Fix for #3312 sigpanic in TargetManager (#3420)

retrieval/targetmanager.go

@@ -40,6 +40,7 @@ type TargetManager struct {
 	// Set of unqiue targets by scrape configuration.
 	targetSets map[string]*targetSet
 	logger     log.Logger
+	starting   chan struct{}
 }
 
 type targetSet struct {
@@ -61,6 +62,7 @@ func NewTargetManager(app Appendable, logger log.Logger) *TargetManager {
 		append:     app,
 		targetSets: map[string]*targetSet{},
 		logger:     logger,
+		starting:   make(chan struct{}),
 	}
 }
 
@@ -74,12 +76,14 @@ func (tm *TargetManager) Run() {
 	tm.reload()
 
 	tm.mtx.Unlock()
+	close(tm.starting)
 
 	tm.wg.Wait()
 }
 
 // Stop all background processing.
 func (tm *TargetManager) Stop() {
+	<-tm.starting
 	level.Info(tm.logger).Log("msg", "Stopping target manager...")
 
 	tm.mtx.Lock()