCache {
-			sessionVars.StmtCtx.AppendWarning(fmt.Errorf("Set operation for '%s' will not take effect", variable.TiDBFoundInPlanCache))
-			return nil
-		}
-		if name == variable.TiDBFoundInBinding {
-			sessionVars.StmtCtx.AppendWarning(fmt.Errorf("Set operation for '%s' will not take effect", variable.TiDBFoundInBinding))
-			return nil
-		}
 		err = variable.SetSessionSystemVar(sessionVars, name, value)
 		if err != nil {
 			return err
diff --git a/session/session_test.go b/session/session_test.go
index 811a3f6f11919..4fcf91414c187 100644
--- a/session/session_test.go
+++ b/session/session_test.go
@@ -3810,3 +3810,12 @@ func (s *testSessionSerialSuite) TestIssue21944(c *C) {
 	_, err := tk1.Exec("set @@tidb_current_ts=1;")
 	c.Assert(err.Error(), Equals, "[variable:1238]Variable 'tidb_current_ts' is a read only variable")
 }
+
+func (s *testSessionSerialSuite) TestIssue21943(c *C) {
+	tk := testkit.NewTestKitWithInit(c, s.store)
+	_, err := tk.Exec("set @@last_plan_from_binding='123';")
+	c.Assert(err.Error(), Equals, "[variable:1238]Variable 'last_plan_from_binding' is a read only variable")
+
+	_, err = tk.Exec("set @@last_plan_from_cache='123';")
+	c.Assert(err.Error(), Equals, "[variable:1238]Variable 'last_plan_from_cache' is a read only variable")
+}
diff --git a/sessionctx/variable/sysvar.go b/sessionctx/variable/sysvar.go
index 7b65bb3d06867..eb57e10f8b4e1 100644
--- a/sessionctx/variable/sysvar.go
+++ b/sessionctx/variable/sysvar.go
@@ -1131,8 +1131,8 @@ var defaultSysVars = []*SysVar{
 	{Scope: ScopeSession, Name: TiDBEnableSlowLog, Value: BoolToOnOff(logutil.DefaultTiDBEnableSlowLog), Type: TypeBool},
 	{Scope: ScopeSession, Name: TiDBQueryLogMaxLen, Value: strconv.Itoa(logutil.DefaultQueryLogMaxLen), Type: TypeInt, MinValue: -1, MaxValue: math.MaxInt64},
 	{Scope: ScopeSession, Name: TiDBCheckMb4ValueInUTF8, Value: BoolToOnOff(config.GetGlobalConfig().CheckMb4ValueInUTF8), Type: TypeBool},
-	{Scope: ScopeSession, Name: TiDBFoundInPlanCache, Value: BoolToOnOff(DefTiDBFoundInPlanCache), Type: TypeBool},
-	{Scope: ScopeSession, Name: TiDBFoundInBinding, Value: BoolToOnOff(DefTiDBFoundInBinding), Type: TypeBool},
+	{Scope: ScopeSession, Name: TiDBFoundInPlanCache, Value: BoolToOnOff(DefTiDBFoundInPlanCache), Type: TypeBool, ReadOnly: true},
+	{Scope: ScopeSession, Name: TiDBFoundInBinding, Value: BoolToOnOff(DefTiDBFoundInBinding), Type: TypeBool, ReadOnly: true},
 	{Scope: ScopeSession, Name: TiDBEnableCollectExecutionInfo, Value: BoolToOnOff(DefTiDBEnableCollectExecutionInfo), Type: TypeBool},
 	{Scope: ScopeGlobal | ScopeSession, Name: TiDBAllowAutoRandExplicitInsert, Value: BoolToOnOff(DefTiDBAllowAutoRandExplicitInsert), Type: TypeBool},
 	{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnableClusteredIndex, Value: BoolToOnOff(DefTiDBEnableClusteredIndex), Type: TypeBool},
diff --git a/sessionctx/variable/varsutil_test.go b/sessionctx/variable/varsutil_test.go
index f8e49d6f2ff67..c5b56363c3338 100644
--- a/sessionctx/variable/varsutil_test.go
+++ b/sessionctx/variable/varsutil_test.go
@@ -466,18 +466,10 @@ func (s *testVarsutilSuite) TestVarsutil(c *C) {
 	c.Assert(err, ErrorMatches, ".*Incorrect argument type to variable 'tidb_stmt_summary_max_sql_length'")
 
 	err = SetSessionSystemVar(v, TiDBFoundInPlanCache, types.NewStringDatum("1"))
-	c.Assert(err, IsNil)
-	val, err = GetSessionSystemVar(v, TiDBFoundInPlanCache)
-	c.Assert(err, IsNil)
-	c.Assert(val, Equals, "OFF")
-	c.Assert(v.systems[TiDBFoundInPlanCache], Equals, "ON")
+	c.Assert(err, ErrorMatches, ".*]Variable 'last_plan_from_cache' is a read only variable")
 
 	err = SetSessionSystemVar(v, TiDBFoundInBinding, types.NewStringDatum("1"))
-	c.Assert(err, IsNil)
-	val, err = GetSessionSystemVar(v, TiDBFoundInBinding)
-	c.Assert(err, IsNil)
-	c.Assert(val, Equals, "OFF")
-	c.Assert(v.systems[TiDBFoundInBinding], Equals, "ON")
+	c.Assert(err, ErrorMatches, ".*]Variable 'last_plan_from_binding' is a read only variable")
 
 	err = SetSessionSystemVar(v, TiDBEnableChangeColumnType, types.NewStringDatum("ON"))
 	c.Assert(err, IsNil)