Exponential Backoff for ByteFIFO (#15724)

This PR is another in the vein of queue improvements. It suggests an
exponential backoff for bytefifo queues to reduce the load from queue
polling. This will mostly be useful for redis queues.

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: Lauris BH <lauris@nix.lv>

modules/queue/queue_bytefifo.go

@@ -114,41 +114,71 @@ func (q *ByteFIFOQueue) Run(atShutdown, atTerminate func(context.Context, func()
 }
 
 func (q *ByteFIFOQueue) readToChan() {
-	for {
+	// handle quick cancels
 	select {
 	case <-q.closed:
 		// tell the pool to shutdown.
 		q.cancel()
 		return
 	default:
-			q.lock.Lock()
-			bs, err := q.byteFIFO.Pop()
-			if err != nil {
-				q.lock.Unlock()
-				log.Error("%s: %s Error on Pop: %v", q.typ, q.name, err)
-				time.Sleep(time.Millisecond * 100)
-				continue
 	}
 
-			if len(bs) == 0 {
-				q.lock.Unlock()
-				time.Sleep(time.Millisecond * 100)
-				continue
+	backOffTime := time.Millisecond * 100
+	maxBackOffTime := time.Second * 3
+	for {
+		success, resetBackoff := q.doPop()
+		if resetBackoff {
+			backOffTime = 100 * time.Millisecond
 		}
 
+		if success {
+			select {
+			case <-q.closed:
+				// tell the pool to shutdown.
+				q.cancel()
+				return
+			default:
+			}
+		} else {
+			select {
+			case <-q.closed:
+				// tell the pool to shutdown.
+				q.cancel()
+				return
+			case <-time.After(backOffTime):
+			}
+			backOffTime += backOffTime / 2
+			if backOffTime > maxBackOffTime {
+				backOffTime = maxBackOffTime
+			}
+		}
+	}
+}
+
+func (q *ByteFIFOQueue) doPop() (success, resetBackoff bool) {
+	q.lock.Lock()
+	defer q.lock.Unlock()
+	bs, err := q.byteFIFO.Pop()
+	if err != nil {
+		log.Error("%s: %s Error on Pop: %v", q.typ, q.name, err)
+		return
+	}
+	if len(bs) == 0 {
+		return
+	}
+
+	resetBackoff = true
+
 	data, err := unmarshalAs(bs, q.exemplar)
 	if err != nil {
 		log.Error("%s: %s Failed to unmarshal with error: %v", q.typ, q.name, err)
-				q.lock.Unlock()
-				time.Sleep(time.Millisecond * 100)
-				continue
+		return
 	}
 
 	log.Trace("%s %s: Task found: %#v", q.typ, q.name, data)
 	q.WorkerPool.Push(data)
-			q.lock.Unlock()
-		}
-	}
+	success = true
+	return
 }
 
 // Shutdown processing from this queue