Restart zero worker if there is still work to do (#18658) (#18672)

* Restart zero worker if there is still work to do (#18658)

Backport #18658

It is possible for the zero worker to timeout before all the work is finished.
This may mean that work may take a long time to complete because a worker will only
be induced on repushing.

Also ensure that requested count is reset after pulls and push mirror sync requests and add some more trace logging to the queue push.

Fix #18607

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

* Update modules/queue/workerpool.go

modules/queue/workerpool.go

@@ -87,6 +87,20 @@ func (p *WorkerPool) Push(data Data) {
 	}
 }
 
+// HasNoWorkerScaling will return true if the queue has no workers, and has no worker boosting
+func (p *WorkerPool) HasNoWorkerScaling() bool {
+	p.lock.Lock()
+	defer p.lock.Unlock()
+	return p.hasNoWorkerScaling()
+}
+
+func (p *WorkerPool) hasNoWorkerScaling() bool {
+	return p.numberOfWorkers == 0 && (p.boostTimeout == 0 || p.boostWorkers == 0 || p.maxNumberOfWorkers == 0)
+}
+
+// zeroBoost will add a temporary boost worker for a no worker queue
+// p.lock must be locked at the start of this function BUT it will be unlocked by the end of this function
+// (This is because addWorkers has to be called whilst unlocked)
 func (p *WorkerPool) zeroBoost() {
 	ctx, cancel := context.WithTimeout(p.baseCtx, p.boostTimeout)
 	mq := GetManager().GetManagedQueue(p.qid)
@@ -277,6 +291,21 @@ func (p *WorkerPool) addWorkers(ctx context.Context, cancel context.CancelFunc,
 				p.cond.Broadcast()
 				cancel()
 			}
+
+			select {
+			case <-p.baseCtx.Done():
+				// Don't warn if the baseCtx is shutdown
+			default:
+				if p.hasNoWorkerScaling() {
+					log.Warn(
+						"Queue: %d is configured to be non-scaling and has no workers - this configuration is likely incorrect.", p.qid)
+				} else if p.numberOfWorkers == 0 && atomic.LoadInt64(&p.numInQueue) > 0 {
+					// OK there are no workers but... there's still work to be done -> Reboost
+					p.zeroBoost()
+					// p.lock will be unlocked by zeroBoost
+					return
+				}
+			}
 			p.lock.Unlock()
 		}()
 	}