[plugins] add alterFinalizer plugin

docs/plugins.md

@@ -175,6 +175,14 @@ config:
   remove: boolean value, if true the annotation is removed, if false the annotation is added or changed, optional
 ```
 
+### alterFinalizer
+Adds or removes a finalizer.
+```yaml
+config:
+  key: the finalizer key to add or remove, required
+  remove: boolean value, if true the finalizer is removed, if false the finalizer is added, optional
+```
+
 ### alterLabel
 Adds, changes or removes a label.
 ```yaml

plugin/impl/finalizer.go

@@ -0,0 +1,45 @@
+// SPDX-FileCopyrightText: 2025 SAP SE or an SAP affiliate company
+// SPDX-License-Identifier: Apache-2.0
+
+package impl
+
+import (
+	"github.com/sapcc/ucfgwrap"
+	"sigs.k8s.io/controller-runtime/pkg/controller/controllerutil"
+
+	"github.com/sapcc/maintenance-controller/plugin"
+)
+
+// AlterFinalizer is a trigger plugin, which can alter properties of the Finalizer CRO of the node.
+type AlterFinalizer struct {
+	Key    string
+	Remove bool
+}
+
+// New creates a new AlterFinalizer instance with the given config.
+func (a *AlterFinalizer) New(config *ucfgwrap.Config) (plugin.Trigger, error) {
+	conf := struct {
+		Key    string `config:"key" validate:"required"`
+		Remove bool   `config:"remove"`
+	}{}
+	if err := config.Unpack(&conf); err != nil {
+		return nil, err
+	}
+	return &AlterFinalizer{Key: conf.Key, Remove: conf.Remove}, nil
+}
+
+func (a *AlterFinalizer) ID() string {
+	return "alterFinalizer"
+}
+
+// Trigger ensures the Finalizer with the provided key is removed if removes is set to true.
+// Otherwise, it sets the Finalizer with the provided key if required.
+func (a *AlterFinalizer) Trigger(params plugin.Parameters) error {
+	if !a.Remove {
+		controllerutil.AddFinalizer(params.Node, a.Key)
+		return nil
+	}
+
+	controllerutil.RemoveFinalizer(params.Node, a.Key)
+	return nil
+}

plugin/impl/finalizer_test.go

@@ -0,0 +1,76 @@
+// SPDX-FileCopyrightText: 2025 SAP SE or an SAP affiliate company
+// SPDX-License-Identifier: Apache-2.0
+
+package impl
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+	"github.com/sapcc/ucfgwrap"
+	v1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+
+	"github.com/sapcc/maintenance-controller/plugin"
+)
+
+var _ = Describe("The Finalizer plugin", func() {
+	Describe("AlterFinalizer", func() {
+		It("fails parsing incorrect config", func() {
+			_, err := ucfgwrap.FromYAML([]byte("invalid_yaml"))
+			errMsg := "type 'string' is not supported on top level of config, only dictionary or list"
+			Expect(err).To(MatchError(errMsg))
+
+			config, err := ucfgwrap.FromYAML([]byte("value: test"))
+			Expect(err).To(Not(HaveOccurred()))
+
+			var base AlterFinalizer
+			_, err = base.New(&config)
+			Expect(err).To(MatchError("string value is not set accessing 'key'"))
+		})
+
+		It("has valid configuration", func() {
+			config, err := ucfgwrap.FromYAML([]byte("key: test.com/finalizer\nremove: true"))
+			Expect(err).To(Not(HaveOccurred()))
+
+			var base AlterFinalizer
+			plugin, err := base.New(&config)
+			Expect(err).To(Succeed())
+			Expect(plugin).To(Equal(&AlterFinalizer{
+				Key:    "test.com/finalizer",
+				Remove: true,
+			}))
+		})
+
+		It("adds finalizer when not present", func() {
+			config, err := ucfgwrap.FromYAML([]byte("key: test.com/finalizer\nremove: false"))
+			Expect(err).To(Not(HaveOccurred()))
+
+			var base AlterFinalizer
+			addFinalizer, err := base.New(&config)
+			Expect(err).To(Not(HaveOccurred()))
+
+			testNode := &v1.Node{ObjectMeta: metav1.ObjectMeta{Name: "test-node"}}
+			err = addFinalizer.Trigger(plugin.Parameters{Node: testNode})
+			Expect(err).To(Not(HaveOccurred()))
+			Expect(testNode.Finalizers).To(ContainElement("test.com/finalizer"))
+		})
+
+		It("removes finalizer when present", func() {
+			testNode := &v1.Node{ObjectMeta: metav1.ObjectMeta{
+				Name:       "test-node",
+				Finalizers: []string{"test.com/finalizer"},
+			}}
+
+			config, err := ucfgwrap.FromYAML([]byte("key: test.com/finalizer\nremove: true"))
+			Expect(err).To(Succeed())
+
+			var base AlterFinalizer
+			removeFinalizer, err := base.New(&config)
+			Expect(err).To(Not(HaveOccurred()))
+
+			err = removeFinalizer.Trigger(plugin.Parameters{Node: testNode})
+			Expect(err).To(Not(HaveOccurred()))
+			Expect(testNode.Finalizers).ToNot(ContainElement("test.com/finalizer"))
+		})
+	})
+})