util/metric: allowed nils in arrays of MetricStruct implementations

It is useful to have arrays of structs just like it is useful to have arrays
of values when there are enums. In the face of enums, zero is often an
uninteresting value and we do not want to instantiate metrics for it. In this
case of individual metrics, this was achieved by leaving the entry nil. This
was not permitted in structs and would lead to panics. This commit fixes that.

Release note: None

Author: Andrew Werner

pkg/util/metric/registry.go

@@ -114,18 +114,18 @@ func (r *Registry) AddMetricStruct(metricStruct interface{}) {
 func (r *Registry) addMetricValue(
 	ctx context.Context, val reflect.Value, name string, skipNil bool,
 ) {
-	switch typ := val.Interface().(type) {
-	case Iterable:
-		if val.Kind() == reflect.Ptr && val.IsNil() {
-			if skipNil {
-				if log.V(2) {
-					log.Infof(ctx, "skipping nil metric field %s", name)
-				}
-			} else {
-				log.Fatalf(ctx, "found nil metric field %s", name)
+	if val.Kind() == reflect.Ptr && val.IsNil() {
+		if skipNil {
+			if log.V(2) {
+				log.Infof(ctx, "skipping nil metric field %s", name)
 			}
-			return
+		} else {
+			log.Fatalf(ctx, "found nil metric field %s", name)
 		}
+		return
+	}
+	switch typ := val.Interface().(type) {
+	case Iterable:
 		r.AddMetric(typ)
 	case Struct:
 		r.AddMetricStruct(typ)

pkg/util/metric/registry_test.go

@@ -88,6 +88,8 @@ func TestRegistry(t *testing.T) {
 		StructHistogram     *Histogram
 		NestedStructGauge   NestedStruct
 		ArrayStructCounters [4]*Counter
+		// Ensure that nil struct values in arrays are safe.
+		NestedStructArray [2]*NestedStruct
 		// A few extra ones: either not exported, or not metric objects.
 		privateStructGauge            *Gauge
 		privateStructGauge64          *GaugeFloat64
@@ -113,6 +115,12 @@ func TestRegistry(t *testing.T) {
 			nil, // skipped
 			NewCounter(Metadata{Name: "array.struct.counter.3"}),
 		},
+		NestedStructArray: [2]*NestedStruct{
+			0: nil, // skipped
+			1: {
+				NestedStructGauge: NewGauge(Metadata{Name: "nested.struct.array.1.gauge"}),
+			},
+		},
 		privateStructGauge:     NewGauge(Metadata{Name: "private.struct.gauge"}),
 		privateStructGauge64:   NewGaugeFloat64(Metadata{Name: "private.struct.gauge64"}),
 		privateStructCounter:   NewCounter(Metadata{Name: "private.struct.counter"}),
@@ -132,19 +140,20 @@ func TestRegistry(t *testing.T) {
 	r.AddMetricStruct(ms)
 
 	expNames := map[string]struct{}{
-		"top.histogram":          {},
-		"top.gauge":              {},
-		"top.floatgauge":         {},
-		"top.counter":            {},
-		"bottom.gauge":           {},
-		"struct.gauge":           {},
-		"struct.gauge64":         {},
-		"struct.counter":         {},
-		"struct.histogram":       {},
-		"nested.struct.gauge":    {},
-		"array.struct.counter.0": {},
-		"array.struct.counter.1": {},
-		"array.struct.counter.3": {},
+		"top.histogram":               {},
+		"top.gauge":                   {},
+		"top.floatgauge":              {},
+		"top.counter":                 {},
+		"bottom.gauge":                {},
+		"struct.gauge":                {},
+		"struct.gauge64":              {},
+		"struct.counter":              {},
+		"struct.histogram":            {},
+		"nested.struct.gauge":         {},
+		"array.struct.counter.0":      {},
+		"array.struct.counter.1":      {},
+		"array.struct.counter.3":      {},
+		"nested.struct.array.1.gauge": {},
 	}
 
 	r.Each(func(name string, _ interface{}) {