storage: add LastSamplePairForFingerprint method

storage/local/chunk.go

@@ -142,6 +142,20 @@ func (cd *chunkDesc) lastTime() clientmodel.Timestamp {
 	return cd.c.newIterator().lastTimestamp()
 }
 
+func (cd *chunkDesc) lastSamplePair() *metric.SamplePair {
+	cd.Lock()
+	defer cd.Unlock()
+
+	if cd.c == nil {
+		return nil
+	}
+	it := cd.c.newIterator()
+	return &metric.SamplePair{
+		Timestamp: it.lastTimestamp(),
+		Value:     it.lastSampleValue(),
+	}
+}
+
 func (cd *chunkDesc) isEvicted() bool {
 	cd.Lock()
 	defer cd.Unlock()

storage/local/interface.go

@@ -40,6 +40,9 @@ type Storage interface {
 	// label matchers. At least one label matcher must be specified that does not
 	// match the empty string.
 	MetricsForLabelMatchers(matchers ...*metric.LabelMatcher) map[clientmodel.Fingerprint]clientmodel.COWMetric
+	// LastSamplePairForFingerprint returns the last sample pair for the provided fingerprint.
+	// If the respective time series is evicted, nil is returned.
+	LastSamplePairForFingerprint(clientmodel.Fingerprint) *metric.SamplePair
 	// Get all of the label values that are associated with a given label name.
 	LabelValuesForLabelName(clientmodel.LabelName) clientmodel.LabelValues
 	// Get the metric associated with the provided fingerprint.

storage/local/storage.go

@@ -301,7 +301,7 @@ func (s *memorySeriesStorage) WaitForIndexing() {
 	s.persistence.waitForIndexing()
 }
 
-// NewIterator implements storage.
+// NewIterator implements Storage.
 func (s *memorySeriesStorage) NewIterator(fp clientmodel.Fingerprint) SeriesIterator {
 	s.fpLocker.Lock(fp)
 	defer s.fpLocker.Unlock(fp)
@@ -321,6 +321,18 @@ func (s *memorySeriesStorage) NewIterator(fp clientmodel.Fingerprint) SeriesIter
 	}
 }
 
+// LastSampleForFingerprint implements Storage.
+func (s *memorySeriesStorage) LastSamplePairForFingerprint(fp clientmodel.Fingerprint) *metric.SamplePair {
+	s.fpLocker.Lock(fp)
+	defer s.fpLocker.Unlock(fp)
+
+	series, ok := s.fpToSeries.get(fp)
+	if !ok {
+		return nil
+	}
+	return series.head().lastSamplePair()
+}
+
 // boundedIterator wraps a SeriesIterator and does not allow fetching
 // data from earlier than the configured start time.
 type boundedIterator struct {