Start supporting strategic merge patch in kyaml merge2.

kyaml/yaml/merge2/smpdirective.go

@@ -0,0 +1,71 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package merge2
+
+import (
+	"fmt"
+
+	"sigs.k8s.io/kustomize/kyaml/yaml"
+)
+
+// A strategic merge patch directive.
+// See https://github.com/kubernetes/community/blob/master/contributors/devel/sig-api-machinery/strategic-merge-patch.md
+//
+//go:generate stringer -type=smpDirective -linecomment
+type smpDirective int
+
+const (
+	smpUnknown smpDirective = iota // unknown
+	smpReplace                     // replace
+	smpDelete                      // delete
+	smpMerge                       // merge
+)
+
+const strategicMergePatchDirectiveKey = "$patch"
+
+// Examine patch for a strategic merge patch directive.
+// If found, return it, and remove the directive from the patch.
+func determineSmpDirective(patch *yaml.RNode) (smpDirective, error) {
+	if patch == nil {
+		return smpMerge, nil
+	}
+	switch patch.YNode().Kind {
+	case yaml.SequenceNode:
+		return determineSequenceNodePatchStrategy(patch)
+	case yaml.MappingNode:
+		return determineMappingNodePatchStrategy(patch)
+	default:
+		return smpUnknown, fmt.Errorf(
+			"no implemented strategic merge patch strategy for '%s' ('%s')",
+			patch.YNode().ShortTag(), patch.MustString())
+	}
+}
+
+// TODO: what should this do?
+func determineSequenceNodePatchStrategy(_ *yaml.RNode) (smpDirective, error) {
+	return smpMerge, nil
+}
+
+func determineMappingNodePatchStrategy(patch *yaml.RNode) (smpDirective, error) {
+	node, err := patch.Pipe(yaml.Get(strategicMergePatchDirectiveKey))
+	if err != nil || node == nil || node.YNode() == nil {
+		return smpMerge, nil
+	}
+	v := node.YNode().Value
+	if v == smpDelete.String() {
+		return smpDelete, elidePatchDirective(patch)
+	}
+	if v == smpReplace.String() {
+		return smpReplace, elidePatchDirective(patch)
+	}
+	if v == smpMerge.String() {
+		return smpMerge, elidePatchDirective(patch)
+	}
+	return smpUnknown, fmt.Errorf(
+		"unknown patch strategy '%s'", v)
+}
+
+func elidePatchDirective(patch *yaml.RNode) error {
+	return patch.PipeE(yaml.Clear(strategicMergePatchDirectiveKey))
+}