Consider not-found pod as permanent when taskrun is done

When the taskrun is "done" i.e. it has a completion time set,
we try to stop sidecars. If the pod cannot be found, it has most
likely been evicted and in any case the sidecar is not running
anymore, so we should not return a transient error.

Fixes tektoncd#3517

Signed-off-by: Andrea Frittoli <andrea.frittoli@uk.ibm.com>

pkg/reconciler/taskrun/taskrun.go

@@ -213,8 +213,9 @@ func (c *Reconciler) stopSidecars(ctx context.Context, tr *v1beta1.TaskRun) (*co
 	}
 
 	if k8serrors.IsNotFound(err) {
-		// failed to get the pod, return error without any sidecars
-		return nil, err
+		// At this stage the TaskRun has been completed if the pod is not found, it won't come back,
+		// it has probably evicted. We can return the error, but we consider it a permanent one.
+		return nil, controller.NewPermanentError(err)
 	} else if err != nil {
 		logger.Errorf("Error stopping sidecars for TaskRun %q: %v", tr.Name, err)
 		tr.Status.MarkResourceFailed(podconvert.ReasonFailedResolution, err)