common/prque: refactor LazyQueue (#21236)

common/prque/lazyqueue.go

@@ -26,9 +26,10 @@ import (
 // LazyQueue is a priority queue data structure where priorities can change over
 // time and are only evaluated on demand.
 // Two callbacks are required:
-// - 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
+//   - 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 {
@@ -36,14 +37,15 @@ type LazyQueue struct {
 	// Items are stored in one of two internal queues ordered by estimated max
 	// priority until the next and the next-after-next refresh. Update and Refresh
 	// always places items in queue[1].
-	queue       [2]*sstack
-	popQueue    *sstack
-	period      time.Duration
-	maxUntil    mclock.AbsTime
-	indexOffset int
-	setIndex    SetIndexCallback
-	priority    PriorityCallback
-	maxPriority MaxPriorityCallback
+	queue                      [2]*sstack
+	popQueue                   *sstack
+	period                     time.Duration
+	maxUntil                   mclock.AbsTime
+	indexOffset                int
+	setIndex                   SetIndexCallback
+	priority                   PriorityCallback
+	maxPriority                MaxPriorityCallback
+	lastRefresh1, lastRefresh2 mclock.AbsTime
 }
 
 type (
@@ -54,14 +56,17 @@ type (
 // NewLazyQueue creates a new lazy queue
 func NewLazyQueue(setIndex SetIndexCallback, priority PriorityCallback, maxPriority MaxPriorityCallback, clock mclock.Clock, refreshPeriod time.Duration) *LazyQueue {
 	q := &LazyQueue{
-		popQueue:    newSstack(nil),
-		setIndex:    setIndex,
-		priority:    priority,
-		maxPriority: maxPriority,
-		clock:       clock,
-		period:      refreshPeriod}
+		popQueue:     newSstack(nil),
+		setIndex:     setIndex,
+		priority:     priority,
+		maxPriority:  maxPriority,
+		clock:        clock,
+		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
 		}
 	}
 }