Fix head chunk persisting and a chunkDesc race condition.

- Head chunk persisting only happens in evictOlderThan, so do it
  there.  (With the previous code, it would never happen.)

- Raw accesses to chunkDesc.chunk are now done via isEvicted (with
  locking).

Change-Id: I48b07b56dfea4899b50df159b4ea566954396fcd

storage/local/series.go

@@ -183,6 +183,13 @@ func (cd *chunkDesc) lastTime() clientmodel.Timestamp {
 	return cd.chunk.lastTime()
 }
 
+func (cd *chunkDesc) isEvicted() bool {
+	cd.Lock()
+	defer cd.Unlock()
+
+	return cd.chunk == nil
+}
+
 func (cd *chunkDesc) contains(t clientmodel.Timestamp) bool {
 	return !t.Before(cd.firstTime()) && !t.After(cd.lastTime())
 }
@@ -201,16 +208,21 @@ func (cd *chunkDesc) open(c chunk) {
 }
 
 // evictOnUnpin evicts the chunk once unpinned. If it is not pinned when this
-// method is called, it evicts the chunk immediately and returns true.
+// method is called, it evicts the chunk immediately and returns true. If the
+// chunk is already evicted when this method is called, it returns true, too.
 func (cd *chunkDesc) evictOnUnpin() bool {
 	cd.Lock()
 	defer cd.Unlock()
 
+	if cd.chunk == nil {
+		// Already evicted.
+		return true
+	}
+	cd.evict = true
 	if cd.refCount == 0 {
 		cd.evictNow()
 		return true
 	}
-	cd.evict = true
 	return false
 }
 
@@ -229,9 +241,9 @@ type memorySeries struct {
 	// (or all) chunkDescs are only on disk. These chunks are all contiguous
 	// and at the tail end.
 	chunkDescsLoaded bool
-	// Whether the current head chunk has already been persisted (or at
-	// least has been scheduled to be persisted). If true, the current head
-	// chunk must not be modified anymore.
+	// Whether the current head chunk has already been scheduled to be
+	// persisted. If true, the current head chunk must not be modified
+	// anymore.
 	headChunkPersisted bool
 }
 
@@ -286,35 +298,48 @@ func (s *memorySeries) add(fp clientmodel.Fingerprint, v *metric.SamplePair, per
 	}
 }
 
-// persistHeadChunk queues the head chunk for persisting if not already done.
-// The caller must have locked the fingerprint of the series.
-func (s *memorySeries) persistHeadChunk(fp clientmodel.Fingerprint, persistQueue chan *persistRequest) {
-	if s.headChunkPersisted {
-		return
-	}
-	s.headChunkPersisted = true
-	persistQueue <- &persistRequest{
-		fingerprint: fp,
-		chunkDesc:   s.head(),
-	}
-}
-
-// evictOlderThan evicts chunks whose latest sample is older than the given
-// timestamp.  It returns true if all chunks in the series were immediately
-// evicted (i.e. all chunks are older than the timestamp, and none of the chunks
-// was pinned).
+// evictOlderThan marks for eviction all chunks whose latest sample is older
+// than the given timestamp. It returns true if all chunks in the series were
+// immediately evicted (i.e. all chunks are older than the timestamp, and none
+// of the chunks was pinned).
+//
+// Special considerations for the head chunk: If it has not been scheduled to be
+// persisted yet but is old enough for eviction, the scheduling happens now. (To
+// do that, the method neets the fingerprint and the persist queue.) It is
+// likely that the actual persisting will not happen soon enough to immediately
+// evict the head chunk, though. Thus, calling evictOlderThan for a series with
+// a non-persisted head chunk will most likely return false, even if no chunk is
+// pinned for other reasons. A series old enough for archiving will usually
+// require at least two eviction runs to become ready for archiving: In the
+// first run, its head chunk is scheduled to be persisted. The next call of
+// evictOlderThan will then return true, provided that the series hasn't
+// received new samples in the meantime, the head chunk has now been persisted,
+// and no chunk is pinned for other reasons.
 //
 // The caller must have locked the fingerprint of the series.
-func (s *memorySeries) evictOlderThan(t clientmodel.Timestamp) bool {
+func (s *memorySeries) evictOlderThan(
+	t clientmodel.Timestamp,
+	fp clientmodel.Fingerprint,
+	persistQueue chan *persistRequest,
+) bool {
 	allEvicted := true
 	// For now, always drop the entire range from oldest to t.
-	for _, cd := range s.chunkDescs {
+	for i, cd := range s.chunkDescs {
 		if !cd.lastTime().Before(t) {
 			return false
 		}
-		if cd.chunk == nil {
+		if cd.isEvicted() {
 			continue
 		}
+		if !s.headChunkPersisted && i == len(s.chunkDescs)-1 {
+			// This is a non-persisted head chunk that is old enough
+			// for eviction. Queue it to be persisted:
+			s.headChunkPersisted = true
+			persistQueue <- &persistRequest{
+				fingerprint: fp,
+				chunkDesc:   cd,
+			}
+		}
 		if !cd.evictOnUnpin() {
 			allEvicted = false
 		}
@@ -323,6 +348,7 @@ func (s *memorySeries) evictOlderThan(t clientmodel.Timestamp) bool {
 }
 
