Return SamplePair istead of *SamplePair consistently

Formalize ZeroSamplePair as return value for non-existing samples.

Change LastSamplePairForFingerprint to return a SamplePair (and not a
pointer to it), which saves allocations in a potentially extremely
frequent call.

storage/local/chunk.go

@@ -194,19 +194,19 @@ func (cd *chunkDesc) maybePopulateLastTime() {
 	}
 }
 
-// lastSamplePair returns the last sample pair of the underlying chunk, or nil
-// if the chunk is evicted. For safe concurrent access, this method requires the
-// fingerprint of the time series to be locked.
+// lastSamplePair returns the last sample pair of the underlying chunk, or
+// ZeroSampleValue if the chunk is evicted. For safe concurrent access, this
+// method requires the fingerprint of the time series to be locked.
 // TODO(beorn7): Move up into memorySeries.
-func (cd *chunkDesc) lastSamplePair() *model.SamplePair {
+func (cd *chunkDesc) lastSamplePair() model.SamplePair {
 	cd.Lock()
 	defer cd.Unlock()
 
 	if cd.c == nil {
-		return nil
+		return ZeroSamplePair
 	}
 	it := cd.c.newIterator()
-	return &model.SamplePair{
+	return model.SamplePair{
 		Timestamp: it.lastTimestamp(),
 		Value:     it.lastSampleValue(),
 	}
@@ -293,8 +293,7 @@ type chunkIterator interface {
 	lastSampleValue() model.SampleValue
 	// Gets the value that is closest before the given time. In case a value
 	// exist at precisely the given time, that value is returned. If no
-	// applicable value exists, a SamplePair with timestamp model.Earliest
-	// and value 0.0 is returned.
+	// applicable value exists, ZeroSamplePair is returned.
 	valueAtOrBeforeTime(model.Time) model.SamplePair
 	// Gets all values contained within a given interval.
 	rangeValues(metric.Interval) []model.SamplePair

storage/local/delta.go

@@ -307,7 +307,7 @@ func (it *deltaEncodedChunkIterator) valueAtOrBeforeTime(t model.Time) model.Sam
 		return it.timestampAtIndex(i).After(t)
 	})
 	if i == 0 {
-		return model.SamplePair{Timestamp: model.Earliest}
+		return ZeroSamplePair
 	}
 	return model.SamplePair{
 		Timestamp: it.timestampAtIndex(i - 1),

storage/local/doubledelta.go

@@ -413,7 +413,7 @@ func (it *doubleDeltaEncodedChunkIterator) valueAtOrBeforeTime(t model.Time) mod
 		return it.timestampAtIndex(i).After(t)
 	})
 	if i == 0 {
-		return model.SamplePair{Timestamp: model.Earliest}
+		return ZeroSamplePair
 	}
 	return model.SamplePair{
 		Timestamp: it.timestampAtIndex(i - 1),

