Re-add tekton.dev/task label to taskruns from ClusterTasks

In #2390 TaskRuns referencing ClusterTasks had their label changed
from tekton.dev/task=<Name> to tekton.dev/clusterTask=<Name>. This
is a backwards-incompatible change for teams relying on that original
task label.

This commit adds both labels to TaskRuns referencing ClusterTasks.
We can then deprecate the tekton.dev/task= label on these TaskRuns
in accordance with Pipelines' API deprecation policy.

Author: Scott

docs/labels.md

@@ -54,7 +54,8 @@ The following labels are added to resources automatically:
   references.
 - `tekton.dev/clusterTask` is added to `TaskRuns` (and propagated to `Pods`) that
   reference an existing `ClusterTask`and contains the name of the `ClusterTask` 
-  that the `TaskRun` references.
+  that the `TaskRun` references. For backwards compatibility, `TaskRuns` that
+  reference a `ClusterTask` will also receive `tekton.dev/task`.
 - `tekton.dev/taskRun` is added to `Pods`, and contains the name of the
   `TaskRun` that created the `Pod`.
 
@@ -88,4 +89,4 @@ the following command:
 
 ```shell
 kubectl get taskruns --all-namespaces -l tekton.dev/clusterTask=test-clustertask
-```
+```

pkg/reconciler/taskrun/taskrun.go

@@ -224,10 +224,9 @@ func (c *Reconciler) reconcile(ctx context.Context, tr *v1alpha1.TaskRun) error
 		tr.ObjectMeta.Labels[key] = value
 	}
 	if tr.Spec.TaskRef != nil {
+		tr.ObjectMeta.Labels[pipeline.GroupName+pipeline.TaskLabelKey] = taskMeta.Name
 		if tr.Spec.TaskRef.Kind == "ClusterTask" {
 			tr.ObjectMeta.Labels[pipeline.GroupName+pipeline.ClusterTaskLabelKey] = taskMeta.Name
-		} else {
-			tr.ObjectMeta.Labels[pipeline.GroupName+pipeline.TaskLabelKey] = taskMeta.Name
 		}
 	}
 

pkg/reconciler/taskrun/taskrun_test.go

@@ -795,6 +795,7 @@ func TestReconcile(t *testing.T) {
 		wantPod: tb.Pod("test-taskrun-with-cluster-task-pod-abcde",
 			tb.PodNamespace("foo"),
 			tb.PodAnnotation(podconvert.ReleaseAnnotation, podconvert.ReleaseAnnotationValue),
+			tb.PodLabel(taskNameLabelKey, "test-cluster-task"),
 			tb.PodLabel(clusterTaskNameLabelKey, "test-cluster-task"),
 			tb.PodLabel(taskRunNameLabelKey, "test-taskrun-with-cluster-task"),
 			tb.PodLabel("app.kubernetes.io/managed-by", "tekton-pipelines"),