relabel: default value for source_labels should be nil and not empty slice (#5059)

* relabel: default value for source_labels should be nil and not empty
slice

Author: kalleep

CHANGELOG.md

@@ -69,6 +69,8 @@ Main (unreleased)
 
 - Allow configuration of `force_attempt_http2` and default it to `true` for otelcol exporters with HTTP client configurations. (@dehaansa)
 
+- Fix default values for relabel rules, this caused issues in e.g. `prometheus.operator.servicemonitors` when using labeldrop. (@kalleep)
+
 v1.12.0
 -----------------
 

internal/component/common/relabel/relabel.go

@@ -318,9 +318,12 @@ func doRelabel(cfg *Config, lb LabelBuilder) (keep bool) {
 func ComponentToPromRelabelConfigs(rcs []*Config) []*relabel.Config {
 	res := make([]*relabel.Config, len(rcs))
 	for i, rc := range rcs {
-		sourceLabels := make([]model.LabelName, len(rc.SourceLabels))
-		for i, sl := range rc.SourceLabels {
-			sourceLabels[i] = model.LabelName(sl)
+		var sourceLabels []model.LabelName
+		if len(rc.SourceLabels) > 0 {
+			sourceLabels = make([]model.LabelName, len(rc.SourceLabels))
+			for i, sl := range rc.SourceLabels {
+				sourceLabels[i] = model.LabelName(sl)
+			}
 		}
 
 		res[i] = &relabel.Config{

internal/component/common/relabel/relabel_test.go

@@ -19,10 +19,12 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/grafana/alloy/syntax"
 	"github.com/grafana/regexp"
 	"github.com/stretchr/testify/require"
 	"gopkg.in/yaml.v2"
 
+	"github.com/prometheus/common/model"
 	"github.com/prometheus/prometheus/model/labels"
 	"github.com/prometheus/prometheus/util/testutil"
 )
@@ -857,6 +859,21 @@ func BenchmarkRelabel(b *testing.B) {
 	}
 }
 
+func TestComponentToPromRelabelConfigs(t *testing.T) {
+	rule := `
+		action = "labeldrop"
+        regex  = "helm_sh_chart"
+	`
+
+	var cfg Config
+	require.NoError(t, syntax.Unmarshal([]byte(rule), &cfg))
+
+	converted := ComponentToPromRelabelConfigs([]*Config{&cfg})
+	for _, r := range converted {
+		require.NoError(t, r.Validate(model.LegacyValidation))
+	}
+}
+
 // MustNewRegexp works like NewRegexp, but panics if the regular expression does not compile.
 func MustNewRegexp(s string) Regexp {
 	re, err := NewRegexp(s)