 // purgeOlderThan returns true if all chunks have been purged.
+//
 // The caller must have locked the fingerprint of the series.
 func (s *memorySeries) purgeOlderThan(t clientmodel.Timestamp) bool {
 	keepIdx := len(s.chunkDescs)
@@ -334,24 +360,19 @@ func (s *memorySeries) purgeOlderThan(t clientmodel.Timestamp) bool {
 	}
 
 	for i := 0; i < keepIdx; i++ {
-		if s.chunkDescs[i].chunk != nil {
-			s.chunkDescs[i].evictOnUnpin()
-		}
+		s.chunkDescs[i].evictOnUnpin()
 	}
 	s.chunkDescs = s.chunkDescs[keepIdx:]
 	return len(s.chunkDescs) == 0
 }
 
 // preloadChunks is an internal helper method.
-// TODO: in this method (and other places), we just fudge around with chunkDesc
-// internals without grabbing the chunkDesc lock. Study how this needs to be
-// protected against other accesses that don't hold the fp lock.
 func (s *memorySeries) preloadChunks(indexes []int, p *persistence) (chunkDescs, error) {
 	loadIndexes := []int{}
 	pinnedChunkDescs := make(chunkDescs, 0, len(indexes))
 	for _, idx := range indexes {
 		pinnedChunkDescs = append(pinnedChunkDescs, s.chunkDescs[idx])
-		if s.chunkDescs[idx].chunk == nil {
+		if s.chunkDescs[idx].isEvicted() {
 			loadIndexes = append(loadIndexes, idx)
 		} else {
 			s.chunkDescs[idx].pin()
@@ -364,7 +385,7 @@ func (s *memorySeries) preloadChunks(indexes []int, p *persistence) (chunkDescs,
 		if err != nil {
 			// Unpin any pinned chunks that were already loaded.
 			for _, cd := range pinnedChunkDescs {
-				if cd.chunk != nil {
+				if !cd.isEvicted() {
 					cd.unpin()
 				}
 			}
@@ -459,7 +480,7 @@ func (s *memorySeries) preloadChunksForRange(from clientmodel.Timestamp, through
 func (s *memorySeries) newIterator(lockFunc, unlockFunc func()) SeriesIterator {
 	chunks := make(chunks, 0, len(s.chunkDescs))
 	for i, cd := range s.chunkDescs {
-		if cd.chunk != nil {
+		if !cd.isEvicted() {
 			if i == len(s.chunkDescs)-1 {
 				chunks = append(chunks, cd.chunk.clone())
 			} else {

storage/local/storage.go

@@ -205,8 +205,10 @@ func (s *memorySeriesStorage) evictMemoryChunks(ttl time.Duration) {
 
 	for m := range s.fingerprintToSeries.iter() {
 		s.fpLocker.Lock(m.fp)
-		if m.series.evictOlderThan(clientmodel.TimestampFromTime(time.Now()).Add(-1 * ttl)) {
-			m.series.persistHeadChunk(m.fp, s.persistQueue)
+		if m.series.evictOlderThan(
+			clientmodel.TimestampFromTime(time.Now()).Add(-1*ttl),
+			m.fp, s.persistQueue,
+		) {
 			s.fingerprintToSeries.del(m.fp)
 			if err := s.persistence.archiveMetric(
 				m.fp, m.series.metric, m.series.firstTime(), m.series.lastTime(),