fix(ndb): correct disruptionsAllowed calculation in NodeDisruptionBudget

internal/controller/nodedisruptionbudget_controller.go

@@ -176,6 +176,12 @@ type NodeDisruptionBudgetResolver struct {
 	Resolver             resolver.Resolver
 }
 
+func computeNodeDisruptionBudgetDisruptionsAllowed(maxDisruptedNodes, minUndisruptedNodes, watchedNodes, currentDisruptions int) int {
+	disruptionsForMax := maxDisruptedNodes - currentDisruptions
+	disruptionsForMin := (watchedNodes - currentDisruptions) - minUndisruptedNodes
+	return int(math.Min(float64(disruptionsForMax), float64(disruptionsForMin)))
+}
+
 // Sync ensure the budget's status is up to date
 func (r *NodeDisruptionBudgetResolver) Sync(ctx context.Context) error {
 	nodeNames, err := r.GetSelectedNodes(ctx)
@@ -192,9 +198,12 @@ func (r *NodeDisruptionBudgetResolver) Sync(ctx context.Context) error {
 
 	r.NodeDisruptionBudget.Status.WatchedNodes = nodes
 	r.NodeDisruptionBudget.Status.CurrentDisruptions = disruptionCount
-	disruptionsForMax := r.NodeDisruptionBudget.Spec.MaxDisruptedNodes - disruptionCount
-	disruptionsForMin := (len(nodes) - disruptionCount) - r.NodeDisruptionBudget.Spec.MinUndisruptedNodes
-	r.NodeDisruptionBudget.Status.DisruptionsAllowed = int(math.Min(float64(disruptionsForMax), float64(disruptionsForMin))) - disruptionCount
+	r.NodeDisruptionBudget.Status.DisruptionsAllowed = computeNodeDisruptionBudgetDisruptionsAllowed(
+		r.NodeDisruptionBudget.Spec.MaxDisruptedNodes,
+		r.NodeDisruptionBudget.Spec.MinUndisruptedNodes,
+		len(nodes),
+		disruptionCount,
+	)
 	r.NodeDisruptionBudget.Status.Disruptions = disruptions
 	return nil
 }

internal/controller/nodedisruptionbudget_math_test.go

@@ -0,0 +1,61 @@
+package controller
+
+import "testing"
+
+func TestComputeNodeDisruptionBudgetDisruptionsAllowed(t *testing.T) {
+	tests := []struct {
+		name                string
+		maxDisruptedNodes   int
+		minUndisruptedNodes int
+		watchedNodes        int
+		currentDisruptions  int
+		expected            int
+	}{
+		{
+			name:                "at max disruption returns zero",
+			maxDisruptedNodes:   1,
+			minUndisruptedNodes: 0,
+			watchedNodes:        34,
+			currentDisruptions:  1,
+			expected:            0,
+		},
+		{
+			name:                "max side is limiting factor",
+			maxDisruptedNodes:   3,
+			minUndisruptedNodes: 0,
+			watchedNodes:        10,
+			currentDisruptions:  2,
+			expected:            1,
+		},
+		{
+			name:                "min undisrupted side is limiting factor",
+			maxDisruptedNodes:   10,
+			minUndisruptedNodes: 10,
+			watchedNodes:        12,
+			currentDisruptions:  1,
+			expected:            1,
+		},
+		{
+			name:                "returns negative when min undisrupted cannot be respected",
+			maxDisruptedNodes:   10,
+			minUndisruptedNodes: 10,
+			watchedNodes:        5,
+			currentDisruptions:  1,
+			expected:            -6,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := computeNodeDisruptionBudgetDisruptionsAllowed(
+				tt.maxDisruptedNodes,
+				tt.minUndisruptedNodes,
+				tt.watchedNodes,
+				tt.currentDisruptions,
+			)
+			if got != tt.expected {
+				t.Fatalf("unexpected disruptions allowed: got=%d expected=%d", got, tt.expected)
+			}
+		})
+	}
+}