Only attempt to flush queue if the underlying worker pool is not finished (#18593)

* Only attempt to flush queue if the underlying worker pool is not finished

There is a possible race whereby a worker pool could be cancelled but yet the
underlying queue is not empty. This will lead to flush-all cycling because it
cannot empty the pool.

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

* Apply suggestions from code review

Co-authored-by: Gusted <williamzijl7@hotmail.com>

Co-authored-by: Gusted <williamzijl7@hotmail.com>

modules/queue/manager.go

@@ -84,6 +84,8 @@ type ManagedPool interface {
 	BoostWorkers() int
 	// SetPoolSettings sets the user updatable settings for the pool
 	SetPoolSettings(maxNumberOfWorkers, boostWorkers int, timeout time.Duration)
+	// Done returns a channel that will be closed when the Pool's baseCtx is closed
+	Done() <-chan struct{}
 }
 
 // ManagedQueueList implements the sort.Interface
@@ -211,6 +213,15 @@ func (m *Manager) FlushAll(baseCtx context.Context, timeout time.Duration) error
 					continue
 				}
 			}
+			if pool, ok := mq.Managed.(ManagedPool); ok {
+				// No point into flushing pools when their base's ctx is already done.
+				select {
+				case <-pool.Done():
+					wg.Done()
+					continue
+				default:
+				}
+			}
 
 			allEmpty = false
 			if flushable, ok := mq.Managed.(Flushable); ok {