Ensure that nil registers are treat as a no-op, even when wrapping.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>

prometheus/promauto/auto_test.go

@@ -0,0 +1,29 @@
+// Copyright 2020 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package promauto
+
+import (
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+func TestWrapNil(t *testing.T) {
+	// A nil registerer should be treated as a no-op by promauto, even when wrapped.
+	registerer := prometheus.WrapRegistererWith(prometheus.Labels{"foo": "bar"}, nil)
+	c := With(registerer).NewCounter(prometheus.CounterOpts{
+		Name: "test",
+	})
+	c.Inc()
+}

prometheus/wrap.go

@@ -81,6 +81,9 @@ type wrappingRegisterer struct {
 }
 
 func (r *wrappingRegisterer) Register(c Collector) error {
+	if r.wrappedRegisterer == nil {
+		return nil
+	}
 	return r.wrappedRegisterer.Register(&wrappingCollector{
 		wrappedCollector: c,
 		prefix:           r.prefix,
@@ -89,6 +92,9 @@ func (r *wrappingRegisterer) Register(c Collector) error {
 }
 
 func (r *wrappingRegisterer) MustRegister(cs ...Collector) {
+	if r.wrappedRegisterer == nil {
+		return
+	}
 	for _, c := range cs {
 		if err := r.Register(c); err != nil {
 			panic(err)
@@ -97,6 +103,9 @@ func (r *wrappingRegisterer) MustRegister(cs ...Collector) {
 }
 
 func (r *wrappingRegisterer) Unregister(c Collector) bool {
+	if r.wrappedRegisterer == nil {
+		return false
+	}
 	return r.wrappedRegisterer.Unregister(&wrappingCollector{
 		wrappedCollector: c,
 		prefix:           r.prefix,