Fixes https://redhat.atlassian.net/browse/OCPBUGS-82584

install/0000_30_machine-api-operator_03_machineset.CustomNoUpgrade.crd.yaml

@@ -660,6 +660,12 @@ spec:
                   labels of the machine template of the MachineSet.
                 format: int32
                 type: integer
+              selector:
+                description: |-
+                  selector is the string form of the label selector in spec.selector, in the same format as
+                  autoscaling/v1 Scale status.selector (for example "app=myapp,role=worker"). It is written by the
+                  machineset controller so the scale subresource's labelSelectorPath resolves for HPAs and other autoscalers.
+                type: string
               observedGeneration:
                 description: observedGeneration reflects the generation of the most
                   recently observed MachineSet.
@@ -708,7 +714,7 @@ spec:
     storage: true
     subresources:
       scale:
-        labelSelectorPath: .status.labelSelector
+        labelSelectorPath: .status.selector
         specReplicasPath: .spec.replicas
         statusReplicasPath: .status.replicas
       status: {}

install/0000_30_machine-api-operator_03_machineset.Default.crd.yaml

@@ -615,6 +615,12 @@ spec:
                   labels of the machine template of the MachineSet.
                 format: int32
                 type: integer
+              selector:
+                description: |-
+                  selector is the string form of the label selector in spec.selector, in the same format as
+                  autoscaling/v1 Scale status.selector (for example "app=myapp,role=worker"). It is written by the
+                  machineset controller so the scale subresource's labelSelectorPath resolves for HPAs and other autoscalers.
+                type: string
               observedGeneration:
                 description: observedGeneration reflects the generation of the most
                   recently observed MachineSet.
@@ -635,7 +641,7 @@ spec:
     storage: true
     subresources:
       scale:
-        labelSelectorPath: .status.labelSelector
+        labelSelectorPath: .status.selector
         specReplicasPath: .spec.replicas
         statusReplicasPath: .status.replicas
       status: {}

install/0000_30_machine-api-operator_03_machineset.DevPreviewNoUpgrade.crd.yaml

@@ -660,6 +660,12 @@ spec:
                   labels of the machine template of the MachineSet.
                 format: int32
                 type: integer
+              selector:
+                description: |-
+                  selector is the string form of the label selector in spec.selector, in the same format as
+                  autoscaling/v1 Scale status.selector (for example "app=myapp,role=worker"). It is written by the
+                  machineset controller so the scale subresource's labelSelectorPath resolves for HPAs and other autoscalers.
+                type: string
               observedGeneration:
                 description: observedGeneration reflects the generation of the most
                   recently observed MachineSet.
@@ -708,7 +714,7 @@ spec:
     storage: true
     subresources:
       scale:
-        labelSelectorPath: .status.labelSelector
+        labelSelectorPath: .status.selector
         specReplicasPath: .spec.replicas
         statusReplicasPath: .status.replicas
       status: {}

install/0000_30_machine-api-operator_03_machineset.OKD.crd.yaml

@@ -615,6 +615,12 @@ spec:
                   labels of the machine template of the MachineSet.
                 format: int32
                 type: integer
+              selector:
+                description: |-
+                  selector is the string form of the label selector in spec.selector, in the same format as
+                  autoscaling/v1 Scale status.selector (for example "app=myapp,role=worker"). It is written by the
+                  machineset controller so the scale subresource's labelSelectorPath resolves for HPAs and other autoscalers.
+                type: string
               observedGeneration:
                 description: observedGeneration reflects the generation of the most
                   recently observed MachineSet.
@@ -635,7 +641,7 @@ spec:
     storage: true
     subresources:
       scale:
-        labelSelectorPath: .status.labelSelector
+        labelSelectorPath: .status.selector
         specReplicasPath: .spec.replicas
         statusReplicasPath: .status.replicas
       status: {}

install/0000_30_machine-api-operator_03_machineset.TechPreviewNoUpgrade.crd.yaml

@@ -660,6 +660,12 @@ spec:
                   labels of the machine template of the MachineSet.
                 format: int32
                 type: integer
+              selector:
+                description: |-
+                  selector is the string form of the label selector in spec.selector, in the same format as
+                  autoscaling/v1 Scale status.selector (for example "app=myapp,role=worker"). It is written by the
+                  machineset controller so the scale subresource's labelSelectorPath resolves for HPAs and other autoscalers.
+                type: string
               observedGeneration:
                 description: observedGeneration reflects the generation of the most
                   recently observed MachineSet.
@@ -708,7 +714,7 @@ spec:
     storage: true
     subresources:
       scale:
-        labelSelectorPath: .status.labelSelector
+        labelSelectorPath: .status.selector
         specReplicasPath: .spec.replicas
         statusReplicasPath: .status.replicas
       status: {}

pkg/controller/machineset/status.go

@@ -67,6 +67,7 @@ func (c *ReconcileMachineSet) calculateStatus(ms *machinev1.MachineSet, filtered
 	newStatus.FullyLabeledReplicas = int32(fullyLabeledReplicasCount)
 	newStatus.ReadyReplicas = int32(readyReplicasCount)
 	newStatus.AvailableReplicas = int32(availableReplicasCount)
+	newStatus.Selector = metav1.FormatLabelSelector(&ms.Spec.Selector)
 	return newStatus
 }
 
@@ -80,6 +81,7 @@ func updateMachineSetStatus(c client.Client, ms *machinev1.MachineSet, newStatus
 		ms.Status.FullyLabeledReplicas == newStatus.FullyLabeledReplicas &&
 		ms.Status.ReadyReplicas == newStatus.ReadyReplicas &&
 		ms.Status.AvailableReplicas == newStatus.AvailableReplicas &&
+		ms.Status.Selector == newStatus.Selector &&
 		reflect.DeepEqual(ms.Status.Conditions, newStatus.Conditions) &&
 		ms.Generation == ms.Status.ObservedGeneration {
 		return ms, nil

pkg/controller/machineset/status_test.go

@@ -0,0 +1,67 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package machineset
+
+import (
+	"testing"
+
+	machinev1 "github.com/openshift/api/machine/v1beta1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+// TestMachineSetStatusSelectorMatchesScaleSubresource verifies that status.selector is the
+// serialized label selector string expected by the CRD scale subresource (labelSelectorPath
+// -> .status.selector), which the apiserver maps to autoscaling/v1 Scale status.selector.
+func TestMachineSetStatusSelectorMatchesScaleSubresource(t *testing.T) {
+	tests := []struct {
+		name string
+		spec metav1.LabelSelector
+	}{
+		{
+			name: "matchLabels",
+			spec: metav1.LabelSelector{
+				MatchLabels: map[string]string{
+					"machine.openshift.io/cluster-api-cluster":    "cluster-id",
+					"machine.openshift.io/cluster-api-machineset": "workers",
+				},
+			},
+		},
+		{
+			name: "matchExpressions",
+			spec: metav1.LabelSelector{
+				MatchExpressions: []metav1.LabelSelectorRequirement{
+					{Key: "role", Operator: metav1.LabelSelectorOpIn, Values: []string{"worker", "infra"}},
+				},
+			},
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			ms := &machinev1.MachineSet{
+				Spec: machinev1.MachineSetSpec{
+					Selector: tc.spec,
+				},
+			}
+			r := &ReconcileMachineSet{}
+			got := r.calculateStatus(ms, nil).Selector
+			want := metav1.FormatLabelSelector(&ms.Spec.Selector)
+			if got != want {
+				t.Fatalf("status.selector = %q, want %q (must match Scale status.selector for HPA)", got, want)
+			}
+		})
+	}
+}