Return a nop iterator for series that have been purged completely.

Change-Id: I6e92cac4472486feefdecba8593c17867e8c710d

storage/local/series.go

@@ -456,13 +456,6 @@ func (s *memorySeries) preloadChunksForRange(from clientmodel.Timestamp, through
 	return s.preloadChunks(pinIndexes, p)
 }
 
-// memorySeriesIterator implements SeriesIterator.
-type memorySeriesIterator struct {
-	lock, unlock func()
-	chunkIt      chunkIterator
-	chunks       chunks
-}
-
 func (s *memorySeries) newIterator(lockFunc, unlockFunc func()) SeriesIterator {
 	chunks := make(chunks, 0, len(s.chunkDescs))
 	for i, cd := range s.chunkDescs {
@@ -504,6 +497,13 @@ func (s *memorySeries) lastTime() clientmodel.Timestamp {
 	return s.head().lastTime()
 }
 
+// memorySeriesIterator implements SeriesIterator.
+type memorySeriesIterator struct {
+	lock, unlock func()
+	chunkIt      chunkIterator
+	chunks       chunks
+}
+
 // GetValueAtTime implements SeriesIterator.
 func (it *memorySeriesIterator) GetValueAtTime(t clientmodel.Timestamp) metric.Values {
 	it.lock()
@@ -628,3 +628,21 @@ func (it *memorySeriesIterator) GetRangeValues(in metric.Interval) metric.Values
 	}
 	return values
 }
+
+// nopSeriesIterator implements Series Iterator. It never returns any values.
+type nopSeriesIterator struct{}
+
+// GetValueAtTime implements SeriesIterator.
+func (_ nopSeriesIterator) GetValueAtTime(t clientmodel.Timestamp) metric.Values {
+	return metric.Values{}
+}
+
+// GetBoundaryValues implements SeriesIterator.
+func (_ nopSeriesIterator) GetBoundaryValues(in metric.Interval) metric.Values {
+	return metric.Values{}
+}
+
+// GetRangeValues implements SeriesIterator.
+func (_ nopSeriesIterator) GetRangeValues(in metric.Interval) metric.Values {
+	return metric.Values{}
+}

storage/local/storage.go

@@ -185,8 +185,12 @@ func (s *memorySeriesStorage) NewIterator(fp clientmodel.Fingerprint) SeriesIter
 
 	series, ok := s.fingerprintToSeries.get(fp)
 	if !ok {
-		// TODO: Could this legitimately happen? Series just got purged?
+		// Oops, no series for fp found. That happens if, after
-		panic("requested iterator for non-existent series")
+		// preloading is done, the whole series is identified as old
+		// enough for purging and hence purged for good. As there is no
+		// data left to iterate over, return an iterator that will never
+		// return any values.
+		return nopSeriesIterator{}
 	}
 	return series.newIterator(
 		func() { s.fpLocker.Lock(fp) },