fix 3merge null values

pkg/lib/update/merge3/tuple.go

@@ -20,7 +20,6 @@ import (
 	"sigs.k8s.io/kustomize/kyaml/kio/filters"
 	"sigs.k8s.io/kustomize/kyaml/kio/kioutil"
 	"sigs.k8s.io/kustomize/kyaml/yaml"
-	"sigs.k8s.io/kustomize/kyaml/yaml/merge3"
 	"sigs.k8s.io/kustomize/kyaml/yaml/walk"
 )
 
@@ -35,7 +34,7 @@ type tuple struct {
 func (t *tuple) merge() (*yaml.RNode, error) {
 	return walk.Walker{
 		// same as in merge3.Merge()
-		Visitor:            merge3.Visitor{},
+		Visitor:            Visitor{},
 		VisitKeysAsScalars: true,
 		Sources:            []*yaml.RNode{t.dest, t.original, t.updated},
 

pkg/lib/update/merge3/visitor.go

@@ -0,0 +1,187 @@
+// Copyright 2026 The kpt 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 merge3
+
+import (
+	"sigs.k8s.io/kustomize/kyaml/openapi"
+	"sigs.k8s.io/kustomize/kyaml/yaml"
+	"sigs.k8s.io/kustomize/kyaml/yaml/walk"
+)
+
+type ConflictStrategy uint
+
+const (
+	// TODO: Support more strategies
+	TakeUpdate ConflictStrategy = 1 + iota
+)
+
+type Visitor struct{}
+
+func (m Visitor) VisitMap(nodes walk.Sources, s *openapi.ResourceSchema) (*yaml.RNode, error) {
+	if yaml.IsMissingOrNull(nodes.Updated()) || m.isDeletedInDest(nodes.Origin(), nodes.Dest()) {
+		// explicitly cleared from either dest or update
+		return walk.ClearNode, nil
+	}
+	if nodes.Dest() == nil && nodes.Updated() == nil {
+		// implicitly cleared missing from both dest and update
+		return walk.ClearNode, nil
+	}
+
+	if nodes.Dest() == nil {
+		// not cleared, but missing from the dest
+		// initialize a new value that can be recursively merged
+		return yaml.NewRNode(&yaml.Node{Kind: yaml.MappingNode}), nil
+	}
+
+	// recursively merge the dest with the original and updated
+	return nodes.Dest(), nil
+}
+
+func (m Visitor) visitAList(nodes walk.Sources, _ *openapi.ResourceSchema) (*yaml.RNode, error) {
+	if yaml.IsMissingOrNull(nodes.Updated()) && !yaml.IsMissingOrNull(nodes.Origin()) {
+		// implicitly cleared from update -- element was deleted
+		return walk.ClearNode, nil
+	}
+	if yaml.IsMissingOrNull(nodes.Dest()) {
+		// not cleared, but missing from the dest
+		// initialize a new value that can be recursively merged
+		return yaml.NewRNode(&yaml.Node{Kind: yaml.SequenceNode}), nil
+	}
+
+	// recursively merge the dest with the original and updated
+	return nodes.Dest(), nil
+}
+
+func (m Visitor) VisitScalar(nodes walk.Sources, s *openapi.ResourceSchema) (*yaml.RNode, error) {
+	if yaml.IsMissingOrNull(nodes.Updated()) || m.isDeletedInDest(nodes.Origin(), nodes.Dest()) {
+		// explicitly cleared from either dest or update
+		return nil, nil
+	}
+	if yaml.IsMissingOrNull(nodes.Updated()) != yaml.IsMissingOrNull(nodes.Origin()) {
+		// value added or removed in update
+		return nodes.Updated(), nil
+	}
+	if yaml.IsMissingOrNull(nodes.Updated()) && yaml.IsMissingOrNull(nodes.Origin()) {
+		// value added or removed in update
+		return nodes.Dest(), nil
+	}
+
+	values, err := m.getStrValues(nodes)
+	if err != nil {
+		return nil, err
