fix panic when fieldspec refers an empty value

api/krusty/namereference_test.go

@@ -0,0 +1,44 @@
+package krusty_test
+
+import (
+	"testing"
+
+	kusttest_test "sigs.k8s.io/kustomize/api/testutils/kusttest"
+)
+
+func TestEmptyFielsSpecValue(t *testing.T) {
+	th := kusttest_test.MakeHarness(t)
+	th.WriteK("/app", `
+generators:
+- generators.yaml
+configurations:
+- kustomizeconfig.yaml
+`)
+	th.WriteF("/app/generators.yaml", `
+apiVersion: builtin
+kind: ConfigMapGenerator
+metadata:
+  name: secret-example
+labels:
+  app.kubernetes.io/name: secret-example
+literals:
+- this_is_a_secret_name=
+`)
+	th.WriteF("/app/kustomizeconfig.yaml", `
+nameReference:
+- kind: Secret
+  version: v1
+  fieldSpecs:
+  - path: data/this_is_a_secret_name
+    kind: ConfigMap
+`)
+	m := th.Run("/app", th.MakeDefaultOptions())
+	th.AssertActualEqualsExpected(m, `
+apiVersion: v1
+data:
+  this_is_a_secret_name: ""
+kind: ConfigMap
+metadata:
+  name: secret-example-7hf4fh868h
+`)
+}

kyaml/yaml/fns.go

@@ -467,6 +467,10 @@ func (s FieldSetter) Filter(rn *RNode) (*RNode, error) {
 		if err := ErrorIfInvalid(rn, yaml.ScalarNode); err != nil {
 			return rn, err
 		}
+		// value is nil or null
+		if IsMissingOrNull(s.Value) {
+			return rn, nil
+		}
 		// only apply the style if there is not an existing style
 		// or we want to override it
 		if !s.OverrideStyle || s.Value.YNode().Style == 0 {

kyaml/yaml/fns_test.go

@@ -395,6 +395,19 @@ bar: buz
 	assert.Equal(t, `baz
 `, assertNoErrorString(t)(k.String()))
 
+	// Empty value
+	node, err = Parse(`
+foo
+`)
+	assert.NoError(t, err)
+	instance = FieldSetter{}
+	k, err = instance.Filter(node)
+	assert.NoError(t, err)
+	assert.Equal(t, `foo
+`, assertNoErrorString(t)(node.String()))
+	assert.Equal(t, `foo
+`, assertNoErrorString(t)(k.String()))
+
 	// Encounter error
 	node, err = Parse(`
 -a