🐛 Fix rollout with unavailable machines

internal/controllers/machinedeployment/machinedeployment_rollout_rollingupdate.go

@@ -247,12 +247,18 @@ func (p *rolloutPlanner) reconcileOldMachineSetsRollingUpdate(ctx context.Contex
 		}
 	}
 
+	// On top of considering maxUnavailable, before scaling down it is also important to consider how unavailability is distributed
+	// across MachineSets, because if there are unavailable machines on the new MachineSet it is necessary to slow down or stop rollout
+	// to prevent to transition all the Machines to a broken state.
+	// In order to do so, compute the number of unavailable machines on the new MachineSet and use it to reduce totalScaleDownCount.
+	newMSUnavailableMachineCount := max(ptr.Deref(p.newMS.Spec.Replicas, 0)-ptr.Deref(p.newMS.Status.AvailableReplicas, 0), 0)
+
 	// Compute the total number of replicas that can be scaled down.
 	// Exit immediately if there is no room for scaling down.
 	// NOTE: this is a quick preliminary check to verify if there is room for scaling down any of the oldMSs; further down the code
 	// will make additional checks to ensure scale down actually happens without breaching MaxUnavailable, and
 	// if necessary, it will reduce the extent of the scale down accordingly.
-	totalScaleDownCount := max(totalSpecReplicas-totalPendingScaleDown-minAvailable, 0)
+	totalScaleDownCount := max(totalSpecReplicas-totalPendingScaleDown-minAvailable-newMSUnavailableMachineCount, 0)
 	if totalScaleDownCount <= 0 {
 		return nil
 	}

internal/controllers/machinedeployment/machinedeployment_rollout_rollingupdate_test.go

@@ -463,6 +463,34 @@ func Test_reconcileOldMachineSetsRollingUpdate(t *testing.T) {
 			},
 			expectedNotes: map[string][]string{},
 		},
+		{
+			name:        "do not scale down if there are unavailable replicas on the new MachineSet (maxSurge 1, maxUnavailable 0), all  machines unavailable",
+			scaleIntent: map[string]int32{},
+			md:          createMD("v2", 3, withRollingUpdateStrategy(1, 0)),
+			newMS:       createMS("ms2", "v2", 1, withStatusReplicas(1), withStatusAvailableReplicas(0)), // one machine created on the new NewMS due to maxSurge, but it is not reaching the available state,
+			oldMSs: []*clusterv1.MachineSet{
+				createMS("ms1", "v1", 3, withStatusReplicas(3), withStatusAvailableReplicas(0)),
+			},
+			expectScaleIntent: map[string]int32{
+				// no new scale down intent for oldMSs (ms1): unavailability on new MS
+			},
+			expectedNotes:              map[string][]string{},
+			skipMaxUnavailabilityCheck: true, // The test case is simulating all machines not unavailable, so this check will fail
+		},
+		{
+			name:        "do not scale down if there are unavailable replicas on the new MachineSet (maxSurge 0, maxUnavailable 1), all  machines unavailable",
+			scaleIntent: map[string]int32{},
+			md:          createMD("v2", 3, withRollingUpdateStrategy(0, 1)),
+			newMS:       createMS("ms2", "v2", 1, withStatusReplicas(1), withStatusAvailableReplicas(0)), // one machine created on the new NewMS after scale down on the old MS, but it is not reaching the available state,
+			oldMSs: []*clusterv1.MachineSet{
+				createMS("ms1", "v1", 2, withStatusReplicas(2), withStatusAvailableReplicas(0)), // OldMS scaled down due to maxUnavailable 1
+			},
+			expectScaleIntent: map[string]int32{
+				// no new scale down intent for oldMSs (ms1): unavailability on new MS
+			},
+			expectedNotes:              map[string][]string{},
+			skipMaxUnavailabilityCheck: true, // The test case is simulating all machines not unavailable, so this check will fail
+		},
 		{
 			name: "do not scale down if there are more replicas than minAvailable replicas, but scale down from current reconcile already takes the availability buffer (newMS is scaling down)",
 			scaleIntent: map[string]int32{