+	}
+
+	if (values.Dest == "" || values.Dest == values.Origin) && values.Origin != values.Update {
+		// if local is nil or is unchanged but there is new update
+		return nodes.Updated(), nil
+	}
+
+	if nodes.Updated().YNode().Value != nodes.Origin().YNode().Value {
+		// value changed in update
+		return nodes.Updated(), nil
+	}
+
+	// unchanged between origin and update, keep the dest
+	return nodes.Dest(), nil
+}
+
+func (m Visitor) visitNAList(nodes walk.Sources) (*yaml.RNode, error) {
+	if yaml.IsMissingOrNull(nodes.Updated()) || m.isDeletedInDest(nodes.Origin(), nodes.Dest()) {
+		// explicitly cleared from either dest or update
+		return walk.ClearNode, nil
+	}
+
+	if yaml.IsMissingOrNull(nodes.Updated()) != yaml.IsMissingOrNull(nodes.Origin()) {
+		// value added or removed in update
+		return nodes.Updated(), nil
+	}
+	if yaml.IsMissingOrNull(nodes.Updated()) && yaml.IsMissingOrNull(nodes.Origin()) {
+		// value not present in source or dest
+		return nodes.Dest(), nil
+	}
+
+	// compare origin and update values to see if they have changed
+	values, err := m.getStrValues(nodes)
+	if err != nil {
+		return nil, err
+	}
+	if values.Update != values.Origin {
+		// value changed in update
+		return nodes.Updated(), nil
+	}
+
+	// unchanged between origin and update, keep the dest
+	return nodes.Dest(), nil
+}
+
+func (m Visitor) isDeletedInDest(origin, dest *yaml.RNode) bool {
+	return !yaml.IsMissingOrNull(origin) && yaml.IsMissingOrNull(dest)
+}
+
+func (m Visitor) VisitList(nodes walk.Sources, s *openapi.ResourceSchema, kind walk.ListKind) (*yaml.RNode, error) {
+	if kind == walk.AssociativeList {
+		return m.visitAList(nodes, s)
+	}
+	// non-associative list
+	return m.visitNAList(nodes)
+}
+
+func (m Visitor) getStrValues(nodes walk.Sources) (strValues, error) {
+	var uStr, oStr, dStr string
+	var err error
+	if nodes.Updated() != nil && nodes.Updated().YNode() != nil {
+		s := nodes.Updated().YNode().Style
+		defer func() {
+			nodes.Updated().YNode().Style = s
+		}()
+		nodes.Updated().YNode().Style = yaml.FlowStyle | yaml.SingleQuotedStyle
+		uStr, err = nodes.Updated().String()
+		if err != nil {
+			return strValues{}, err
+		}
+	}
+	if nodes.Origin() != nil && nodes.Origin().YNode() != nil {
+		s := nodes.Origin().YNode().Style
+		defer func() {
+			nodes.Origin().YNode().Style = s
+		}()
+		nodes.Origin().YNode().Style = yaml.FlowStyle | yaml.SingleQuotedStyle
+		oStr, err = nodes.Origin().String()
+		if err != nil {
+			return strValues{}, err
+		}
+	}
+	if nodes.Dest() != nil && nodes.Dest().YNode() != nil {
+		s := nodes.Dest().YNode().Style
+		defer func() {
+			nodes.Dest().YNode().Style = s
+		}()
+		nodes.Dest().YNode().Style = yaml.FlowStyle | yaml.SingleQuotedStyle
+		dStr, err = nodes.Dest().String()
+		if err != nil {
+			return strValues{}, err
+		}
+	}
+
+	return strValues{Origin: oStr, Update: uStr, Dest: dStr}, nil
+}
+
+type strValues struct {
+	Origin string
+	Update string
+	Dest   string
+}
+
+var _ walk.Visitor = Visitor{}