Refactored decoder responsibilities (#1402)

- improved comment handling
- yaml decoder now responsible for leading content work around

pkg/yqlib/decoder_xml.go

@@ -25,10 +25,11 @@ func NewXMLDecoder(prefs XmlPreferences) Decoder {
 	}
 }
 
-func (dec *xmlDecoder) Init(reader io.Reader) {
+func (dec *xmlDecoder) Init(reader io.Reader) error {
 	dec.reader = reader
 	dec.readAnything = false
 	dec.finished = false
+	return nil
 }
 
 func (dec *xmlDecoder) createSequence(nodes []*xmlNode) (*yaml.Node, error) {
@@ -118,32 +119,36 @@ func (dec *xmlDecoder) convertToYamlNode(n *xmlNode) (*yaml.Node, error) {
 	return scalar, nil
 }
 
-func (dec *xmlDecoder) Decode(rootYamlNode *yaml.Node) error {
+func (dec *xmlDecoder) Decode() (*CandidateNode, error) {
 	if dec.finished {
-		return io.EOF
+		return nil, io.EOF
 	}
 	root := &xmlNode{}
 	// cant use xj - it doesn't keep map order.
 	err := dec.decodeXML(root)
 
 	if err != nil {
-		return err
+		return nil, err
 	}
 	firstNode, err := dec.convertToYamlNode(root)
 
 	if err != nil {
-		return err
+		return nil, err
 	} else if firstNode.Tag == "!!null" {
 		dec.finished = true
 		if dec.readAnything {
-			return io.EOF
+			return nil, io.EOF
 		}
 	}
 	dec.readAnything = true
-	rootYamlNode.Kind = yaml.DocumentNode
-	rootYamlNode.Content = []*yaml.Node{firstNode}
 	dec.finished = true
-	return nil
+
+	return &CandidateNode{
+		Node: &yaml.Node{
+			Kind:    yaml.DocumentNode,
+			Content: []*yaml.Node{firstNode},
+		},
+	}, nil
 }
 
 type xmlNode struct {