Automatically create named directories for each output resource in a …

…TaskRun.

TaskRuns are expected to place files related to an Output in a directory named
for each Output. Right now, we expect Task authors to create this directory before
placing files there. This is onerous and error-prone, so we should automatically create
them.

There are a few approaches to doing this:
- a mkdir init container
- a volume
- a mkdir pod container

I chose to use the final one, a mkdir pod container for consistency with Input resources.
This is prepended to the Task steps.

pkg/apis/pipeline/v1alpha1/artifact_pvc.go

@@ -90,7 +90,7 @@ func GetPvcMount(name string) corev1.VolumeMount {
 // CreateDirContainer returns a container step to create a dir
 func CreateDirContainer(name, destinationPath string) corev1.Container {
 	return corev1.Container{
-		Name:    names.SimpleNameGenerator.RestrictLengthWithRandomSuffix(fmt.Sprintf("create-dir-%s", name)),
+		Name:    names.SimpleNameGenerator.RestrictLengthWithRandomSuffix(fmt.Sprintf("create-dir-%s", strings.ToLower(name))),
 		Image:   *BashNoopImage,
 		Command: []string{"/ko-app/bash"},
 		Args:    []string{"-args", strings.Join([]string{"mkdir", "-p", destinationPath}, " ")},

pkg/reconciler/v1alpha1/taskrun/resources/output_resource.go

@@ -129,6 +129,10 @@ func AddOutputResources(
 			resourceVolumes = append(resourceVolumes, as.GetSecretsVolumes()...)
 		}
 
+		// Add containers to mkdir each output directory. This should run before the build steps themselves.
+		mkdirStep := []corev1.Container{v1alpha1.CreateDirContainer(boundResource.Name, sourcePath)}
+		taskSpec.Steps = append(mkdirStep, taskSpec.Steps...)
+
 		taskSpec.Steps = append(taskSpec.Steps, resourceContainers...)
 		taskSpec.Volumes = append(taskSpec.Volumes, resourceVolumes...)