Delete Labels & IssueLabels on Repo Delete too (#15039)

* Doctor: find IssueLabels without existing label

* on Repo Delete: delete labels & issue_labels too

* performance nits

* Add Migration: Delete orphaned IssueLabels

* Migration v174: use Sync2

* USE sess !!!

* better func name

* code format & comment

* RAW SQL

* Update models/migrations/v176.go

* next try?

models/migrations/v177.go

@@ -0,0 +1,42 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"fmt"
+
+	"xorm.io/xorm"
+)
+
+func deleteOrphanedIssueLabels(x *xorm.Engine) error {
+	type IssueLabel struct {
+		ID      int64 `xorm:"pk autoincr"`
+		IssueID int64 `xorm:"UNIQUE(s)"`
+		LabelID int64 `xorm:"UNIQUE(s)"`
+	}
+
+	sess := x.NewSession()
+	defer sess.Close()
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
+	if err := sess.Sync2(new(IssueLabel)); err != nil {
+		return fmt.Errorf("Sync2: %v", err)
+	}
+
+	if _, err := sess.Exec(`DELETE FROM issue_label WHERE issue_label.id IN (
+		SELECT ill.id FROM (
+			SELECT il.id
+			FROM issue_label AS il
+				LEFT JOIN label ON il.label_id = label.id
+			WHERE
+				label.id IS NULL
+		) AS ill)`); err != nil {
+		return err
+	}
+
+	return sess.Commit()
+}