labels: simplify call to get Labels from Builder

It took a `Labels` where the memory could be re-used, but in practice
this hardly ever benefitted. Especially after converting `relabel.Process`
to `relabel.ProcessBuilder`.

Comparing the parameter to `nil` was a bug; `EmptyLabels` is not `nil`
so the slice was reallocated multiple times by `append`.

Lastly `Builder.Labels()` now estimates that the final size will depend
on labels added and deleted.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

cmd/promtool/rules.go

@@ -163,7 +163,7 @@ func (importer *ruleImporter) importRule(ctx context.Context, ruleExpr, ruleName
 				})
 
 				lb.Set(labels.MetricName, ruleName)
-				lbls := lb.Labels(labels.EmptyLabels())
+				lbls := lb.Labels()
 
 				for _, value := range sample.Values {
 					if err := app.add(ctx, lbls, timestamp.FromTime(value.Timestamp.Time()), float64(value.Value)); err != nil {

model/labels/labels.go

@@ -570,24 +570,14 @@ func contains(s []Label, n string) bool {
 	return false
 }
 
-// Labels returns the labels from the builder, adding them to res if non-nil.
-// Argument res can be the same as b.base, if caller wants to overwrite that slice.
+// Labels returns the labels from the builder.
 // If no modifications were made, the original labels are returned.
-func (b *Builder) Labels(res Labels) Labels {
+func (b *Builder) Labels() Labels {
 	if len(b.del) == 0 && len(b.add) == 0 {
 		return b.base
 	}
 
-	if res == nil {
-		// In the general case, labels are removed, modified or moved
-		// rather than added.
-		res = make(Labels, 0, len(b.base))
-	} else {
-		res = res[:0]
-	}
-	// Justification that res can be the same slice as base: in this loop
-	// we move forward through base, and either skip an element or assign
-	// it to res at its current position or an earlier position.
+	res := make(Labels, 0, len(b.base)-len(b.del)+len(b.add))
 	for _, l := range b.base {
 		if slices.Contains(b.del, l.Name) || contains(b.add, l.Name) {
 			continue

model/labels/labels_string.go

@@ -158,7 +158,7 @@ func (ls Labels) MatchLabels(on bool, names ...string) Labels {
 		b.Del(MetricName)
 		b.Del(names...)
 	}
-	return b.Labels(EmptyLabels())
+	return b.Labels()
 }
 
 // Hash returns a hash value for the label set.
@@ -624,10 +624,9 @@ func contains(s []Label, n string) bool {
 	return false
 }
 
-// Labels returns the labels from the builder, adding them to res if non-nil.
-// Argument res can be the same as b.base, if caller wants to overwrite that slice.
+// Labels returns the labels from the builder.
 // If no modifications were made, the original labels are returned.
-func (b *Builder) Labels(res Labels) Labels {
+func (b *Builder) Labels() Labels {
 	if len(b.del) == 0 && len(b.add) == 0 {
 		return b.base
 	}
@@ -637,7 +636,7 @@ func (b *Builder) Labels(res Labels) Labels {
 	a, d := 0, 0
 
 	bufSize := len(b.base.data) + labelsSize(b.add)
-	buf := make([]byte, 0, bufSize) // TODO: see if we can re-use the buffer from res.
+	buf := make([]byte, 0, bufSize)
 	for pos := 0; pos < len(b.base.data); {
 		oldPos := pos
 		var lName string

model/labels/labels_test.go

@@ -596,7 +596,7 @@ func TestBuilder(t *testing.T) {
 				b.Keep(tcase.keep...)
 			}
 			b.Del(tcase.del...)
-			require.Equal(t, tcase.want, b.Labels(EmptyLabels()))
+			require.Equal(t, tcase.want, b.Labels())
 
