Add YAML anchor/alias expansion.

api/resmap/resmap.go

@@ -214,6 +214,35 @@ type ResMap interface {
 	// namespaces. Cluster wide objects are never excluded.
 	SubsetThatCouldBeReferencedByResource(*resource.Resource) ResMap
 
+	// DeAnchor replaces YAML aliases with structured data copied from anchors.
+	// This cannot be undone; if desired, call DeepCopy first.
+	// Subsequent marshalling to YAML will no longer have anchor
+	// definitions ('&') or aliases ('*').
+	//
+	// Anchors are not expected to work across YAML 'documents'.
+	// If three resources are loaded from one file containing three YAML docs:
+	//
+	//   {resourceA}
+	//   ---
+	//   {resourceB}
+	//   ---
+	//   {resourceC}
+	//
+	// then anchors defined in A cannot be seen from B and C and vice versa.
+	// OTOH, cross-resource links (a field in B referencing fields in A) will
+	// work if the resources are gathered in a ResourceList:
+	//
+	//   apiVersion: config.kubernetes.io/v1
+	//   kind: ResourceList
+	//   metadata:
+	//     name: someList
+	//   items:
+	//   - {resourceA}
+	//   - {resourceB}
+	//   - {resourceC}
+	//
+	DeAnchor() error
+
 	// DeepCopy copies the ResMap and underlying resources.
 	DeepCopy() ResMap
 

api/resmap/reswrangler.go

@@ -607,6 +607,16 @@ func (m *resWrangler) ToRNodeSlice() []*kyaml.RNode {
 	return result
 }
 
+// DeAnchor implements ResMap.
+func (m *resWrangler) DeAnchor() (err error) {
+	for i := range m.rList {
+		if err = m.rList[i].DeAnchor(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 // ApplySmPatch applies the patch, and errors on Id collisions.
 func (m *resWrangler) ApplySmPatch(
 	selectedSet *resource.IdSet, patch *resource.Resource) error {

api/resmap/reswrangler_test.go

@@ -902,6 +902,100 @@ rules:
 	}
 }
 
+func TestDeAnchorSingleDoc(t *testing.T) {
+	input := `apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: wildcard
+data:
+  color: &color-used blue
+  feeling: *color-used
+`
+	rm, err := rmF.NewResMapFromBytes([]byte(input))
+	assert.NoError(t, err)
+	assert.NoError(t, rm.DeAnchor())
+	yaml, err := rm.AsYaml()
+	assert.NoError(t, err)
+	assert.Equal(t, strings.TrimSpace(`
+apiVersion: v1
+data:
+  color: blue
+  feeling: blue
+kind: ConfigMap
+metadata:
+  name: wildcard
+`), strings.TrimSpace(string(yaml)))
+}
+
+// Anchor references don't cross YAML document boundaries.
+func TestDeAnchorMultiDoc(t *testing.T) {
+	input := `apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: betty
+data:
+  color: &color-used blue
+  feeling: *color-used
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: bob
+data:
+  color: red
+  feeling: *color-used
+`
+	_, err := rmF.NewResMapFromBytes([]byte(input))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "unknown anchor 'color-used' referenced")
+}
+
+// Anchor references cross list elements in a ResourceList.
+func TestDeAnchorResourceList(t *testing.T) {
+	input := `apiVersion: config.kubernetes.io/v1
+kind: ResourceList
+metadata:
+  name: aShortList
+items:
+- apiVersion: v1
+  kind: ConfigMap
+  metadata:
+    name: betty
+  data:
+    color: &color-used blue
+    feeling: *color-used
+- apiVersion: v1
+  kind: ConfigMap
+  metadata:
+    name: bob
+  data:
+    color: red
+    feeling: *color-used
+`
+	rm, err := rmF.NewResMapFromBytes([]byte(input))
+	assert.NoError(t, err)
+	assert.NoError(t, rm.DeAnchor())
+	yaml, err := rm.AsYaml()
+	assert.NoError(t, err)
+	assert.Equal(t, strings.TrimSpace(`
+apiVersion: v1
+data:
+  color: blue
+  feeling: blue
+kind: ConfigMap
+metadata:
+  name: betty
+---
+apiVersion: v1
+data:
+  color: red
+  feeling: blue
+kind: ConfigMap
+metadata:
+  name: bob
+`), strings.TrimSpace(string(yaml)))
+}
+
 func TestApplySmPatch_General(t *testing.T) {
 	const (
 		myDeployment      = "Deployment"