nfd-updater: events: enable timer-only flow

By default the nfd-topology-updater has state-directories
notification enabled by default. In theory, we can have only
timer-based updates, but if the option is given to disable the
state-directories event source, then all the update mechanism
is mistakenly disabled, including the timer-based updates.

The two updaters mechanism should be decoupled.
So this PR changes this to make sure we can enable just and only
the timer-based updates.

Signed-off-by: Francesco Romani <[email protected]>

pkg/nfd-topology-updater/kubeletnotifier/kubeletnotifier.go

@@ -54,11 +54,14 @@ type Info struct {
 }
 
 func New(sleepInterval time.Duration, dest chan<- Info, kubeletStateDir string) (*Notifier, error) {
-	devicePluginsDir := path.Join(kubeletStateDir, devicePluginsDirName)
-	ch, err := createFSWatcherEvent([]string{kubeletStateDir, devicePluginsDir})
-	if err != nil {
-		return nil, err
+	if kubeletStateDir != "" {
+		devicePluginsDir := path.Join(kubeletStateDir, devicePluginsDirName)
+		ch, err := createFSWatcherEvent([]string{kubeletStateDir, devicePluginsDir})
+		if err != nil {
+			return nil, err
+		}
 	}
+
 	return &Notifier{
 		sleepInterval: sleepInterval,
 		dest:          dest,
@@ -67,13 +70,17 @@ func New(sleepInterval time.Duration, dest chan<- Info, kubeletStateDir string)
 }
 
 func (n *Notifier) Run() {
-	timeEvents := make(<-chan time.Time)
+	var timeEvents <-chan time.Time
+
 	if n.sleepInterval > 0 {
 		ticker := time.NewTicker(n.sleepInterval)
 		defer ticker.Stop()
 		timeEvents = ticker.C
 	}
 
+	// it's safe to keep the channels we don't need nil:
+	// https://dave.cheney.net/2014/03/19/channel-axioms
+	// "A receive from a nil channel blocks forever"
 	for {
 		select {
 		case <-timeEvents:

pkg/nfd-topology-updater/nfd-topology-updater.go

@@ -85,13 +85,12 @@ type nfdTopologyUpdater struct {
 // NewTopologyUpdater creates a new NfdTopologyUpdater instance.
 func NewTopologyUpdater(args Args, resourcemonitorArgs resourcemonitor.Args) (NfdTopologyUpdater, error) {
 	eventSource := make(chan kubeletnotifier.Info)
-	if args.KubeletStateDir != "" {
-		ntf, err := kubeletnotifier.New(resourcemonitorArgs.SleepInterval, eventSource, args.KubeletStateDir)
-		if err != nil {
-			return nil, err
-		}
-		go ntf.Run()
+
+	ntf, err := kubeletnotifier.New(resourcemonitorArgs.SleepInterval, eventSource, args.KubeletStateDir)
+	if err != nil {
+		return nil, err
 	}
+	go ntf.Run()
 
 	kubeletConfigFunc, err := getKubeletConfigFunc(resourcemonitorArgs.KubeletConfigURI, resourcemonitorArgs.APIAuthTokenFile)
 	if err != nil {