Move login related structs and functions to models/login (#17093)

* Move login related structs and functions to models/login

* Fix test

* Fix lint

* Fix lint

* Fix lint of windows

* Fix lint

* Fix test

* Fix test

* Only load necessary fixtures when preparing unit tests envs

* Fix lint

* Fix test

* Fix test

* Fix error log

* Fix error log

* Fix error log

* remove unnecessary change

* fix error log

* merge main branch

services/auth/login_source.go

@@ -0,0 +1,41 @@
+// 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 auth
+
+import (
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/login"
+)
+
+// DeleteLoginSource deletes a LoginSource record in DB.
+func DeleteLoginSource(source *login.Source) error {
+	count, err := db.GetEngine(db.DefaultContext).Count(&models.User{LoginSource: source.ID})
+	if err != nil {
+		return err
+	} else if count > 0 {
+		return login.ErrSourceInUse{
+			ID: source.ID,
+		}
+	}
+
+	count, err = db.GetEngine(db.DefaultContext).Count(&models.ExternalLoginUser{LoginSourceID: source.ID})
+	if err != nil {
+		return err
+	} else if count > 0 {
+		return login.ErrSourceInUse{
+			ID: source.ID,
+		}
+	}
+
+	if registerableSource, ok := source.Cfg.(login.RegisterableSource); ok {
+		if err := registerableSource.UnregisterSource(); err != nil {
+			return err
+		}
+	}
+
+	_, err = db.GetEngine(db.DefaultContext).ID(source.ID).Delete(new(login.Source))
+	return err
+}