refactor: use SetScalar in imagetag filter

This change refactors imagetag to reuse the abstraction provided by
filtersutil. This change is intended to make imagetag more consistent
with other filters by using the same layer of abstraction (filtersutil),
and to prepare for upcoming changes which are planned to be implemented
at the filtersutil layer.

api/filters/imagetag/imagetag.go

@@ -4,7 +4,6 @@
 package imagetag
 
 import (
-	"sigs.k8s.io/kustomize/api/filters/filtersutil"
 	"sigs.k8s.io/kustomize/api/filters/fsslice"
 	"sigs.k8s.io/kustomize/api/types"
 	"sigs.k8s.io/kustomize/kyaml/kio"
@@ -41,7 +40,9 @@ func (f Filter) filter(node *yaml.RNode) (*yaml.RNode, error) {
 	}
 	if err := node.PipeE(fsslice.Filter{
 		FsSlice:  f.FsSlice,
-		SetValue: updateImageTagFn(f.ImageTag),
+		SetValue: imageTagUpdater{
+			ImageTag: f.ImageTag,
+		}.SetImageValue,
 	}); err != nil {
 		return nil, err
 	}
@@ -59,11 +60,3 @@ func (f Filter) isOnDenyList(node *yaml.RNode) bool {
 	// https://github.com/kubernetes-sigs/kustomize/issues/890
 	return meta.Kind == `CustomResourceDefinition`
 }
-
-func updateImageTagFn(imageTag types.Image) filtersutil.SetFn {
-	return func(node *yaml.RNode) error {
-		return node.PipeE(imageTagUpdater{
-			ImageTag: imageTag,
-		})
-	}
-}

api/filters/imagetag/updater.go

@@ -4,6 +4,7 @@
 package imagetag
 
 import (
+	"sigs.k8s.io/kustomize/api/filters/filtersutil"
 	"sigs.k8s.io/kustomize/api/image"
 	"sigs.k8s.io/kustomize/api/types"
 	"sigs.k8s.io/kustomize/kyaml/yaml"
@@ -17,15 +18,15 @@ type imageTagUpdater struct {
 	ImageTag types.Image `yaml:"imageTag,omitempty"`
 }
 
-func (u imageTagUpdater) Filter(rn *yaml.RNode) (*yaml.RNode, error) {
+func (u imageTagUpdater) SetImageValue(rn *yaml.RNode) error {
 	if err := yaml.ErrorIfInvalid(rn, yaml.ScalarNode); err != nil {
-		return nil, err
+		return err
 	}
 
 	value := rn.YNode().Value
 
 	if !image.IsImageMatched(value, u.ImageTag.Name) {
-		return rn, nil
+		return nil
 	}
 
 	name, tag := image.Split(value)
@@ -39,5 +40,12 @@ func (u imageTagUpdater) Filter(rn *yaml.RNode) (*yaml.RNode, error) {
 		tag = "@" + u.ImageTag.Digest
 	}
 
-	return rn.Pipe(yaml.FieldSetter{StringValue: name + tag})
+	return filtersutil.SetScalar(name + tag)(rn)
+}
+
+func (u imageTagUpdater) Filter(rn *yaml.RNode) (*yaml.RNode, error) {
+	if err := u.SetImageValue(rn); err != nil {
+		return nil, err
+	}
+	return rn, nil
 }