varsutil: tiny cleanup (#2884)

Extract a function to check if the option is on.

sessionctx/varsutil/varsutil.go

@@ -108,24 +108,29 @@ func SetSessionSystemVar(vars *variable.SessionVars, name string, value types.Da
 			return errors.Trace(err)
 		}
 	case variable.AutocommitVar:
-		isAutocommit := strings.EqualFold(sVal, "ON") || sVal == "1"
+		isAutocommit := tidbOptOn(sVal)
 		vars.SetStatusFlag(mysql.ServerStatusAutocommit, isAutocommit)
 		if isAutocommit {
 			vars.SetStatusFlag(mysql.ServerStatusInTrans, false)
 		}
 	case variable.TiDBSkipConstraintCheck:
-		vars.SkipConstraintCheck = (sVal == "1")
+		vars.SkipConstraintCheck = tidbOptOn(sVal)
 	case variable.TiDBSkipDDLWait:
-		vars.SkipDDLWait = (sVal == "1")
+		vars.SkipDDLWait = tidbOptOn(sVal)
 	case variable.TiDBOptAggPushDown:
-		vars.AllowAggPushDown = strings.EqualFold(sVal, "ON") || sVal == "1"
+		vars.AllowAggPushDown = tidbOptOn(sVal)
 	case variable.TiDBOptInSubqUnFolding:
-		vars.AllowInSubqueryUnFolding = strings.EqualFold(sVal, "ON") || sVal == "1"
+		vars.AllowInSubqueryUnFolding = tidbOptOn(sVal)
 	}
 	vars.Systems[name] = sVal
 	return nil
 }
 
+// For all tidb session variable options, we use "ON"/1 to turn on the options.
+func tidbOptOn(opt string) bool {
+	return strings.EqualFold(opt, "ON") || opt == "1"
+}
+
 func parseTimeZone(s string) *time.Location {
 	if s == "SYSTEM" {
 		// TODO: Support global time_zone variable, it should be set to global time_zone value.

sessionctx/varsutil/varsutil_test.go

@@ -33,6 +33,28 @@ var _ = Suite(&testVarsutilSuite{})
 type testVarsutilSuite struct {
 }
 
+func (s *testVarsutilSuite) TestTiDBOptOn(c *C) {
+	defer testleak.AfterTest(c)()
+	tbl := []struct {
+		val string
+		on  bool
+	}{
+		{"ON", true},
+		{"on", true},
+		{"On", true},
+		{"1", true},
+		{"off", false},
+		{"No", false},
+		{"0", false},
+		{"1.1", false},
+		{"", false},
+	}
+	for _, t := range tbl {
+		on := tidbOptOn(t.val)
+		c.Assert(on, Equals, t.on)
+	}
+}
+
 func (s *testVarsutilSuite) TestVarsutil(c *C) {
 	defer testleak.AfterTest(c)()
 	v := variable.NewSessionVars()