sessionctx: refactor set to be in struct (#23383)

sessionctx/variable/session.go

@@ -1351,7 +1351,9 @@ func (s *SessionVars) ClearStmtVars() {
 	s.stmtVars = make(map[string]string)
 }
 
-// SetSystemVar sets the value of a system variable.
+// SetSystemVar sets the value of a system variable for session scope.
+// Validation has already been performed, and the values have been normalized.
+// i.e. oN / on / 1 => ON
 func (s *SessionVars) SetSystemVar(name string, val string) error {
 	switch name {
 	case TxnIsolationOneShot:
@@ -1750,8 +1752,6 @@ func (s *SessionVars) SetSystemVar(name string, val string) error {
 		s.EnableIndexMergeJoin = TiDBOptOn(val)
 	case TiDBTrackAggregateMemoryUsage:
 		s.TrackAggregateMemoryUsage = TiDBOptOn(val)
-	case TiDBMultiStatementMode:
-		s.MultiStatementMode = TiDBOptMultiStmt(val)
 	case TiDBEnableExchangePartition:
 		s.TiDBEnableExchangePartition = TiDBOptOn(val)
 	case TiDBAllowFallbackToTiKV:
@@ -1764,6 +1764,11 @@ func (s *SessionVars) SetSystemVar(name string, val string) error {
 		}
 	case TiDBIntPrimaryKeyDefaultAsClustered:
 		s.IntPrimaryKeyDefaultAsClustered = TiDBOptOn(val)
+	default:
+		sv := GetSysVar(name)
+		if err := sv.SetSessionFromHook(s, val); err != nil {
+			return err
+		}
 	}
 	s.systems[name] = val
 	return nil

sessionctx/variable/sysvar.go

@@ -111,10 +111,20 @@ type SysVar struct {
 	AllowAutoValue bool
 	// Validation is a callback after the type validation has been performed
 	Validation func(*SessionVars, string, string, ScopeFlag) (string, error)
+	// SetSession is called after validation
+	SetSession func(*SessionVars, string) error
 	// IsHintUpdatable indicate whether it's updatable via SET_VAR() hint (optional)
 	IsHintUpdatable bool
 }
 
+// SetSessionFromHook calls the SetSession func if it exists.
+func (sv *SysVar) SetSessionFromHook(s *SessionVars, val string) error {
+	if sv.SetSession != nil {
+		return sv.SetSession(s, val)
+	}
+	return nil
+}
+
 // ValidateFromType provides automatic validation based on the SysVar's type
 func (sv *SysVar) ValidateFromType(vars *SessionVars, value string, scope ScopeFlag) (string, error) {
 	// Some sysvars are read-only. Attempting to set should always fail.
@@ -804,7 +814,10 @@ var defaultSysVars = []*SysVar{
 	}},
 	{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnableIndexMergeJoin, Value: BoolToOnOff(DefTiDBEnableIndexMergeJoin), Type: TypeBool},
 	{Scope: ScopeGlobal | ScopeSession, Name: TiDBTrackAggregateMemoryUsage, Value: BoolToOnOff(DefTiDBTrackAggregateMemoryUsage), Type: TypeBool},
-	{Scope: ScopeGlobal | ScopeSession, Name: TiDBMultiStatementMode, Value: Off, Type: TypeEnum, PossibleValues: []string{Off, On, Warn}},
+	{Scope: ScopeGlobal | ScopeSession, Name: TiDBMultiStatementMode, Value: Off, Type: TypeEnum, PossibleValues: []string{Off, On, Warn}, SetSession: func(s *SessionVars, val string) error {
+		s.MultiStatementMode = TiDBOptMultiStmt(val)
+		return nil
+	}},
 	{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnableExchangePartition, Value: BoolToOnOff(DefTiDBEnableExchangePartition), Type: TypeBool},
 
 	/* tikv gc metrics */