 			// Check what happens when we call Range and mutate the builder.
 			b.Range(func(l Label) {
@@ -604,7 +604,7 @@ func TestBuilder(t *testing.T) {
 					b.Del(l.Name)
 				}
 			})
-			require.Equal(t, tcase.want.BytesWithoutLabels(nil, "aaa", "bbb"), b.Labels(tcase.base).Bytes(nil))
+			require.Equal(t, tcase.want.BytesWithoutLabels(nil, "aaa", "bbb"), b.Labels().Bytes(nil))
 		})
 	}
 }
@@ -669,7 +669,7 @@ func BenchmarkLabels_Hash(b *testing.B) {
 					// Label ~20B name, 50B value.
 					b.Set(fmt.Sprintf("abcdefghijabcdefghijabcdefghij%d", i), fmt.Sprintf("abcdefghijabcdefghijabcdefghijabcdefghijabcdefghij%d", i))
 				}
-				return b.Labels(EmptyLabels())
+				return b.Labels()
 			}(),
 		},
 		{
@@ -680,7 +680,7 @@ func BenchmarkLabels_Hash(b *testing.B) {
 					// Label ~50B name, 50B value.
 					b.Set(fmt.Sprintf("abcdefghijabcdefghijabcdefghijabcdefghijabcdefghij%d", i), fmt.Sprintf("abcdefghijabcdefghijabcdefghijabcdefghijabcdefghij%d", i))
 				}
-				return b.Labels(EmptyLabels())
+				return b.Labels()
 			}(),
 		},
 		{
@@ -729,7 +729,7 @@ func BenchmarkBuilder(b *testing.B) {
 		for _, l := range m {
 			builder.Set(l.Name, l.Value)
 		}
-		l = builder.Labels(EmptyLabels())
+		l = builder.Labels()
 	}
 	require.Equal(b, 9, l.Len())
 }

model/relabel/relabel.go

@@ -211,7 +211,7 @@ func Process(lbls labels.Labels, cfgs ...*Config) (ret labels.Labels, keep bool)
 	if !ProcessBuilder(lb, cfgs...) {
 		return labels.EmptyLabels(), false
 	}
-	return lb.Labels(lbls), true
+	return lb.Labels(), true
 }
 
 // ProcessBuilder is like Process, but the caller passes a labels.Builder

notifier/notifier.go

@@ -359,7 +359,7 @@ func (n *Manager) Send(alerts ...*Alert) {
 			}
 		})
 
-		a.Labels = lb.Labels(a.Labels)
+		a.Labels = lb.Labels()
 	}
 
 	alerts = n.relabelAlerts(alerts)

promql/engine.go

@@ -2193,7 +2193,7 @@ func resultMetric(lhs, rhs labels.Labels, op parser.ItemType, matching *parser.V
 		}
 	}
 
-	ret := enh.lb.Labels(labels.EmptyLabels())
+	ret := enh.lb.Labels()
 	enh.resultMetric[str] = ret
 	return ret
 }
@@ -2233,7 +2233,7 @@ func (ev *evaluator) VectorscalarBinop(op parser.ItemType, lhs Vector, rhs Scala
 }
 
 func dropMetricName(l labels.Labels) labels.Labels {
-	return labels.NewBuilder(l).Del(labels.MetricName).Labels(labels.EmptyLabels())
+	return labels.NewBuilder(l).Del(labels.MetricName).Labels()
 }
 
 // scalarBinop evaluates a binary operation between two Scalars.
@@ -2367,7 +2367,7 @@ func (ev *evaluator) aggregation(op parser.ItemType, grouping []string, without
 		if op == parser.COUNT_VALUES {
 			enh.resetBuilder(metric)
 			enh.lb.Set(valueLabel, strconv.FormatFloat(s.V, 'f', -1, 64))
-			metric = enh.lb.Labels(labels.EmptyLabels())
+			metric = enh.lb.Labels()
 
 			// We've changed the metric so we have to recompute the grouping key.
 			recomputeGroupingKey = true
@@ -2389,10 +2389,10 @@ func (ev *evaluator) aggregation(op parser.ItemType, grouping []string, without
 			if without {
 				enh.lb.Del(grouping...)
 				enh.lb.Del(labels.MetricName)
-				m = enh.lb.Labels(labels.EmptyLabels())
+				m = enh.lb.Labels()
 			} else if len(grouping) > 0 {
 				enh.lb.Keep(grouping...)
-				m = enh.lb.Labels(labels.EmptyLabels())
+				m = enh.lb.Labels()
 			} else {
 				m = labels.EmptyLabels()
 			}

