variables last_plan_from_cache & last_plan_from_binding should be read-only

[SunRunAway]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> set @@last_plan_from_binding='123';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+-----------------------------------------------------------------+
| Level   | Code | Message                                                         |
+---------+------+-----------------------------------------------------------------+
| Warning | 1105 | Set operation for 'last_plan_from_binding' will not take effect |
+---------+------+-----------------------------------------------------------------+
1 row in set (0.00 sec)

2. What did you expect to see? (Required)

mysql> set @@last_plan_from_binding='123';
ERROR 1105 (HY000): Variable 'version_comment' is a read only variable

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[morgo]

I think this is easy for new contributors. The SysVar struct supports the ability to set a sysvar as read-only:

diff --git a/sessionctx/variable/sysvar.go b/sessionctx/variable/sysvar.go
index eb6710392..00d3034c3 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},

This is checked when changing a global variable, but not yet a sessionvar. It is also an easy fix:

diff --git a/executor/set.go b/executor/set.go
index f1944bc1e..41a6c06db 100644
--- a/executor/set.go
+++ b/executor/set.go
@@ -130,7 +130,7 @@ func (e *SetExecutor) setSysVariable(name string, v *expression.VarAssignment) e
        if sysVar == nil {
                return variable.ErrUnknownSystemVar.GenWithStackByArgs(name)
        }
-       if sysVar.Scope == variable.ScopeNone {
+       if sysVar.Scope == variable.ScopeNone || sysVar.ReadOnly {
                return errors.Errorf("Variable '%s' is a read only variable", name)
        }
        var valStr string

The code that sets the current warnings can also be removed.

[ti-srebot]

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