From 95e242353b05683818b9db3a8556c309afd8ac9b Mon Sep 17 00:00:00 2001
From: Morten Torkildsen <mortent@google.com>
Date: Thu, 10 Jun 2021 17:42:47 -0700
Subject: [PATCH] Allow users to customize handling of deleted resources for
 merge3

---
 kyaml/kio/filters/merge3.go | 91 +++++++++++++++++++++++++++++++------
 1 file changed, 76 insertions(+), 15 deletions(-)

diff --git a/kyaml/kio/filters/merge3.go b/kyaml/kio/filters/merge3.go
index 30932800c..d767ea46b 100644
--- a/kyaml/kio/filters/merge3.go
+++ b/kyaml/kio/filters/merge3.go
@@ -30,6 +30,36 @@ type ResourceMatcher interface {
 	IsSameResource(node1, node2 *yaml.RNode) bool
 }
 
+// ResourceMergeStrategy is the return type from the Handle function in the
+// ResourceHandler interface. It determines which version of a resource should
+// be included in the output (if any).
+type ResourceMergeStrategy int
+
+const (
+	// Merge means the output to dest should be the 3-way merge of original,
+	// updated and dest.
+	Merge ResourceMergeStrategy = iota
+	// KeepDest means the version of the resource in dest should be the output.
+	KeepDest
+	// KeepUpdated means the version of the resource in updated should be the
+	// output.
+	KeepUpdated
+	// KeepOriginal means the version of the resource in original should be the
+	// output.
+	KeepOriginal
+	// Skip means the resource should not be included in the output.
+	Skip
+)
+
+// ResourceHandler interface is used to determine what should be done for a
+// resource once the versions in original, updated and dest has been
+// identified based on the ResourceMatcher. This allows users to customize
+// what should be the result in dest if a resource has been deleted from
+// upstream.
+type ResourceHandler interface {
+	Handle(original, updated, dest *yaml.RNode) ResourceMergeStrategy
+}
+
 // Merge3 performs a 3-way merge on the original, updated, and destination packages.
 type Merge3 struct {
 	OriginalPath   string
@@ -37,6 +67,7 @@ type Merge3 struct {
 	DestPath       string
 	MatchFilesGlob []string
 	Matcher        ResourceMatcher
+	Handler        ResourceHandler
 }
 
 func (m Merge3) Merge() error {
@@ -78,6 +109,11 @@ func (m Merge3) Filter(nodes []*yaml.RNode) ([]*yaml.RNode, error) {
 	if matcher == nil {
 		matcher = &DefaultGVKNNMatcher{MergeOnPath: true}
 	}
+	handler := m.Handler
+	if handler == nil {
+		handler = &DefaultResourceHandler{}
+	}
+
 	tl := tuples{matcher: matcher}
 	for i := range nodes {
 		if err := tl.add(nodes[i]); err != nil {
@@ -89,21 +125,9 @@ func (m Merge3) Filter(nodes []*yaml.RNode) ([]*yaml.RNode, error) {
 	var output []*yaml.RNode
 	for i := range tl.list {
 		t := tl.list[i]
-		switch {
-		case t.original == nil && t.updated == nil && t.dest != nil:
-			// added locally -- keep dest
-			output = append(output, t.dest)
-		case t.updated != nil && t.dest == nil:
-			// added in the update -- add update
-			output = append(output, t.updated)
-		case t.original != nil && t.updated == nil:
-			// deleted in the update
-		// don't include the resource in the output
-		case t.original != nil && t.dest == nil:
-			// deleted locally
-			// don't include the resource in the output
-		default:
-			// dest and updated are non-nil -- merge them
+		strategy := handler.Handle(t.original, t.updated, t.dest)
+		switch strategy {
+		case Merge:
 			node, err := t.merge()
 			if err != nil {
 				return nil, err
@@ -111,6 +135,14 @@ func (m Merge3) Filter(nodes []*yaml.RNode) ([]*yaml.RNode, error) {
 			if node != nil {
 				output = append(output, node)
 			}
+		case KeepDest:
+			output = append(output, t.dest)
+		case KeepUpdated:
+			output = append(output, t.updated)
+		case KeepOriginal:
+			output = append(output, t.original)
+		case Skip:
+			// do nothing
 		}
 	}
 	return output, nil
@@ -245,3 +277,32 @@ func (t *tuple) merge() (*yaml.RNode, error) {
 func duplicateError(source, filePath string) error {
 	return fmt.Errorf(`found duplicate %q resources in file %q, please refer to "update" documentation for the fix`, source, filePath)
 }
+
+// DefaultResourceHandler is the default implementation of the ResourceHandler
+// interface. It uses the following rules:
+// * Keep dest if resource only exists in dest.
+// * Keep updated if resource added in updated.
+// * Delete dest if updated has been deleted.
+// * Don't add the resource back if removed from dest.
+// * Otherwise merge.
+type DefaultResourceHandler struct{}
+
+func (*DefaultResourceHandler) Handle(original, updated, dest *yaml.RNode) ResourceMergeStrategy {
+	switch {
+	case original == nil && updated == nil && dest != nil:
+		// added locally -- keep dest
+		return KeepDest
+	case updated != nil && dest == nil:
+		// added in the update -- add update
+		return KeepUpdated
+	case original != nil && updated == nil:
+		// deleted in the update
+		return Skip
+	case original != nil && dest == nil:
+		// deleted locally
+		return Skip
+	default:
+		// dest and updated are non-nil -- merge them
+		return Merge
+	}
+}