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

prometheus/promauto/auto_test.go

@@ -0,0 +1,25 @@
+// 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 TestNil(t *testing.T) {
+	// A nil registerer should be treated as a no-op by promauto.
+	With(nil).NewCounter(prometheus.CounterOpts{Name: "test"}).Inc()
+}

prometheus/wrap.go

@@ -28,7 +28,8 @@ import (
 // registered with the wrapped Registerer in a modified way. The modified
 // Collector adds the provided Labels to all Metrics it collects (as
 // ConstLabels). The Metrics collected by the unmodified Collector must not
-// duplicate any of those labels.
+// duplicate any of those labels. Wrapping a nil value is valid, resulting
+// in a no-op Registerer.
 //
 // WrapRegistererWith provides a way to add fixed labels to a subset of
 // Collectors. It should not be used to add fixed labels to all metrics exposed.
@@ -51,6 +52,7 @@ func WrapRegistererWith(labels Labels, reg Registerer) Registerer {
 // Registerer. Collectors registered with the returned Registerer will be
 // registered with the wrapped Registerer in a modified way. The modified
 // Collector adds the provided prefix to the name of all Metrics it collects.
+// Wrapping a nil value is valid, resulting in a no-op Registerer.
 //
 // WrapRegistererWithPrefix is useful to have one place to prefix all metrics of
 // a sub-system. To make this work, register metrics of the sub-system with the
@@ -81,6 +83,9 @@ 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 +94,9 @@ 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 +105,9 @@ 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,

prometheus/wrap_test.go

@@ -321,3 +321,12 @@ func TestWrap(t *testing.T) {
 	}
 
 }
+
+func TestNil(t *testing.T) {
+	// A wrapped nil registerer should be treated as a no-op, and not panic.
+	c := NewCounter(CounterOpts{Name: "test"})
+	err := WrapRegistererWith(Labels{"foo": "bar"}, nil).Register(c)
+	if err != nil {
+		t.Fatal("registering failed:", err)
+	}
+}