common/prque: refactor LazyQueue (#21236)

common/prque/lazyqueue.go

@@ -29,6 +29,7 @@ import (
 //   - priority evaluates the actual priority of an item
 //   - maxPriority gives an upper estimate for the priority in any moment between
 //     now and the given absolute time
+//
 // If the upper estimate is exceeded then Update should be called for that item.
 // A global Refresh function should also be called periodically.
 type LazyQueue struct {
@@ -44,6 +45,7 @@ type LazyQueue struct {
 	setIndex                   SetIndexCallback
 	priority                   PriorityCallback
 	maxPriority                MaxPriorityCallback
+	lastRefresh1, lastRefresh2 mclock.AbsTime
 }
 
 type (
@@ -59,9 +61,12 @@ func NewLazyQueue(setIndex SetIndexCallback, priority PriorityCallback, maxPrior
 		priority:     priority,
 		maxPriority:  maxPriority,
 		clock:        clock,
-		period:      refreshPeriod}
+		period:       refreshPeriod,
+		lastRefresh1: clock.Now(),
+		lastRefresh2: clock.Now(),
+	}
 	q.Reset()
-	q.Refresh()
+	q.refresh(clock.Now())
 	return q
 }
 
@@ -71,9 +76,19 @@ func (q *LazyQueue) Reset() {
 	q.queue[1] = newSstack(q.setIndex1)
 }
 
-// Refresh should be called at least with the frequency specified by the refreshPeriod parameter
+// Refresh performs queue re-evaluation if necessary
 func (q *LazyQueue) Refresh() {
-	q.maxUntil = q.clock.Now() + mclock.AbsTime(q.period)
+	now := q.clock.Now()
+	for time.Duration(now-q.lastRefresh2) >= q.period*2 {
+		q.refresh(now)
+		q.lastRefresh2 = q.lastRefresh1
+		q.lastRefresh1 = now
+	}
+}
+
+// refresh re-evaluates items in the older queue and swaps the two queues
+func (q *LazyQueue) refresh(now mclock.AbsTime) {
+	q.maxUntil = now + mclock.AbsTime(q.period)
 	for q.queue[0].Len() != 0 {
 		q.Push(heap.Pop(q.queue[0]).(*item).value)
 	}
@@ -139,6 +154,7 @@ func (q *LazyQueue) MultiPop(callback func(data interface{}, priority int64) boo
 				}
 				return
 			}
+			nextIndex = q.peekIndex() // re-check because callback is allowed to push items back
 		}
 	}
 }