Auto-correct desired replicas on missing webhook_job completion event

While testing #1179, I discovered that ARC sometimes stop resyncing RunnerReplicaSet when the desired replicas is greater than the actual number of runner pods.
This seems to happen when ARC missed receiving a workflow_job completion event but it has no way to decide if it is either (1) something went wrong on ARC or (2) a loadbalancer in the middle or GitHub or anything not ARC went wrong. It needs a standard to decide it.

So I added a hard-coded 30 seconds timeout(can be made customizable later) to prevent runner pod recreation.

controllers/constants.go

@@ -45,4 +45,14 @@ const (
 	registrationTimeout = 10 * time.Minute
 
 	defaultRegistrationCheckInterval = time.Minute
+
+	// DefaultRunnerPodRecreationDelayAfterWebhookScale is the delay until syncing the runners with the desired replicas
+	// after a webhook-based scale up.
+	// This is used to prevent ARC from recreating completed runner pods that are deleted soon without being used at all.
+	// In other words, this is used as a timer to wait for the completed runner to emit the next `workflow_job` webhook event to decrease the desired replicas.
+	// So if we set 30 seconds for this, you are basically saying that you would assume GitHub and your installation of ARC to
+	// emit and propagate a workflow_job completion event down to the RunnerSet or RunnerReplicaSet, vha ARC's github webhook server and HRA, in approximately 30 seconds.
+	// In case it actually took more than DefaultRunnerPodRecreationDelayAfterWebhookScale for the workflow_job completion event to arrive,
+	// ARC will recreate the completed runner(s), assuming something went wrong in either GitHub, your K8s cluster, or ARC, so ARC needs to resync anyway.
+	DefaultRunnerPodRecreationDelayAfterWebhookScale = 30 * time.Second
 )

controllers/runner_pod_owner.go

@@ -328,9 +328,31 @@ func syncRunnerPodsOwners(ctx context.Context, c client.Client, log logr.Logger,
 
 	maybeRunning := pending + running
 
-	if newDesiredReplicas > maybeRunning && ephemeral && lastSyncTime != nil && effectiveTime != nil && lastSyncTime.After(effectiveTime.Time) {
-		log.V(2).Info("Detected that some ephemeral runners have disappeared. Usually this is due to that ephemeral runner completions so ARC does not create new runners until EffectiveTime is updated.", "lastSyncTime", metav1.Time{Time: *lastSyncTime}, "effectiveTime", *effectiveTime, "desired", newDesiredReplicas, "pending", pending, "running", running)
-	} else if newDesiredReplicas > maybeRunning {
+	wantMoreRunners := newDesiredReplicas > maybeRunning
+	alreadySyncedAfterEffectiveTime := lastSyncTime != nil && effectiveTime != nil && lastSyncTime.After(effectiveTime.Time)
+	runnerPodRecreationDelayAfterWebhookScale := lastSyncTime != nil && time.Now().Before(lastSyncTime.Add(DefaultRunnerPodRecreationDelayAfterWebhookScale))
+
+	log = log.WithValues(
+		"lastSyncTime", lastSyncTime,
+		"effectiveTime", effectiveTime,
+		"templateHashDesired", desiredTemplateHash,
+		"replicasDesired", newDesiredReplicas,
+		"replicasPending", pending,
+		"replicasRunning", running,
+		"replicasMaybeRunning", maybeRunning,
+		"templateHashObserved", hashes,
+	)
+
+	if wantMoreRunners && alreadySyncedAfterEffectiveTime && runnerPodRecreationDelayAfterWebhookScale {
+		log.V(2).Info(
+			"Detected that some ephemeral runners have disappeared. " +
+				"Usually this is due to that ephemeral runner completions " +
+				"so ARC does not create new runners until EffectiveTime is updated, or DefaultRunnerPodRecreationDelayAfterWebhookScale is elapsed.")
+	} else if wantMoreRunners {
+		if alreadySyncedAfterEffectiveTime && !runnerPodRecreationDelayAfterWebhookScale {
+			log.V(2).Info("Adding more replicas because DefaultRunnerPodRecreationDelayAfterWebhookScale has been passed")
+		}
+
 		num := newDesiredReplicas - maybeRunning
 
 		for i := 0; i < num; i++ {
@@ -342,10 +364,6 @@ func syncRunnerPodsOwners(ctx context.Context, c client.Client, log logr.Logger,
 
 		log.V(1).Info("Created replica(s)",
 			"created", num,
-			"templateHashDesired", desiredTemplateHash,
-			"replicasDesired", newDesiredReplicas,
-			"replicasMaybeRunning", maybeRunning,
-			"templateHashObserved", hashes,
 		)
 
 		return nil, nil