Refactor update checker to use AppState (#17387)

We have the `AppState` module now, it can store app related data easily. We do not need to create separate tables for each feature.

So the update checker can use `AppState` instead of a one-row dedicate table.

And the code of update checker is moved from `models` to `modules`.

models/migrations/migrations.go

@@ -351,9 +351,11 @@ var migrations = []Migration{
 	// v198 -> v199
 	NewMigration("Add issue content history table", addTableIssueContentHistory),
 	// v199 -> v200
-	NewMigration("Add remote version table", addRemoteVersionTable),
+	NewMigration("No-op (remote version is using AppState now)", addRemoteVersionTableNoop),
 	// v200 -> v201
 	NewMigration("Add table app_state", addTableAppState),
+	// v201 -> v202
+	NewMigration("Drop table remote_version (if exists)", dropTableRemoteVersion),
 }
 
 // GetCurrentDBVersion returns the current db version

models/migrations/v199.go

@@ -5,19 +5,10 @@
 package migrations
 
 import (
-	"fmt"
-
 	"xorm.io/xorm"
 )
 
-func addRemoteVersionTable(x *xorm.Engine) error {
-	type RemoteVersion struct {
-		ID      int64  `xorm:"pk autoincr"`
-		Version string `xorm:"VARCHAR(50)"`
-	}
-
-	if err := x.Sync2(new(RemoteVersion)); err != nil {
-		return fmt.Errorf("Sync2: %v", err)
-	}
+func addRemoteVersionTableNoop(x *xorm.Engine) error {
+	// we used to use a table `remote_version` to store information for updater, now we use `AppState`, so this migration task is a no-op now.
 	return nil
 }

models/migrations/v201.go

@@ -0,0 +1,15 @@
+// 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 (
+	"xorm.io/xorm"
+)
+
+func dropTableRemoteVersion(x *xorm.Engine) error {
+	// drop the orphaned table introduced in `v199`, now the update checker also uses AppState, do not need this table
+	_ = x.DropTables("remote_version")
+	return nil
+}