Fix alert expiry crashbug.

No, the array representation of a binary heap is of course *not* sorted.

Change-Id: Ib18c9b7e1bee24391f98d73135ac19c77026b168

manager/manager.go

@@ -15,7 +15,6 @@ package manager
 
 import (
 	"container/heap"
-	"sort"
 	"strings"
 	"sync"
 	"time"
@@ -98,6 +97,15 @@ func (aggs aggregatesByNextNotification) Less(i, j int) bool {
 	return aggs.AlertAggregates[i].NextNotification.Before(aggs.AlertAggregates[j].NextNotification)
 }
 
+// rebuildFrom rebuilds the aggregatesByNextNotification index from a provided
+// authoritative AlertAggregates slice.
+func (aggs *aggregatesByNextNotification) rebuildFrom(aa AlertAggregates) {
+	aggs.AlertAggregates = aggs.AlertAggregates[:0]
+	for _, a := range aa {
+		aggs.Push(a)
+	}
+}
+
 func (aggs AlertAggregates) Swap(i, j int) {
 	aggs[i], aggs[j] = aggs[j], aggs[i]
 }
@@ -264,36 +272,23 @@ func (s *memoryAlertManager) removeExpiredAggregates() {
 	// aggregates remain in the heap.
 	for {
 		if len(s.aggregatesByLastRefreshed.AlertAggregates) == 0 {
-			return
+			break
 		}
 
 		agg := heap.Pop(&s.aggregatesByLastRefreshed).(*AlertAggregate)
 
 		if time.Since(agg.LastRefreshed) > s.minRefreshInterval {
 			delete(s.aggregates, agg.Alert.Fingerprint())
-
-			// Also remove the aggregate from the last-notification-time index.
-			n := len(s.aggregatesByNextNotification.AlertAggregates)
-			i := sort.Search(n, func(i int) bool {
-				return !agg.NextNotification.After(s.aggregatesByNextNotification.AlertAggregates[i].NextNotification)
-			})
-			if i == n {
-				panic("Missing alert aggregate in aggregatesByNextNotification index")
-			} else {
-				for j := i; j < n; j++ {
-					if s.aggregatesByNextNotification.AlertAggregates[j] == agg {
-						heap.Remove(&s.aggregatesByNextNotification, j)
-						break
-					}
-				}
-			}
-
 			s.needsNotificationRefresh = true
 		} else {
 			heap.Push(&s.aggregatesByLastRefreshed, agg)
-			return
+			break
 		}
 	}
+
+	if s.needsNotificationRefresh {
+		s.aggregatesByNextNotification.rebuildFrom(s.aggregatesByLastRefreshed.AlertAggregates)
+	}
 }
 
 // Check whether one of the filtered (uninhibited, unsilenced) alerts should