Handle closed target provider channel

This fixes the case where a target provider closes the update
channel and exits before the context is canceled.
This should only be true for the static provider but it's safer
to generally handle this case.

retrieval/targetmanager.go

@@ -274,28 +274,28 @@ func (ts *targetSet) runProviders(ctx context.Context, providers map[string]Targ
 		updates := make(chan []*config.TargetGroup)
 
 		go func(name string, prov TargetProvider) {
-			var initial []*config.TargetGroup
-
 			select {
 			case <-ctx.Done():
-				wg.Done()
-				return
-			case initial = <-updates:
+			case initial, ok := <-updates:
+				// Handle the case that a target provider exits and closes the channel
+				// before the context is done.
+				if !ok {
+					break
+				}
 				// First set of all targets the provider knows.
+				for _, tgroup := range initial {
+					targets, err := targetsFromGroup(tgroup, ts.config)
+					if err != nil {
+						log.With("target_group", tgroup).Errorf("Target update failed: %s", err)
+						continue
+					}
+					ts.tgroups[name+"/"+tgroup.Source] = targets
+				}
 			case <-time.After(5 * time.Second):
 				// Initial set didn't arrive. Act as if it was empty
 				// and wait for updates later on.
 			}
 
-			for _, tgroup := range initial {
-				targets, err := targetsFromGroup(tgroup, ts.config)
-				if err != nil {
-					log.With("target_group", tgroup).Errorf("Target update failed: %s", err)
-					continue
-				}
-				ts.tgroups[name+"/"+tgroup.Source] = targets
-			}
-
 			wg.Done()
 
 			// Start listening for further updates.
@@ -303,7 +303,12 @@ func (ts *targetSet) runProviders(ctx context.Context, providers map[string]Targ
 				select {
 				case <-ctx.Done():
 					return
-				case tgs := <-updates:
+				case tgs, ok := <-updates:
+					// Handle the case that a target provider exits and closes the channel
+					// before the context is done.
+					if !ok {
+						return
+					}
 					for _, tg := range tgs {
 						if err := ts.update(name, tg); err != nil {
 							log.With("target_group", tg).Errorf("Target update failed: %s", err)