fix: runner defer executed too early

The recent separation of a blocking Run() and a non-blocking
RunBackground() introduced a bug that prematurely executed
registered defer functions on the runner.
This is a critical error that is now fixed by
having RunBackground return the defer function as well.

Author: ezdac

rolling-shutter/keyper/epochkghandler/trigger_test.go

@@ -44,7 +44,7 @@ func TestHandleDecryptionTriggerIntegration(t *testing.T) {
 		Messaging:     messaging,
 		Trigger:       decrTrigChan,
 	}
-	group := service.RunBackground(
+	group, cleanup := service.RunBackground(
 		ctx,
 		ksh,
 	)
@@ -58,6 +58,7 @@ func TestHandleDecryptionTriggerIntegration(t *testing.T) {
 	decrTrigChan <- broker.NewEvent(trig)
 	close(decrTrigChan)
 	err = group.Wait()
+	cleanup()
 	assert.NilError(t, err)
 
 	// send decryption key share when first trigger is received

rolling-shutter/keyperimpl/rollup/trigger_test.go

@@ -47,10 +47,11 @@ func TestHandleDecryptionTriggerIntegration(t *testing.T) {
 	assert.NilError(t, err)
 	sender.AddMessageHandler(triggerHandler)
 
-	group := service.RunBackground(
+	group, cleanup := service.RunBackground(
 		ctx,
 		sender,
 	)
+	defer cleanup()
 
 	testsetup.InitializeEon(ctx, t, dbpool, config, keyperIndex)
 

rolling-shutter/medley/service/service.go

@@ -65,18 +65,22 @@ func (r *runner) runShutdownFuncs() {
 }
 
 func Run(ctx context.Context, services ...Service) error {
-	group := RunBackground(ctx, services...)
+	group, deferFn := RunBackground(ctx, services...)
+	defer deferFn()
 	return group.Wait()
 }
 
-func RunBackground(ctx context.Context, services ...Service) *errgroup.Group {
+// RunBackground runs the services within the context of an errgroup.Group.
+// It returns the errgroup.Group as well as the cleanup function to be deferred.
+// This is a low-level executor of services, the defer function is expected
+// to be called!
+func RunBackground(ctx context.Context, services ...Service) (*errgroup.Group, func()) {
 	group, ctx := errgroup.WithContext(ctx)
 	r := runner{group: group, ctx: ctx}
 	group.Go(func() error {
 		return r.StartService(services...)
 	})
-	defer r.runShutdownFuncs()
-	return group
+	return group, r.runShutdownFuncs
 }
 
 // notifyTermination creates a context that is canceled, when the process receives SIGINT or