fix setting nil data values via --data-value flag

pkg/yamlmeta/plain.go

@@ -7,11 +7,11 @@ import (
 	"github.com/k14s/ytt/pkg/yamlmeta/internal/yaml.v2"
 )
 
-func PlainMarshal(val interface{}) ([]byte, error) {
-	return yaml.Marshal(val)
+func PlainMarshal(in interface{}) ([]byte, error) {
+	return yaml.Marshal(in)
 }
 
-func PlainUnmarshal(data []byte, val interface{}) error {
+func PlainUnmarshal(data []byte, out interface{}) error {
 	docSet, err := NewParser(true).ParseBytes(data, "")
 	if err != nil {
 		return err
@@ -21,9 +21,14 @@ func PlainUnmarshal(data []byte, val interface{}) error {
 		return fmt.Errorf("Expected to find exactly one YAML document")
 	}
 
-	decodedVal := docSet.Items[0].AsInterface(InterfaceConvertOpts{})
+	newVal := docSet.Items[0].AsInterface(InterfaceConvertOpts{})
 
-	reflect.Indirect(reflect.ValueOf(val)).Set(reflect.ValueOf(decodedVal))
+	outVal := reflect.ValueOf(out)
+	if newVal == nil {
+		outVal.Elem().Set(reflect.Zero(outVal.Elem().Type()))
+	} else {
+		outVal.Elem().Set(reflect.ValueOf(newVal))
+	}
 
 	return nil
 }

pkg/yamlmeta/plain_test.go

@@ -0,0 +1,47 @@
+package yamlmeta_test
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/k14s/ytt/pkg/yamlmeta"
+)
+
+var _ = fmt.Sprintf
+
+func TestPlainUnmarshalInt(t *testing.T) {
+	var val interface{} = "abc" // set to some previous value
+
+	err := yamlmeta.PlainUnmarshal([]byte("123"), &val)
+	if err != nil {
+		t.Fatalf("Expected to succeed: %s", err)
+	}
+	if !reflect.DeepEqual(val, 123) {
+		t.Fatalf("Expected to be nil: val=%#v type=%T", val, val)
+	}
+}
+
+func TestPlainUnmarshalNil(t *testing.T) {
+	var val interface{} = 123 // set to some previous value
+
+	err := yamlmeta.PlainUnmarshal([]byte("null"), &val)
+	if err != nil {
+		t.Fatalf("Expected to succeed: %s", err)
+	}
+	if !reflect.DeepEqual(val, nil) {
+		t.Fatalf("Expected to be nil: val=%#v type=%T", val, val)
+	}
+}
+
+func TestPlainUnmarshalMap(t *testing.T) {
+	var val interface{} = 123 // set to some previous value
+
+	err := yamlmeta.PlainUnmarshal([]byte(`{"a":123}`), &val)
+	if err != nil {
+		t.Fatalf("Expected to succeed: %s", err)
+	}
+	if !reflect.DeepEqual(val, map[interface{}]interface{}{"a": 123}) {
+		t.Fatalf("Expected to be nil: val=%#v type=%T", val, val)
+	}
+}