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`.

modules/updatechecker/update_checker.go

@@ -0,0 +1,98 @@
+// 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 updatechecker
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"net/http"
+
+	"code.gitea.io/gitea/modules/appstate"
+	"code.gitea.io/gitea/modules/proxy"
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/hashicorp/go-version"
+)
+
+// CheckerState stores the remote version from the JSON endpoint
+type CheckerState struct {
+	LatestVersion string
+}
+
+// Name returns the name of the state item for update checker
+func (r *CheckerState) Name() string {
+	return "update-checker"
+}
+
+// GiteaUpdateChecker returns error when new version of Gitea is available
+func GiteaUpdateChecker(httpEndpoint string) error {
+	httpClient := &http.Client{
+		Transport: &http.Transport{
+			Proxy: proxy.Proxy(),
+		},
+	}
+
+	req, err := http.NewRequest("GET", httpEndpoint, nil)
+	if err != nil {
+		return err
+	}
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	body, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return err
+	}
+
+	type respType struct {
+		Latest struct {
+			Version string `json:"version"`
+		} `json:"latest"`
+	}
+	respData := respType{}
+	err = json.Unmarshal(body, &respData)
+	if err != nil {
+		return err
+	}
+
+	return UpdateRemoteVersion(respData.Latest.Version)
+
+}
+
+// UpdateRemoteVersion updates the latest available version of Gitea
+func UpdateRemoteVersion(version string) (err error) {
+	return appstate.AppState.Set(&CheckerState{LatestVersion: version})
+}
+
+// GetRemoteVersion returns the current remote version (or currently installed verson if fail to fetch from DB)
+func GetRemoteVersion() string {
+	item := new(CheckerState)
+	if err := appstate.AppState.Get(item); err != nil {
+		return ""
+	}
+	return item.LatestVersion
+}
+
+// GetNeedUpdate returns true whether a newer version of Gitea is available
+func GetNeedUpdate() bool {
+	curVer, err := version.NewVersion(setting.AppVer)
+	if err != nil {
+		// return false to fail silently
+		return false
+	}
+	remoteVerStr := GetRemoteVersion()
+	if remoteVerStr == "" {
+		// no remote version is known
+		return false
+	}
+	remoteVer, err := version.NewVersion(remoteVerStr)
+	if err != nil {
+		// return false to fail silently
+		return false
+	}
+	return curVer.LessThan(remoteVer)
+}