storage/local/interface.go

@@ -42,10 +42,12 @@ type Storage interface {
 	// label matchers. At least one label matcher must be specified that does not
 	// match the empty string.
 	MetricsForLabelMatchers(...*metric.LabelMatcher) map[model.Fingerprint]metric.Metric
-	// LastSamplePairForFingerprint returns the last sample pair for the
-	// provided fingerprint. If the respective time series does not exist or
-	// has an evicted head chunk, nil is returned.
-	LastSamplePairForFingerprint(model.Fingerprint) *model.SamplePair
+	// LastSamplePairForFingerprint returns the last sample pair that has
+	// been ingested for the provided fingerprint. If this instance of the
+	// Storage has never ingested a sample for the provided fingerprint (or
+	// the last ingestion is so long ago that the series has been archived),
+	// ZeroSamplePair is returned.
+	LastSamplePairForFingerprint(model.Fingerprint) model.SamplePair
 	// Get all of the label values that are associated with a given label name.
 	LabelValuesForLabelName(model.LabelName) model.LabelValues
 	// Get the metric associated with the provided fingerprint.
@@ -73,8 +75,7 @@ type Storage interface {
 type SeriesIterator interface {
 	// Gets the value that is closest before the given time. In case a value
 	// exist at precisely the given time, that value is returned. If no
-	// applicable value exists, a SamplePair with timestamp model.Earliest
-	// and value 0.0 is returned.
+	// applicable value exists, ZeroSamplePair is returned.
 	ValueAtOrBeforeTime(model.Time) model.SamplePair
 	// Gets the boundary values of an interval: the first and last value
 	// within a given interval.
@@ -94,3 +95,10 @@ type Preloader interface {
 	// Close unpins any previously requested series data from memory.
 	Close()
 }
+
+// ZeroSamplePair is the pseudo zero-value of model.SamplePair used by the local
+// package to signal a non-existing sample. It is a SamplePair with timestamp
+// model.Earliest and value 0.0. Note that the natural zero value of SamplePair
+// has a timestamp of 0, which is possible to appear in a real SamplePair and
+// thus not suitable to signal a non-existing SamplePair.
+var ZeroSamplePair = model.SamplePair{Timestamp: model.Earliest}

storage/local/series.go

@@ -479,7 +479,7 @@ func (it *memorySeriesIterator) ValueAtOrBeforeTime(t model.Time) model.SamplePa
 	}
 
 	if len(it.chunks) == 0 {
-		return model.SamplePair{Timestamp: model.Earliest}
+		return ZeroSamplePair
 	}
 
 	// Find the last chunk where firstTime() is before or equal to t.
@@ -489,7 +489,7 @@ func (it *memorySeriesIterator) ValueAtOrBeforeTime(t model.Time) model.SamplePa
 	})
 	if i == len(it.chunks) {
 		// Even the first chunk starts after t.
-		return model.SamplePair{Timestamp: model.Earliest}
+		return ZeroSamplePair
 	}
 	it.chunkIt = it.chunkIterator(l - i)
 	return it.chunkIt.valueAtOrBeforeTime(t)
@@ -598,7 +598,7 @@ type nopSeriesIterator struct{}
 
 // ValueAtTime implements SeriesIterator.
 func (i nopSeriesIterator) ValueAtOrBeforeTime(t model.Time) model.SamplePair {
-	return model.SamplePair{Timestamp: model.Earliest}
+	return ZeroSamplePair
 }
 
 // BoundaryValues implements SeriesIterator.

storage/local/storage.go

@@ -346,13 +346,13 @@ func (s *memorySeriesStorage) WaitForIndexing() {
 }
 
 // LastSampleForFingerprint implements Storage.
-func (s *memorySeriesStorage) LastSamplePairForFingerprint(fp model.Fingerprint) *model.SamplePair {
+func (s *memorySeriesStorage) LastSamplePairForFingerprint(fp model.Fingerprint) model.SamplePair {
 	s.fpLocker.Lock(fp)
 	defer s.fpLocker.Unlock(fp)
 
 	series, ok := s.fpToSeries.get(fp)
 	if !ok {
-		return nil
+		return ZeroSamplePair
 	}
 	return series.head().lastSamplePair()
 }
@@ -367,7 +367,7 @@ type boundedIterator struct {
 // ValueAtOrBeforeTime implements the SeriesIterator interface.
 func (bit *boundedIterator) ValueAtOrBeforeTime(ts model.Time) model.SamplePair {
 	if ts < bit.start {
-		return model.SamplePair{Timestamp: model.Earliest}
+		return ZeroSamplePair
 	}
 	return bit.it.ValueAtOrBeforeTime(ts)
 }

web/federate.go

@@ -67,7 +67,7 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 
 		sp := h.storage.LastSamplePairForFingerprint(fp)
 		// Discard if sample does not exist or lays before the staleness interval.
-		if sp == nil || sp.Timestamp.Before(minTimestamp) {
+		if sp.Timestamp.Before(minTimestamp) {
 			continue
 		}