Extracting html templates into a library (#16946)

* Extracting html templates into a library

* Moving tests to the right place

* Fixing tests

* Addressing PR review comments

* Addressing PR review comments

* Replacing attomic with RWMutex

* Returning errors as channel for Templates watcher

* Address PR review comments

* Other small fixes

* Simplifying NewWithWatcher

* Addressing PR review comments

* Making error handling on rendering templates more robust

* Fixing tests

* Changing how we return errors

* Fixing shadow variables

* Addressing PR review comments

* Logging errors from the outside of sendNotificationEmail

* Fixing lock in shutdown

* Fixing the resource copy for commands tests temporary directories

* Removing unused import

* A couple of tiny fixes

app/server.go

@@ -58,6 +58,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/shared/filestore"
 	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v5/shared/templates"
 	"github.com/mattermost/mattermost-server/v5/store"
 	"github.com/mattermost/mattermost-server/v5/store/localcachelayer"
 	"github.com/mattermost/mattermost-server/v5/store/retrylayer"
@@ -65,6 +66,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/store/sqlstore"
 	"github.com/mattermost/mattermost-server/v5/store/timerlayer"
 	"github.com/mattermost/mattermost-server/v5/utils"
+	"github.com/mattermost/mattermost-server/v5/utils/fileutils"
 )
 
 var MaxNotificationsPerChannelDefault int64 = 1000000
@@ -133,7 +135,7 @@ type Server struct {
 
 	newStore func() (store.Store, error)
 
-	htmlTemplateWatcher     *utils.HTMLTemplateWatcher
+	htmlTemplateWatcher     *templates.Container
 	sessionCache            cache.Cache
 	seenPendingPostIdsCache cache.Cache
 	statusCache             cache.Cache
@@ -391,9 +393,19 @@ func NewServer(options ...Option) (*Server, error) {
 		}
 	}
 
-	if htmlTemplateWatcher, err2 := utils.NewHTMLTemplateWatcher("templates"); err2 != nil {
-		mlog.Error("Failed to parse server templates", mlog.Err(err2))
+	templatesDir, ok := fileutils.FindDir("templates")
+	if !ok {
+		mlog.Error("Failed find server templates", mlog.String("directory", "templates"))
 	} else {
+		htmlTemplateWatcher, errorsChan, err2 := templates.NewWithWatcher(templatesDir)
+		if err2 != nil {
+			return nil, errors.Wrap(err2, "cannot initialize server templates")
+		}
+		s.Go(func() {
+			for err2 := range errorsChan {
+				mlog.Warn("Server templates error", mlog.Err(err2))
+			}
+		})
 		s.htmlTemplateWatcher = htmlTemplateWatcher
 	}
 
@@ -865,13 +877,10 @@ func (s *Server) Shutdown() {
 	// Push notification hub needs to be shutdown after HTTP server
 	// to prevent stray requests from generating a push notification after it's shut down.
 	s.StopPushNotificationsHubWorkers()
+	s.htmlTemplateWatcher.Close()
 
 	s.WaitForGoroutines()
 
-	if s.htmlTemplateWatcher != nil {
-		s.htmlTemplateWatcher.Close()
-	}
-
 	if s.advancedLogListenerCleanup != nil {
 		s.advancedLogListenerCleanup()
 		s.advancedLogListenerCleanup = nil