Remove global variables and fix racy test (#18733)

https://community.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=65b44de0-1a9c-4643-84c1-5d803593f314

This PR improves on two things at once:
- It fixes the race condition by removing global variables.
- But instead of moving them to Server, it moves them inside Channels,
thereby making one more step towards the multi-product architecture.

```release-note
NONE
```

app/channels.go

@@ -4,6 +4,7 @@
 package app
 
 import (
+	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/services/imageproxy"
 )
 
@@ -12,6 +13,13 @@ type Channels struct {
 	srv *Server
 
 	imageProxy *imageproxy.ImageProxy
+
+	// cached counts that are used during notice condition validation
+	cachedPostCount   int64
+	cachedUserCount   int64
+	cachedDBMSVersion string
+	// previously fetched notices
+	cachedNotices model.ProductNotices
 }
 
 func init() {

app/product_notices.go

@@ -29,14 +29,6 @@ const MinSecondsBetweenRepeatViewings = 60 * 60
 // http request cache
 var noticesCache = utils.RequestCache{}
 
-// cached counts that are used during notice condition validation
-var cachedPostCount int64
-var cachedUserCount int64
-
-var cachedDBMSVersion string
-
-// previously fetched notices
-var cachedNotices model.ProductNotices
 var rcStripRegexp = regexp.MustCompile(`(.*?)(-rc\d+)(.*?)`)
 
 func cleanupVersion(originalVersion string) string {
@@ -269,7 +261,7 @@ func (a *App) GetProductNotices(c *request.Context, userID, teamID string, clien
 
 	filteredNotices := make([]model.NoticeMessage, 0)
 
-	for noticeIndex, notice := range cachedNotices {
+	for noticeIndex, notice := range a.ch.cachedNotices {
 		// check if the notice has been viewed already
 		var view *model.ProductNoticeViewState
 		for viewIndex, v := range views {
@@ -297,17 +289,17 @@ func (a *App) GetProductNotices(c *request.Context, userID, teamID string, clien
 			userID,
 			client,
 			clientVersion,
-			cachedPostCount,
-			cachedUserCount,
+			a.ch.cachedPostCount,
+			a.ch.cachedUserCount,
 			isSystemAdmin,
 			isTeamAdmin,
 			isCloud,
 			sku,
 			dbName,
-			cachedDBMSVersion,
+			a.ch.cachedDBMSVersion,
 			searchEngineName,
 			searchEngineVersion,
-			&cachedNotices[noticeIndex])
+			&a.ch.cachedNotices[noticeIndex])
 		if err != nil {
 			return nil, model.NewAppError("GetProductNotices", "api.system.update_notices.validating_failed", nil, err.Error(), http.StatusBadRequest)
 		}
@@ -337,7 +329,7 @@ func (a *App) UpdateViewedProductNotices(userID string, noticeIds []string) *mod
 // user as viewed in order to avoid showing them imminently on first login
 func (a *App) UpdateViewedProductNoticesForNewUser(userID string) {
 	var noticeIds []string
-	for _, notice := range cachedNotices {
+	for _, notice := range a.ch.cachedNotices {
 		noticeIds = append(noticeIds, notice.ID)
 	}
 	if err := a.Srv().Store.ProductNotices().View(userID, noticeIds); err != nil {
@@ -351,33 +343,33 @@ func (a *App) UpdateProductNotices() *model.AppError {
 	skip := *a.Srv().Config().AnnouncementSettings.NoticesSkipCache
 	mlog.Debug("Will fetch notices from", mlog.String("url", url), mlog.Bool("skip_cache", skip))
 	var err error
-	cachedPostCount, err = a.Srv().Store.Post().AnalyticsPostCount("", false, false)
+	a.ch.cachedPostCount, err = a.Srv().Store.Post().AnalyticsPostCount("", false, false)
 	if err != nil {
 		mlog.Warn("Failed to fetch post count", mlog.String("error", err.Error()))
 	}
 
-	cachedUserCount, err = a.Srv().Store.User().Count(model.UserCountOptions{IncludeDeleted: true})
+	a.ch.cachedUserCount, err = a.Srv().Store.User().Count(model.UserCountOptions{IncludeDeleted: true})
 	if err != nil {
 		mlog.Warn("Failed to fetch user count", mlog.String("error", err.Error()))
 	}
 
-	cachedDBMSVersion, err = a.Srv().Store.GetDbVersion(false)
+	a.ch.cachedDBMSVersion, err = a.Srv().Store.GetDbVersion(false)
 	if err != nil {
 		mlog.Warn("Failed to get DBMS version", mlog.String("error", err.Error()))
 	}
 
-	cachedDBMSVersion = strings.Split(cachedDBMSVersion, " ")[0] // get rid of trailing strings attached to the version
+	a.ch.cachedDBMSVersion = strings.Split(a.ch.cachedDBMSVersion, " ")[0] // get rid of trailing strings attached to the version
 
 	data, err := utils.GetURLWithCache(url, &noticesCache, skip)
 	if err != nil {
 		return model.NewAppError("UpdateProductNotices", "api.system.update_notices.fetch_failed", nil, err.Error(), http.StatusBadRequest)
 	}
-	cachedNotices, err = model.UnmarshalProductNotices(data)
+	a.ch.cachedNotices, err = model.UnmarshalProductNotices(data)
 	if err != nil {
 		return model.NewAppError("UpdateProductNotices", "api.system.update_notices.parse_failed", nil, err.Error(), http.StatusBadRequest)
 	}
 
-	if err := a.Srv().Store.ProductNotices().ClearOldNotices(cachedNotices); err != nil {
+	if err := a.Srv().Store.ProductNotices().ClearOldNotices(a.ch.cachedNotices); err != nil {
 		return model.NewAppError("UpdateProductNotices", "api.system.update_notices.clear_failed", nil, err.Error(), http.StatusBadRequest)
 	}
 	return nil