Cache signature of an empty label set

This is an optimization of labelsToSignature to avoid excess allocations
when the label set is empty.

Change-Id: If2d59bbc3ae6d4457e2ded197b6f4e7c67e6a173

prometheus/signature.go

@@ -14,9 +14,16 @@ import (
 	"github.com/prometheus/client_golang/model"
 )
 
+// cache the signature of an empty label set.
+var emptyLabelSignature = fnv.New64a().Sum64()
+
 // LabelsToSignature provides a way of building a unique signature
 // (i.e., fingerprint) for a given label set sequence.
 func labelsToSignature(labels map[string]string) uint64 {
+	if len(labels) == 0 {
+		return emptyLabelSignature
+	}
+
 	names := make(model.LabelNames, 0, len(labels))
 	for name := range labels {
 		names = append(names, model.LabelName(name))

prometheus/signature_test.go

@@ -7,6 +7,7 @@
 package prometheus
 
 import (
+	"runtime"
 	"testing"
 )
 
@@ -38,6 +39,25 @@ func TestLabelToSignature(t *testing.T) {
 	testLabelsToSignature(t)
 }
 
+func TestEmptyLabelSignature(t *testing.T) {
+	input := []map[string]string{nil, {}}
+
+	var ms runtime.MemStats
+	runtime.ReadMemStats(&ms)
+
+	alloc := ms.Alloc
+
+	for _, labels := range input {
+		labelsToSignature(labels)
+	}
+
+	runtime.ReadMemStats(&ms)
+
+	if got := ms.Alloc; alloc != got {
+		t.Fatal("expected labelsToSignature with empty labels not to perform allocations")
+	}
+}
+
 func BenchmarkLabelToSignature(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		testLabelsToSignature(b)