promql/functions.go

@@ -960,7 +960,7 @@ func funcHistogramQuantile(vals []parser.Value, args parser.Expressions, enh *Ev
 		if !ok {
 			sample.Metric = labels.NewBuilder(sample.Metric).
 				Del(excludedLabels...).
-				Labels(labels.EmptyLabels())
+				Labels()
 
 			mb = &metricWithBuckets{sample.Metric, nil}
 			enh.signatureToMetricWithBuckets[string(enh.lblBuf)] = mb
@@ -1080,7 +1080,7 @@ func funcLabelReplace(vals []parser.Value, args parser.Expressions, enh *EvalNod
 				if len(res) > 0 {
 					lb.Set(dst, string(res))
 				}
-				outMetric = lb.Labels(labels.EmptyLabels())
+				outMetric = lb.Labels()
 				enh.Dmn[h] = outMetric
 			}
 		}
@@ -1148,7 +1148,7 @@ func funcLabelJoin(vals []parser.Value, args parser.Expressions, enh *EvalNodeHe
 				lb.Set(dst, strval)
 			}
 
-			outMetric = lb.Labels(labels.EmptyLabels())
+			outMetric = lb.Labels()
 			enh.Dmn[h] = outMetric
 		}
 
@@ -1414,7 +1414,7 @@ func createLabelsForAbsentFunction(expr parser.Expr) labels.Labels {
 		}
 	}
 
-	return b.Labels(labels.EmptyLabels())
+	return b.Labels()
 }
 
 func stringFromArg(e parser.Expr) string {

promql/parser/generated_parser.y

@@ -567,7 +567,7 @@ label_matcher   : IDENTIFIER match_op STRING
  */
 
 metric          : metric_identifier label_set
-                        { b := labels.NewBuilder($2); b.Set(labels.MetricName, $1.Val); $$ = b.Labels(labels.EmptyLabels()) }
+                        { b := labels.NewBuilder($2); b.Set(labels.MetricName, $1.Val); $$ = b.Labels() }
                 | label_set
                         {$$ = $1}
                 ;

promql/parser/generated_parser.y.go

@@ -1494,7 +1494,7 @@ yydefault:
 		{
 			b := labels.NewBuilder(yyDollar[2].labels)
 			b.Set(labels.MetricName, yyDollar[1].item.Val)
-			yyVAL.labels = b.Labels(labels.EmptyLabels())
+			yyVAL.labels = b.Labels()
 		}
 	case 96:
 		yyDollar = yyS[yypt-1 : yypt+1]

rules/alerting.go

@@ -234,7 +234,7 @@ func (r *AlertingRule) sample(alert *Alert, ts time.Time) promql.Sample {
 	lb.Set(alertStateLabel, alert.State.String())
 
 	s := promql.Sample{
-		Metric: lb.Labels(labels.EmptyLabels()),
+		Metric: lb.Labels(),
 		Point:  promql.Point{T: timestamp.FromTime(ts), V: 1},
 	}
 	return s
@@ -252,7 +252,7 @@ func (r *AlertingRule) forStateSample(alert *Alert, ts time.Time, v float64) pro
 	lb.Set(labels.AlertName, r.name)
 
 	s := promql.Sample{
-		Metric: lb.Labels(labels.EmptyLabels()),
+		Metric: lb.Labels(),
 		Point:  promql.Point{T: timestamp.FromTime(ts), V: v},
 	}
 	return s
@@ -381,7 +381,7 @@ func (r *AlertingRule) Eval(ctx context.Context, ts time.Time, query QueryFunc,
 		})
 		annotations := sb.Labels()
 
-		lbs := lb.Labels(labels.EmptyLabels())
+		lbs := lb.Labels()
 		h := lbs.Hash()
 		resultFPs[h] = struct{}{}
 

rules/recording.go

@@ -93,7 +93,7 @@ func (rule *RecordingRule) Eval(ctx context.Context, ts time.Time, query QueryFu
 			lb.Set(l.Name, l.Value)
 		})
 
-		sample.Metric = lb.Labels(sample.Metric)
+		sample.Metric = lb.Labels()
 	}
 
 	// Check that the rule does not produce identical metrics after applying

scrape/scrape.go

@@ -692,7 +692,7 @@ func mutateSampleLabels(lset labels.Labels, target *Target, honor bool, rc []*re
 		}
 	}
 
