MCO-1927: Add OSImageStream v1alpha1 API

Introduces OSImageStream as a catalog resource that enables discovery
of available OS image streams for MachineConfigPools. The resource is
populated by the MachineConfigOperator from release metadata and provides
OS and OS Extensions image URLs referenced by digest.

The change adds the the osImageStream field to MachineConfigPoolSpec,
allowing individual pools to override cluster-wide OS images with a
specific OS stream. The field is optional and gated behind the
OSStreams feature gate. When omitted, pools use the cluster-wide
default OS images.

Author: pablintino

machineconfiguration/v1/tests/machineconfigpools.machineconfiguration.openshift.io/OSStreams.yaml

@@ -0,0 +1,78 @@
+apiVersion: apiextensions.k8s.io/v1 # Hack because controller-gen complains if we don't have this
+name: "[TechPreview] OSStreams"
+crdName: machineconfigpools.machineconfiguration.openshift.io
+featureGates:
+- OSStreams
+tests:
+  onCreate:
+    - name: Should be able to create a minimal MachineConfigPool
+      initial: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec: {} # No spec is required for a MachineConfigPool
+      expected: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec: {}
+    - name: Should be able to select a non-default osImageStream
+      initial: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec:
+          osImageStream: "rhel10-coreos"
+      expected: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec:
+          osImageStream: "rhel10-coreos"
+    - name: If given, the osImageStream must be non-empty
+      initial: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec:
+          osImageStream: ""
+      expectedError: "spec.osImageStream in body should be at least 1 chars long"
+    - name: If given, the osImageStream should be composed only by alphanumeric chars, hyphens and dots
+      initial: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec:
+          osImageStream: "rhel10#1-coreos"
+      expectedError: "spec.osImageStream: Invalid value: \"string\": The osImageStream must start with a letter and contain only alphanumeric characters, hyphens ('-'), and dots ('.')."
+    - name: If given, the osImageStream should not exceed 70 characters
+      initial: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec:
+          osImageStream: "rhel-coreos32131e2ddf34f44r3r3r4f43tg54frg54tg45454g4g45gt34f43g54g45g3"
+      expectedError: "Too long: may not be more than 70 bytes"
+  onUpdate:
+    - name: If given, the osImageStream can be removed
+      initial: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec:
+          osImageStream: "rhel10-coreos"
+      updated: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec: {}
+      expected: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec: {}
+    - name: If not present, the osImageStream can be added to the pool
+      initial: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec: {}
+      updated: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec:
+          osImageStream: "rhel10-coreos"
+      expected: |
+        apiVersion: machineconfiguration.openshift.io/v1
+        kind: MachineConfigPool
+        spec:
+          osImageStream: "rhel10-coreos"

machineconfiguration/v1/types.go

@@ -452,6 +452,22 @@ type MachineConfigPoolSpec struct {
 	// +listMapKey=name
 	// +kubebuilder:validation:MaxItems=100
 	PinnedImageSets []PinnedImageSetRef `json:"pinnedImageSets,omitempty"`
+
+	// osImageStream specifies an OS stream to be used for the pool.
+	//
+	// When set, this value overrides the cluster-wide OS images for the pool with
+	// the OS and Extensions associated to the specified stream.
+	// When omitted or empty, the pool uses the cluster-wide default OS images.
+	//
+	// The stream name must start with a letter and contain only alphanumeric
+	// characters, hyphens ('-'), and dots ('.'), with a maximum length of 70 characters.
+	//
+	// +optional
+	// +openshift:enable:FeatureGate=OSStreams
+	// +kubebuilder:validation:MinLength=1
+	// +kubebuilder:validation:MaxLength=70
+	// +kubebuilder:validation:XValidation:rule=`self.matches('^[a-zA-Z][a-zA-Z0-9.-]*$')`,message="The osImageStream must start with a letter and contain only alphanumeric characters, hyphens ('-'), and dots ('.')."
+	OSImageStream string `json:"osImageStream,omitempty"`
 }
 
 type PinnedImageSetRef struct {