Check that metric_relabel_configs has non-nil configuration in runtime config

CHANGELOG.md

@@ -34,6 +34,7 @@
 * [BUGFIX] Update `github.com/thanos-io/objstore` to address issue with Multipart PUT on s3-compatible Object Storage. #3802 #3821
 * [BUGFIX] Distributor, Query-scheduler: Make sure ring metrics include a `cortex_` prefix as expected by dashboards. #3809
 * [BUGFIX] Querier: canceled requests are no longer reported as "consistency check" failures. #3837
+* [BUGFIX] Distributor: don't panic when `metric_relabel_configs` in overrides contains null element. #3868
 
 ### Mixin
 

pkg/util/validation/limits.go

@@ -272,7 +272,7 @@ func (l *Limits) UnmarshalYAML(value *yaml.Node) error {
 		return err
 	}
 
-	return nil
+	return l.validate()
 }
 
 // UnmarshalJSON implements the json.Unmarshaler interface.
@@ -295,6 +295,16 @@ func (l *Limits) UnmarshalJSON(data []byte) error {
 		return err
 	}
 
+	return l.validate()
+}
+
+func (l *Limits) validate() error {
+	for _, cfg := range l.MetricRelabelConfigs {
+		if cfg == nil {
+			return fmt.Errorf("invalid metric_relabel_configs")
+		}
+	}
+
 	return nil
 }
 

pkg/util/validation/limits_test.go

@@ -627,3 +627,22 @@ func TestCustomTrackerConfigDeserialize(t *testing.T) {
 	assert.False(t, overrides["user"].ActiveSeriesCustomTrackersConfig.Empty())
 	assert.Equal(t, expectedConfig.String(), overrides["user"].ActiveSeriesCustomTrackersConfig.String())
 }
+
+func TestUnmarshalInvalidMetricRelabelConfig(t *testing.T) {
+	t.Run("yaml", func(t *testing.T) {
+		limits := Limits{}
+		cfg := `
+metric_relabel_configs:
+  -
+`
+		err := yaml.Unmarshal([]byte(cfg), &limits)
+		require.ErrorContains(t, err, "invalid metric_relabel_configs")
+	})
+
+	t.Run("json", func(t *testing.T) {
+		limits := Limits{}
+		cfg := `{"metric_relabel_configs": [null]}`
+		err := json.Unmarshal([]byte(cfg), &limits)
+		require.ErrorContains(t, err, "invalid metric_relabel_configs")
+	})
+}