-	res := lb.Labels(labels.EmptyLabels())
+	res := lb.Labels()
 
 	if len(rc) > 0 {
 		res, _ = relabel.Process(res, rc...)
@@ -726,7 +726,7 @@ func mutateReportSampleLabels(lset labels.Labels, target *Target) labels.Labels
 		lb.Set(l.Name, l.Value)
 	})
 
-	return lb.Labels(labels.EmptyLabels())
+	return lb.Labels()
 }
 
 // appender returns an appender for ingested samples from the target.

scrape/target.go

@@ -380,7 +380,7 @@ func PopulateLabels(lb *labels.Builder, cfg *config.ScrapeConfig, noDefaultPort
 		}
 	}
 
-	preRelabelLabels := lb.Labels(labels.EmptyLabels())
+	preRelabelLabels := lb.Labels()
 	keep := relabel.ProcessBuilder(lb, cfg.RelabelConfigs...)
 
 	// Check if the target was dropped.
@@ -476,7 +476,7 @@ func PopulateLabels(lb *labels.Builder, cfg *config.ScrapeConfig, noDefaultPort
 		lb.Set(model.InstanceLabel, addr)
 	}
 
-	res = lb.Labels(labels.EmptyLabels())
+	res = lb.Labels()
 	err = res.Validate(func(l labels.Label) error {
 		// Check label values are valid, drop the target if not.
 		if !model.LabelValue(l.Value).IsValid() {

scrape/target_test.go

@@ -140,7 +140,7 @@ func newTestTarget(targetURL string, deadline time.Duration, lbls labels.Labels)
 	lb.Set(model.AddressLabel, strings.TrimPrefix(targetURL, "http://"))
 	lb.Set(model.MetricsPathLabel, "/metrics")
 
-	return &Target{labels: lb.Labels(labels.EmptyLabels())}
+	return &Target{labels: lb.Labels()}
 }
 
 func TestNewHTTPBearerToken(t *testing.T) {

storage/remote/read.go

@@ -278,5 +278,5 @@ func (sf seriesFilter) Labels() labels.Labels {
 	b := labels.NewBuilder(sf.Series.Labels())
 	// todo: check if this is too inefficient.
 	b.Del(sf.toFilter...)
-	return b.Labels(labels.EmptyLabels())
+	return b.Labels()
 }

tsdb/compact_test.go

@@ -1569,20 +1569,20 @@ func TestSparseHistogramSpaceSavings(t *testing.T) {
 							for it.Next() {
 								numOldSeriesPerHistogram++
 								b := it.At()
-								lbls := labels.NewBuilder(ah.baseLabels).Set("le", fmt.Sprintf("%.16f", b.Upper)).Labels(labels.EmptyLabels())
+								lbls := labels.NewBuilder(ah.baseLabels).Set("le", fmt.Sprintf("%.16f", b.Upper)).Labels()
 								refs[itIdx], err = oldApp.Append(refs[itIdx], lbls, ts, float64(b.Count))
 								require.NoError(t, err)
 								itIdx++
 							}
 							baseName := ah.baseLabels.Get(labels.MetricName)
 							// _count metric.
-							countLbls := labels.NewBuilder(ah.baseLabels).Set(labels.MetricName, baseName+"_count").Labels(labels.EmptyLabels())
+							countLbls := labels.NewBuilder(ah.baseLabels).Set(labels.MetricName, baseName+"_count").Labels()
 							_, err = oldApp.Append(0, countLbls, ts, float64(h.Count))
 							require.NoError(t, err)
 							numOldSeriesPerHistogram++
 
 							// _sum metric.
-							sumLbls := labels.NewBuilder(ah.baseLabels).Set(labels.MetricName, baseName+"_sum").Labels(labels.EmptyLabels())
+							sumLbls := labels.NewBuilder(ah.baseLabels).Set(labels.MetricName, baseName+"_sum").Labels()
 							_, err = oldApp.Append(0, sumLbls, ts, h.Sum)
 							require.NoError(t, err)
 							numOldSeriesPerHistogram++