TaskDependentResource: support preview when the task isn't ready

upup/pkg/fi/changes.go

@@ -116,6 +116,11 @@ func equalFieldValues(a, e reflect.Value) bool {
 		if ok && (e.Kind() == reflect.Ptr || e.Kind() == reflect.Interface) && !e.IsNil() {
 			eResource, ok := e.Interface().(Resource)
 			if ok {
+				if hasIsReady, ok := eResource.(HasIsReady); ok {
+					if !hasIsReady.IsReady() {
+						return false
+					}
+				}
 				same, err := ResourcesMatch(aResource, eResource)
 				if err != nil {
 					klog.Fatalf("error while comparing resources: %v", err)

upup/pkg/fi/resources.go

@@ -30,6 +30,11 @@ type Resource interface {
 	Open() (io.Reader, error)
 }
 
+// HasIsReady is implemented by Resources that are derived (and thus may not be ready at comparison time)
+type HasIsReady interface {
+	IsReady() bool
+}
+
 type TemplateResource interface {
 	Resource
 	Curry(args []string) TemplateResource
@@ -264,14 +269,20 @@ type TaskDependentResource struct {
 
 var _ Resource = &TaskDependentResource{}
 var _ HasDependencies = &TaskDependentResource{}
+var _ HasIsReady = &TaskDependentResource{}
 
 func (r *TaskDependentResource) Open() (io.Reader, error) {
 	if r.Resource == nil {
-		return nil, fmt.Errorf("resource opened before it is ready")
+		return nil, fmt.Errorf("resource opened before it is ready (task=%v)", r.Task)
 	}
 	return r.Resource.Open()
 }
 
 func (r *TaskDependentResource) GetDependencies(tasks map[string]Task) []Task {
 	return []Task{r.Task}
 }
+
+// IsReady implements HasIsReady::IsReady
+func (r *TaskDependentResource) IsReady() bool {
+	return r.Resource != nil
+}