feat: Support for scaling from/to zero (#465)

This is an attempt to support scaling from/to zero.

The basic idea is that we create a one-off "registration-only" runner pod on RunnerReplicaSet being scaled to zero, so that there is one "offline" runner, which enables GitHub Actions to queue jobs instead of discarding those.

GitHub Actions seems to immediately throw away the new job when there are no runners at all. Generally, having runners of any status, `busy`, `idle`, or `offline` would prevent GitHub actions from failing jobs. But retaining `busy` or `idle` runners means that we need to keep runner pods running, which conflicts with our desired to scale to/from zero, hence we retain `offline` runners.

In this change, I enhanced the runnerreplicaset controller to create a registration-only runner on very beginning of its reconciliation logic, only when a runnerreplicaset is scaled to zero. The runner controller creates the registration-only runner pod, waits for it to become "offline", and then removes the runner pod. The runner on GitHub stays `offline`, until the runner resource on K8s is deleted. As we remove the registration-only runner pod as soon as it registers, this doesn't block cluster-autoscaler.

Related to #447

controllers/runnerreplicaset_controller.go

@@ -68,6 +68,65 @@ func (r *RunnerReplicaSetReconciler) Reconcile(req ctrl.Request) (ctrl.Result, e
 		return ctrl.Result{}, nil
 	}
 
+	registrationOnlyRunnerNeeded := rs.Spec.Replicas != nil && *rs.Spec.Replicas == 0
+	registrationOnlyRunner := v1alpha1.Runner{}
+	registrationOnlyRunnerNsName := req.NamespacedName
+	registrationOnlyRunnerNsName.Name = registrationOnlyRunnerNameFor(rs.Name)
+
+	registrationOnlyRunnerExists := false
+	if err := r.Get(
+		ctx,
+		registrationOnlyRunnerNsName,
+		&registrationOnlyRunner,
+	); err != nil {
+		if !kerrors.IsNotFound(err) {
+			return ctrl.Result{}, err
+		}
+	} else {
+		registrationOnlyRunnerExists = true
+	}
+
+	if registrationOnlyRunnerNeeded {
+		if registrationOnlyRunnerExists {
+			if registrationOnlyRunner.Status.Phase == "" {
+				log.Info("Still waiting for the registration-only runner to be registered")
+
+				return ctrl.Result{}, nil
+			}
+		} else {
+			// A registration-only runner does not exist and is needed, hence create it.
+
+			runnerForScaleFromToZero, err := r.newRunner(rs)
+			if err != nil {
+				return ctrl.Result{}, fmt.Errorf("failed to create runner for scale from/to zero: %v", err)
+			}
+
+			runnerForScaleFromToZero.ObjectMeta.Name = registrationOnlyRunnerNsName.Name
+			runnerForScaleFromToZero.ObjectMeta.GenerateName = ""
+			runnerForScaleFromToZero.ObjectMeta.Labels = nil
+			metav1.SetMetaDataAnnotation(&runnerForScaleFromToZero.ObjectMeta, annotationKeyRegistrationOnly, "true")
+
+			if err := r.Client.Create(ctx, &runnerForScaleFromToZero); err != nil {
+				log.Error(err, "Failed to create runner for scale from/to zero")
+
+				return ctrl.Result{}, err
+			}
+
+			// We can continue to deleting runner pods only after the
+			// registration-only runner gets registered.
+			return ctrl.Result{}, nil
+		}
+	} else {
+		// A registration-only runner exists and is not needed, hence delete it.
+		if registrationOnlyRunnerExists {
+			if err := r.Client.Delete(ctx, &registrationOnlyRunner); err != nil {
+				log.Error(err, "Retrying soon because we failed to delete registration-only runner")
+
+				return ctrl.Result{Requeue: true}, nil
+			}
+		}
+	}
+
 	selector, err := metav1.LabelSelectorAsSelector(rs.Spec.Selector)
 	if err != nil {
 		return ctrl.Result{}, err
@@ -95,7 +154,7 @@ func (r *RunnerReplicaSetReconciler) Reconcile(req ctrl.Request) (ctrl.Result, e
 	for _, r := range allRunners.Items {
 		// This guard is required to avoid the RunnerReplicaSet created by the controller v0.17.0 or before
 		// to not treat all the runners in the namespace as its children.
-		if metav1.IsControlledBy(&r, &rs) {
+		if metav1.IsControlledBy(&r, &rs) && !metav1.HasAnnotation(r.ObjectMeta, annotationKeyRegistrationOnly) {
 			myRunners = append(myRunners, r)
 
 			available += 1
@@ -265,3 +324,7 @@ func (r *RunnerReplicaSetReconciler) SetupWithManager(mgr ctrl.Manager) error {
 		Named(name).
 		Complete(r)
 }
+
+func registrationOnlyRunnerNameFor(rsName string) string {
+	return rsName + "-registration-only"
+}