refactor: run the redirect simulation over a rule list

CheckRedirect loads the stored rules and hands them to
checkRedirectRules, which needs no database, so the loop and chain rules
can be unit-tested. No behaviour change.

classes/redirect_graph.go

@@ -166,13 +166,22 @@ func classHas(ranges []rune, c rune) bool {
 // chain when it takes more than one hop or when another rule's target
 // leads into r. A 410 rule ends every chain and is never a loop.
 func (s *Service) CheckRedirect(ctx context.Context, db *gorm.DB, r models.Redirect) (RedirectCheck, error) {
-	var check RedirectCheck
 	if r.Status == models.Status410 {
-		return check, nil
+		return RedirectCheck{}, nil
 	}
 	rules, err := LoadRedirects(db.WithContext(ctx))
 	if err != nil {
-		return check, err
+		return RedirectCheck{}, err
+	}
+	return checkRedirectRules(s.SiteURL(), rules, r)
+}
+
+// checkRedirectRules is CheckRedirect over the stored rules given, for the
+// site URL site.
+func checkRedirectRules(site string, rules []models.Redirect, r models.Redirect) (RedirectCheck, error) {
+	var check RedirectCheck
+	if r.Status == models.Status410 {
+		return check, nil
 	}
 	if r.ID == 0 {
 		r.ID = candidateID
@@ -187,7 +196,6 @@ func (s *Service) CheckRedirect(ctx context.Context, db *gorm.DB, r models.Redir
 		rules = append(rules, r)
 	}
 	set := NewRedirectSet(rules)
-	site := s.SiteURL()
 	path, query, ok := redirectProbe(r)
 	if ok {
 		ch, err := set.Follow(site, path, query, MaxRedirectHops)