DRA taint eviction: avoid nil panic

The timed worker queue actually can have nil entries in its map if the work was
kicked off immediately. This looks like an unnecessary special case (it would
be fine to call AfterFunc with a duration <= 0 and it would do the right
thing), but to avoid more sweeping changes the fix consists of documenting this
special behavior and adding a nil check.

pkg/controller/tainteviction/timed_workers.go

@@ -57,6 +57,7 @@ type TimedWorker struct {
 }
 
 // createWorker creates a TimedWorker that will execute `f` not earlier than `fireAt`.
+// Returns nil if the work was started immediately and doesn't need a timer.
 func createWorker(ctx context.Context, args *WorkArgs, createdAt time.Time, fireAt time.Time, f func(ctx context.Context, fireAt time.Time, args *WorkArgs) error, clock clock.WithDelayedExecution) *TimedWorker {
 	delay := fireAt.Sub(createdAt)
 	logger := klog.FromContext(ctx)
@@ -90,6 +91,7 @@ func (w *TimedWorker) Cancel() {
 type TimedWorkerQueue struct {
 	sync.Mutex
 	// map of workers keyed by string returned by 'KeyFromWorkArgs' from the given worker.
+	// Entries may be nil if the work didn't need a timer and is already running.
 	workers  map[string]*TimedWorker
 	workFunc func(ctx context.Context, fireAt time.Time, args *WorkArgs) error
 	clock    clock.WithDelayedExecution
@@ -145,6 +147,10 @@ func (q *TimedWorkerQueue) UpdateWork(ctx context.Context, args *WorkArgs, creat
 	q.Lock()
 	defer q.Unlock()
 	if worker, exists := q.workers[key]; exists {
+		if worker == nil {
+			logger.V(4).Info("Keeping existing work, already in progress", "item", key)
+			return
+		}
 		if worker.FireAt.Compare(fireAt) == 0 {
 			logger.V(4).Info("Keeping existing work, same time", "item", key, "createTime", worker.CreatedAt, "firedTime", worker.FireAt)
 			return