Adding upstream version 2.2.2.

Signed-off-by: Daniel Baumann <daniel@debian.org>

janitor.go

@@ -0,0 +1,146 @@
+package gocache
+
+import (
+	"log"
+	"time"
+)
+
+const (
+	// JanitorShiftTarget is the target number of expired keys to find during passive clean up duty
+	// before pausing the passive expired keys eviction process
+	JanitorShiftTarget = 25
+
+	// JanitorMaxIterationsPerShift is the maximum number of nodes to traverse before pausing
+	//
+	// This is to prevent the janitor from traversing the entire cache, which could take a long time
+	// to complete depending on the size of the cache.
+	//
+	// By limiting it to a small number, we are effectively reducing the impact of passive eviction.
+	JanitorMaxIterationsPerShift = 1000
+
+	// JanitorMinShiftBackOff is the minimum interval between each iteration of steps
+	// defined by JanitorMaxIterationsPerShift
+	JanitorMinShiftBackOff = 50 * time.Millisecond
+
+	// JanitorMaxShiftBackOff is the maximum interval between each iteration of steps
+	// defined by JanitorMaxIterationsPerShift
+	JanitorMaxShiftBackOff = 500 * time.Millisecond
+)
+
+// StartJanitor starts the janitor on a different goroutine
+// The janitor's job is to delete expired keys in the background, in other words, it takes care of passive eviction.
+// It can be stopped by calling Cache.StopJanitor.
+// If you do not start the janitor, expired keys will only be deleted when they are accessed through Get, GetByKeys, or
+// GetAll.
+func (cache *Cache) StartJanitor() error {
+	if cache.stopJanitor != nil {
+		return ErrJanitorAlreadyRunning
+	}
+	cache.stopJanitor = make(chan bool)
+	go func() {
+		// rather than starting from the tail on every run, we can try to start from the last traversed entry
+		var lastTraversedNode *Entry
+		totalNumberOfExpiredKeysInPreviousRunFromTailToHead := 0
+		backOff := JanitorMinShiftBackOff
+		for {
+			select {
+			case <-time.After(backOff):
+				// Passive clean up duty
+				cache.mutex.Lock()
+				if cache.tail != nil {
+					start := time.Now()
+					steps := 0
+					expiredEntriesFound := 0
+					current := cache.tail
+					if lastTraversedNode != nil {
+						// Make sure the lastTraversedNode is still in the cache, otherwise we might be traversing nodes that were already deleted.
+						// Furthermore, we need to make sure that the entry from the cache has the same pointer as the lastTraversedNode
+						// to verify that there isn't just a new cache entry with the same key (i.e. in case lastTraversedNode got evicted)
+						if entryFromCache, isInCache := cache.get(lastTraversedNode.Key); isInCache && entryFromCache == lastTraversedNode {
+							current = lastTraversedNode
+						}
+					}
+					if current == cache.tail {
+						if Debug {
+							log.Printf("There are currently %d entries in the cache. The last walk resulted in finding %d expired keys", len(cache.entries), totalNumberOfExpiredKeysInPreviousRunFromTailToHead)
+						}
+						totalNumberOfExpiredKeysInPreviousRunFromTailToHead = 0
+					}
+					for current != nil {
+						// since we're walking from the tail to the head, we get the previous reference
+						var previous *Entry
+						steps++
+						if current.Expired() {
+							expiredEntriesFound++
+							// Because delete will remove the previous reference from the entry, we need to store the
+							// previous reference before we delete it
+							previous = current.previous
+							cache.delete(current.Key)
+							cache.stats.ExpiredKeys++
+						}
+						if current == cache.head {
+							lastTraversedNode = nil
+							break
+						}
+						// Travel to the current node's previous node only if no specific previous node has been specified
+						if previous != nil {
+							current = previous
+						} else {
+							current = current.previous
+						}
+						lastTraversedNode = current
+						if steps == JanitorMaxIterationsPerShift || expiredEntriesFound >= JanitorShiftTarget {
+							if expiredEntriesFound > 0 {
+								backOff = JanitorMinShiftBackOff
+							} else {
+								if backOff*2 <= JanitorMaxShiftBackOff {
+									backOff *= 2
+								} else {
+									backOff = JanitorMaxShiftBackOff
+								}
+							}
+							break
+						}
+					}
+					if Debug {
+						log.Printf("traversed %d nodes and found %d expired entries in %s before stopping\n", steps, expiredEntriesFound, time.Since(start))
+					}
+					totalNumberOfExpiredKeysInPreviousRunFromTailToHead += expiredEntriesFound
+				} else {
+					if backOff*2 < JanitorMaxShiftBackOff {
+						backOff *= 2
+					} else {
+						backOff = JanitorMaxShiftBackOff
+					}
+				}
+				cache.mutex.Unlock()
+			case <-cache.stopJanitor:
+				cache.stopJanitor <- true
+				return
+			}
+		}
+	}()
+	//if Debug {
+	//	go func() {
+	//		var m runtime.MemStats
+	//		for {
+	//			runtime.ReadMemStats(&m)
+	//			log.Printf("Alloc=%vMB; HeapReleased=%vMB; Sys=%vMB; HeapInUse=%vMB; HeapObjects=%v; HeapObjectsFreed=%v; GC=%v; cache.memoryUsage=%vMB; cacheSize=%d\n", m.Alloc/1024/1024, m.HeapReleased/1024/1024, m.Sys/1024/1024, m.HeapInuse/1024/1024, m.HeapObjects, m.Frees, m.NumGC, cache.memoryUsage/1024/1024, cache.Count())
+	//			time.Sleep(3 * time.Second)
+	//		}
+	//	}()
+	//}
+	return nil
+}
+
+// StopJanitor stops the janitor
+func (cache *Cache) StopJanitor() {
+	if cache.stopJanitor != nil {
+		// Tell the janitor to stop, and then wait for the janitor to reply on the same channel that it's stopping
+		// This may seem a bit odd, but this allows us to avoid a data race condition when trying to set
+		// cache.stopJanitor to nil
+		cache.stopJanitor <- true
+		<-cache.stopJanitor
+		cache.stopJanitor = nil
+	}
+}