Check for nil handler in legacy pipeline (#4584)

* fix crash when pipeline handler is not found

Signed-off-by: Justin Kolberg <amd.prophet@gmail.com>

* Add test for bugfix

Signed-off-by: Eric Chlebek <eric@sensu.io>

Co-authored-by: Eric Chlebek <eric@sensu.io>

CHANGELOG.md

@@ -14,6 +14,9 @@ Versioning](http://semver.org/spec/v2.0.0.html).
 - Add `sensu_go_event_metric_points_processed` counter metric and
 included it in tessen reporting.
 
+### Fixed
+- Referencing a non-existent handler in a pipeline no longer results in a crash.
+
 ## [6.6.3] - 2021-12-15
 
 ### Added

backend/pipeline/handler/legacy.go

@@ -59,13 +59,19 @@ func (l *LegacyAdapter) Handle(ctx context.Context, ref *corev2.ResourceReferenc
 	fields := utillogging.EventFields(event, false)
 	fields["pipeline"] = corev2.ContextPipeline(ctx)
 	fields["pipeline_workflow"] = corev2.ContextPipelineWorkflow(ctx)
+	fields["handler"] = ref.Name
 
 	tctx, cancel := context.WithTimeout(ctx, l.StoreTimeout)
 	handler, err := l.Store.GetHandlerByName(tctx, ref.Name)
 	cancel()
 	if err != nil {
 		return fmt.Errorf("failed to fetch handler from store: %v", err)
 	}
+	if handler == nil {
+		logger.WithFields(fields).
+			Error("handler not found, skipping handler execution")
+		return nil
+	}
 
 	switch handler.Type {
 	case "pipe":

backend/pipeline/handler/legacy_test.go

@@ -34,6 +34,19 @@ func init() {
 	logrus.SetOutput(ioutil.Discard)
 }
 
+func TestNilHandlerBug_GH4584(t *testing.T) {
+	// tests to make sure Handle() doesn't panic when the handler is not found.
+	mockStore := new(mockstore.MockStore)
+	mockStore.On("GetHandlerByName", mock.Anything, mock.Anything).Return((*corev2.Handler)(nil), nil)
+	adapter := &LegacyAdapter{
+		Store: mockStore,
+	}
+	err := adapter.Handle(context.Background(), &corev2.ResourceReference{}, corev2.FixtureEvent("foo", "bar"), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestHelperHandlerProcess(t *testing.T) {
 	if os.Getenv("GO_WANT_HELPER_HANDLER_PROCESS") != "1" {
 		return