*: add scope check when get system variables

ast/expressions.go

@@ -944,6 +944,8 @@ type VariableExpr struct {
 	IsGlobal bool
 	// IsSystem indicates whether this variable is a system variable in current session.
 	IsSystem bool
+	// ExplicitScope indicates whether this variable scope is explicit set.
+	ExplicitScope bool
 	// Value is the variable value.
 	Value ExprNode
 }

parser/parser.y

@@ -5008,6 +5008,7 @@ SystemVariable:
 	{
 		v := strings.ToLower($1)
 		var isGlobal bool
+	    explicitScope := true
 		if strings.HasPrefix(v, "@@global.") {
 			isGlobal = true
 			v = strings.TrimPrefix(v, "@@global.")
@@ -5016,9 +5017,9 @@ SystemVariable:
 		} else if strings.HasPrefix(v, "@@local.") {
 			v = strings.TrimPrefix(v, "@@local.")
 		} else if strings.HasPrefix(v, "@@") {
-			v = strings.TrimPrefix(v, "@@")
+			v, explicitScope = strings.TrimPrefix(v, "@@"), false
 		}
-		$$ = &ast.VariableExpr{Name: v, IsGlobal: isGlobal, IsSystem: true}
+		$$ = &ast.VariableExpr{Name: v, IsGlobal: isGlobal, IsSystem: true, ExplicitScope: explicitScope}
 	}
 
 UserVariable:

plan/expression_rewriter.go

@@ -781,6 +781,13 @@ func (er *expressionRewriter) rewriteVariable(v *ast.VariableExpr) {
 	}
 	var val string
 	var err error
+	if v.ExplicitScope {
+		err = variable.ValidateGetSystemVar(name, v.IsGlobal)
+		if err != nil {
+			er.err = errors.Trace(err)
+			return
+		}
+	}
 	if v.IsGlobal {
 		val, err = variable.GetGlobalSystemVar(sessionVars, name)
 	} else {

session/session_test.go

@@ -371,6 +371,33 @@ func (s *testSessionSuite) TestGlobalVarAccessor(c *C) {
 	tk.MustExec("set @@global.autocommit=1")
 }
 
+func (s *testSessionSuite) TestGetSysVariables(c *C) {
+	tk := testkit.NewTestKitWithInit(c, s.store)
+
+	// Test ScopeSession
+	tk.MustExec("select @@warning_count")
+	tk.MustExec("select @@session.warning_count")
+	tk.MustExec("select @@local.warning_count")
+	_, err := tk.Exec("select @@global.warning_count")
+	c.Assert(terror.ErrorEqual(err, variable.ErrIncorrectScope), IsTrue)
+
+	// Test ScopeGlobal
+	tk.MustExec("select @@max_connections")
+	tk.MustExec("select @@global.max_connections")
+	_, err = tk.Exec("select @@session.max_connections")
+	c.Assert(terror.ErrorEqual(err, variable.ErrIncorrectScope), IsTrue)
+	_, err = tk.Exec("select @@local.max_connections")
+	c.Assert(terror.ErrorEqual(err, variable.ErrIncorrectScope), IsTrue)
+
+	// Test ScopeNone
+	tk.MustExec("select @@performance_schema_max_mutex_classes")
+	tk.MustExec("select @@global.performance_schema_max_mutex_classes")
+	_, err = tk.Exec("select @@session.performance_schema_max_mutex_classes")
+	c.Assert(terror.ErrorEqual(err, variable.ErrIncorrectScope), IsTrue)
+	_, err = tk.Exec("select @@local.performance_schema_max_mutex_classes")
+	c.Assert(terror.ErrorEqual(err, variable.ErrIncorrectScope), IsTrue)
+}
+
 func (s *testSessionSuite) TestRetryResetStmtCtx(c *C) {
 	tk := testkit.NewTestKitWithInit(c, s.store)
 	tk.MustExec("create table retrytxn (a int unique, b int)")

sessionctx/variable/sysvar.go

@@ -69,7 +69,7 @@ const (
 var (
 	UnknownStatusVar   = terror.ClassVariable.New(CodeUnknownStatusVar, "unknown status variable")
 	UnknownSystemVar   = terror.ClassVariable.New(CodeUnknownSystemVar, "unknown system variable '%s'")
-	ErrIncorrectScope  = terror.ClassVariable.New(CodeIncorrectScope, "Incorrect variable scope")
+	ErrIncorrectScope  = terror.ClassVariable.New(CodeIncorrectScope, "Variable '%s' is a %s variable")
 	ErrUnknownTimeZone = terror.ClassVariable.New(CodeUnknownTimeZone, "unknown or incorrect time zone: %s")
 	ErrReadOnly        = terror.ClassVariable.New(CodeReadOnly, "variable is read only")
 )

sessionctx/variable/varsutil.go

@@ -137,6 +137,25 @@ func SetSessionSystemVar(vars *SessionVars, name string, value types.Datum) erro
 	return vars.SetSystemVar(name, sVal)
 }
 
+// ValidateGetSystemVar check if system variable exists and validate it's scope when get system variable.
+func ValidateGetSystemVar(name string, isGlobal bool) error {
+	sysVar, exists := SysVars[name]
+	if !exists {
+		return UnknownSystemVar.GenByArgs(name)
+	}
+	switch sysVar.Scope {
+	case ScopeGlobal, ScopeNone:
+		if !isGlobal {
+			return ErrIncorrectScope.GenByArgs(name, "GLOBAL")
+		}
+	case ScopeSession:
+		if isGlobal {
+			return ErrIncorrectScope.GenByArgs(name, "SESSION")
+		}
+	}
+	return nil
+}
+
 // TiDBOptOn could be used for all tidb session variable options, we use "ON"/1 to turn on those options.
 func TiDBOptOn(opt string) bool {
 	return strings.EqualFold(opt, "ON") || opt == "1"