Improve mirror iterator (#18928)

* Improve mirror iterator

* fix test

models/repo/pushmirror.go

@@ -101,10 +101,11 @@ func GetPushMirrorsByRepoID(repoID int64) ([]*PushMirror, error) {
 }
 
 // PushMirrorsIterate iterates all push-mirror repositories.
-func PushMirrorsIterate(f func(idx int, bean interface{}) error) error {
+func PushMirrorsIterate(limit int, f func(idx int, bean interface{}) error) error {
 	return db.GetEngine(db.DefaultContext).
 		Where("last_update + (`interval` / ?) <= ?", time.Second, time.Now().Unix()).
 		And("`interval` != 0").
 		OrderBy("last_update ASC").
+		Limit(limit).
 		Iterate(new(PushMirror), f)
 }