Fix collectorID calculation for Unregister()

prometheus/registry.go

@@ -361,15 +361,15 @@ func (r *Registry) Unregister(c Collector) bool {
 var (
 descChan = make(chan *Desc, capDescChan)
 descIDs = map[uint64]struct{}{}
-collectorID uint64 // Just a sum of the desc IDs.
+collectorID uint64 // All desc IDs XOR'd together.
 )
 go func() {
 c.Describe(descChan)
 close(descChan)
 }()
 for desc := range descChan {
 if _, exists := descIDs[desc.id]; !exists {
-collectorID += desc.id
+collectorID ^= desc.id
 descIDs[desc.id] = struct{}{}
 }
 }

prometheus/registry_test.go

@@ -863,6 +863,23 @@ func TestAlreadyRegistered(t *testing.T) {
 }
 }
 
+// TestRegisterUnregisterCollector ensures registering and unregistering a
+// collector doesn't leave any dangling metrics.
+// We use NewGoCollector as a nice concrete example of a collector with
+// multiple metrics.
+func TestRegisterUnregisterCollector(t *testing.T) {
+col := prometheus.NewGoCollector()
+
+reg := prometheus.NewRegistry()
+reg.MustRegister(col)
+reg.Unregister(col)
+if metrics, err := reg.Gather(); err != nil {
+t.Error("error gathering sample metric")
+} else if len(metrics) != 0 {
+t.Error("should have unregistered metric")
+}
+}
+
 // TestHistogramVecRegisterGatherConcurrency is an end-to-end test that
 // concurrently calls Observe on random elements of a HistogramVec while the
 // same HistogramVec is registered concurrently and the Gather method of the