Merge pull request #472 from prometheus/beorn7/registry

Fix a few minor things...

prometheus/collector.go

@@ -40,7 +40,8 @@ type Collector interface {
 	// Collector may yield any Metric it sees fit in its Collect method.
 	//
 	// This method idempotently sends the same descriptors throughout the
-	// lifetime of the Collector.
+	// lifetime of the Collector. It may be called concurrently and
+	// therefore must be implemented in a concurrency safe way.
 	//
 	// If a Collector encounters an error while executing this method, it
 	// must send an invalid descriptor (created with NewInvalidDesc) to

prometheus/registry.go

@@ -107,9 +107,6 @@ type Registerer interface {
 	// Collector, and for providing a Collector that will not cause
 	// inconsistent metrics on collection. (This would lead to scrape
 	// errors.)
-	//
-	// It is in general not safe to register the same Collector multiple
-	// times concurrently.
 	Register(Collector) error
 	// MustRegister works like Register but registers any number of
 	// Collectors and panics upon the first registration that causes an
@@ -273,7 +270,12 @@ func (r *Registry) Register(c Collector) error {
 		close(descChan)
 	}()
 	r.mtx.Lock()
-	defer r.mtx.Unlock()
+	defer func() {
+		// Drain channel in case of premature return to not leak a goroutine.
+		for range descChan {
+		}
+		r.mtx.Unlock()
+	}()
 	// Conduct various tests...
 	for desc := range descChan {