Extract initial FP candidate build logic into candidateFPsForLabelMatchers method

No functional changes otherwise

storage/local/storage.go

@@ -563,43 +563,43 @@ func (s *MemorySeriesStorage) MetricsForLabelMatchers(
 	return metrics, nil
 }
 
-func (s *MemorySeriesStorage) metricsForLabelMatchers(
-	from, through model.Time,
+// returns candidate FPs for given matchers and remaining matchers to be checked
+func (s *MemorySeriesStorage) candidateFPsForLabelMatchers(
 	matchers ...*metric.LabelMatcher,
-) (map[model.Fingerprint]metric.Metric, error) {
+) (map[model.Fingerprint]struct{}, []*metric.LabelMatcher, error) {
 	sort.Sort(metric.LabelMatchers(matchers))
 
 	if len(matchers) == 0 || matchers[0].MatchesEmptyString() {
 		// No matchers at all or even the best matcher matches the empty string.
-		return nil, nil
+		return nil, nil, nil
 	}
 
 	var (
 		matcherIdx   int
-		remainingFPs map[model.Fingerprint]struct{}
+		candidateFPs map[model.Fingerprint]struct{}
 	)
 
 	// Equal matchers.
-	for ; matcherIdx < len(matchers) && (remainingFPs == nil || len(remainingFPs) > fpEqualMatchThreshold); matcherIdx++ {
+	for ; matcherIdx < len(matchers) && (candidateFPs == nil || len(candidateFPs) > fpEqualMatchThreshold); matcherIdx++ {
 		m := matchers[matcherIdx]
 		if m.Type != metric.Equal || m.MatchesEmptyString() {
 			break
 		}
-		remainingFPs = s.fingerprintsForLabelPair(
+		candidateFPs = s.fingerprintsForLabelPair(
 			model.LabelPair{
 				Name:  m.Name,
 				Value: m.Value,
 			},
 			nil,
-			remainingFPs,
+			candidateFPs,
 		)
-		if len(remainingFPs) == 0 {
-			return nil, nil
+		if len(candidateFPs) == 0 {
+			return nil, nil, nil
 		}
 	}
 
 	// Other matchers.
-	for ; matcherIdx < len(matchers) && (remainingFPs == nil || len(remainingFPs) > fpOtherMatchThreshold); matcherIdx++ {
+	for ; matcherIdx < len(matchers) && (candidateFPs == nil || len(candidateFPs) > fpOtherMatchThreshold); matcherIdx++ {
 		m := matchers[matcherIdx]
 		if m.MatchesEmptyString() {
 			break
@@ -607,11 +607,11 @@ func (s *MemorySeriesStorage) metricsForLabelMatchers(
 
 		lvs, err := s.LabelValuesForLabelName(context.TODO(), m.Name)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		lvs = m.Filter(lvs)
 		if len(lvs) == 0 {
-			return nil, nil
+			return nil, nil, nil
 		}
 		fps := map[model.Fingerprint]struct{}{}
 		for _, lv := range lvs {
@@ -621,18 +621,30 @@ func (s *MemorySeriesStorage) metricsForLabelMatchers(
 					Value: lv,
 				},
 				fps,
-				remainingFPs,
+				candidateFPs,
 			)
 		}
-		remainingFPs = fps
-		if len(remainingFPs) == 0 {
-			return nil, nil
+		candidateFPs = fps
+		if len(candidateFPs) == 0 {
+			return nil, nil, nil
 		}
 	}
+	return candidateFPs, matchers[matcherIdx:], nil
+}
+
+func (s *MemorySeriesStorage) metricsForLabelMatchers(
+	from, through model.Time,
+	matchers ...*metric.LabelMatcher,
+) (map[model.Fingerprint]metric.Metric, error) {
+
+	candidateFPs, matchersToCheck, err := s.candidateFPsForLabelMatchers(matchers...)
+	if err != nil {
+		return nil, err
+	}
 
 	result := map[model.Fingerprint]metric.Metric{}
 FP_LOOP:
-	for fp := range remainingFPs {
+	for fp := range candidateFPs {
 		s.fpLocker.Lock(fp)
 		met, _, ok := s.metricForRange(fp, from, through)
 		s.fpLocker.Unlock(fp)
@@ -641,7 +653,7 @@ FP_LOOP:
 			continue FP_LOOP
 		}
 
-		for _, m := range matchers[matcherIdx:] {
+		for _, m := range matchersToCheck {
 			if !m.Match(met[m.Name]) {
 				continue FP_LOOP
 			}