Use maxUnavailable for the critical-op PDB to stop idle alert noise

The critical-op PDB is created with minAvailable equal to
numberOfInstances while its selector (critical-operation=true) matches
no pods during normal operation. This leaves status.desiredHealthy at N
and currentHealthy at 0 permanently, so monitoring stacks fire alerts
like kube-prometheus-stack's KubePdbNotEnoughHealthyPods for every idle
cluster (#3020).

maxUnavailable: 0 provides the same protection while a critical
operation is running - no voluntary evictions of labeled pods - but
keeps the budget satisfied (desiredHealthy 0) when nothing matches.
When PDBs are disabled or there are no instances, the budget relaxes to
maxUnavailable 100% instead of minAvailable 0.

Fixes #3020

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkg/cluster/k8sres.go

@@ -2315,12 +2315,18 @@ func (c *Cluster) generatePrimaryPodDisruptionBudget() *policyv1.PodDisruptionBu
 }
 
 func (c *Cluster) generateCriticalOpPodDisruptionBudget() *policyv1.PodDisruptionBudget {
-	minAvailable := intstr.FromInt32(c.Spec.NumberOfInstances)
+	// MaxUnavailable: 0 blocks voluntary disruption of any pod carrying the
+	// critical-operation label, while keeping the budget satisfied when no
+	// pod matches (status.desiredHealthy stays 0 outside critical
+	// operations). The previous MinAvailable: N spec left desiredHealthy at
+	// N with zero matching pods during normal operation, permanently firing
+	// alerts like kube-prometheus-stack's KubePdbNotEnoughHealthyPods (#3020).
+	maxUnavailable := intstr.FromInt32(0)
 	pdbEnabled := c.OpConfig.EnablePodDisruptionBudget
 
-	// if PodDisruptionBudget is disabled or if there are no DB pods, set the budget to 0.
+	// if PodDisruptionBudget is disabled or if there are no DB pods, allow all disruptions.
 	if (pdbEnabled != nil && !(*pdbEnabled)) || c.Spec.NumberOfInstances <= 0 {
-		minAvailable = intstr.FromInt(0)
+		maxUnavailable = intstr.FromString("100%")
 	}
 
 	labels := c.labelsSet(false)
@@ -2335,7 +2341,7 @@ func (c *Cluster) generateCriticalOpPodDisruptionBudget() *policyv1.PodDisruptio
 			OwnerReferences: c.ownerReferences(),
 		},
 		Spec: policyv1.PodDisruptionBudgetSpec{
-			MinAvailable: &minAvailable,
+			MaxUnavailable: &maxUnavailable,
 			Selector: &metav1.LabelSelector{
 				MatchLabels: labels,
 			},

pkg/cluster/k8sres_test.go

@@ -2556,6 +2556,17 @@ func TestGeneratePodDisruptionBudget(t *testing.T) {
 		}
 	}
 
+	hasMaxUnavailable := func(expected string) func(cluster *Cluster, podDisruptionBudget *policyv1.PodDisruptionBudget) error {
+		return func(cluster *Cluster, podDisruptionBudget *policyv1.PodDisruptionBudget) error {
+			actual := podDisruptionBudget.Spec.MaxUnavailable.String()
+			if actual != expected {
+				return fmt.Errorf("PodDisruptionBudget MaxUnavailable is incorrect, got %s, expected %s",
+					actual, expected)
+			}
+			return nil
+		}
+	}
+
 	hasMinAvailable := func(expectedMinAvailable int) func(cluster *Cluster, podDisruptionBudget *policyv1.PodDisruptionBudget) error {
 		return func(cluster *Cluster, podDisruptionBudget *policyv1.PodDisruptionBudget) error {
 			actual := podDisruptionBudget.Spec.MinAvailable.IntVal
@@ -2749,7 +2760,7 @@ func TestGeneratePodDisruptionBudget(t *testing.T) {
 			check: []func(cluster *Cluster, podDisruptionBudget *policyv1.PodDisruptionBudget) error{
 				testPodDisruptionBudgetOwnerReference,
 				hasName("postgres-myapp-database-critical-op-pdb"),
-				hasMinAvailable(3),
+				hasMaxUnavailable("0"),
 				testLabelsAndSelectors(false),
 			},
 		},
@@ -2766,7 +2777,7 @@ func TestGeneratePodDisruptionBudget(t *testing.T) {
 			check: []func(cluster *Cluster, podDisruptionBudget *policyv1.PodDisruptionBudget) error{
 				testPodDisruptionBudgetOwnerReference,
 				hasName("postgres-myapp-database-critical-op-pdb"),
-				hasMinAvailable(0),
+				hasMaxUnavailable("100%"),
 				testLabelsAndSelectors(false),
 			},
 		},
@@ -2783,7 +2794,7 @@ func TestGeneratePodDisruptionBudget(t *testing.T) {
 			check: []func(cluster *Cluster, podDisruptionBudget *policyv1.PodDisruptionBudget) error{
 				testPodDisruptionBudgetOwnerReference,
 				hasName("postgres-myapp-database-critical-op-pdb"),
-				hasMinAvailable(0),
+				hasMaxUnavailable("100%"),
 				testLabelsAndSelectors(false),
 			},
 		},
@@ -2800,7 +2811,7 @@ func TestGeneratePodDisruptionBudget(t *testing.T) {
 			check: []func(cluster *Cluster, podDisruptionBudget *policyv1.PodDisruptionBudget) error{
 				testPodDisruptionBudgetOwnerReference,
 				hasName("postgres-myapp-database-critical-op-pdb"),
-				hasMinAvailable(3),
+				hasMaxUnavailable("0"),
 				testLabelsAndSelectors(false),
 			},
 		},