add a precondition before heap operations

updater.go

@@ -86,17 +86,19 @@ func (h *updateHeap) updateTask() {
 		h.mu.Lock()
 		hlen := h.Len()
 		now := time.Now()
-		for i := 0; i < hlen; i++ {
-			entry := heap.Pop(h).(entry)
-			if now.After(entry.ts) {
-				entry.ts = now.Add(entry.s.update())
-				heap.Push(h, entry)
-			} else {
-				heap.Push(h, entry)
-				break
+		if hlen > 0 && now.After(h.entries[0].ts) {
+			for i := 0; i < hlen; i++ {
+				entry := heap.Pop(h).(entry)
+				if now.After(entry.ts) {
+					entry.ts = now.Add(entry.s.update())
+					heap.Push(h, entry)
+				} else {
+					heap.Push(h, entry)
+					break
+				}
 			}
 		}
-		if h.Len() > 0 {
+		if hlen > 0 {
 			timer = time.After(h.entries[0].ts.Sub(now))
 		}
 		h.mu.Unlock()