Allow references to ClusterTasks in v1 Pipeline Tasks

We cannot yet remove support for ClusterTasks, so they should continue
to be referenceable from v1 Pipelines. This commit removes an error
message associated with referencing ClusterTasks in v1 Pipelines.

pkg/apis/pipeline/v1/pipeline_validation.go

@@ -131,6 +131,7 @@ func (pt PipelineTask) Validate(ctx context.Context) (errs *apis.FieldError) {
 	taskKinds := map[TaskKind]bool{
 		"":                 true,
 		NamespacedTaskKind: true,
+		ClusterTaskRefKind: true,
 	}
 	// Pipeline task having taskRef/taskSpec with APIVersion is classified as custom task
 	switch {

pkg/apis/pipeline/v1/pipeline_validation_test.go

@@ -77,6 +77,14 @@ func TestPipeline_Validate_Success(t *testing.T) {
 				Tasks: []PipelineTask{{Name: "foo", TaskRef: &TaskRef{Name: "bar", Kind: NamespacedTaskKind}}},
 			},
 		},
+	}, {
+		name: "valid reference to clusterTask",
+		p: &Pipeline{
+			ObjectMeta: metav1.ObjectMeta{Name: "pipeline"},
+			Spec: PipelineSpec{
+				Tasks: []PipelineTask{{Name: "foo", TaskRef: &TaskRef{Name: "bar", Kind: ClusterTaskRefKind}}},
+			},
+		},
 	}}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

pkg/apis/pipeline/v1/taskref_types.go

@@ -42,6 +42,9 @@ type TaskKind string
 const (
 	// NamespacedTaskKind indicates that the task type has a namespaced scope.
 	NamespacedTaskKind TaskKind = "Task"
+	// ClusterTaskRefKind is the task type for a reference to a task with cluster scope.
+	// ClusterTasks are not supported in v1, but v1 types may reference ClusterTasks.
+	ClusterTaskRefKind TaskKind = "ClusterTask"
 )
 
 // IsCustomTask checks whether the reference is to a Custom Task