Basic support for serializing v2 torrent file (#968)

* Support marshalling FileTree

* Omit Pieces in torrent info if empty

Otherwise some BitTorrent client (e.g. libttorrent) will not be able to
parse v2 torrent file.

metainfo/file-tree.go

@@ -49,6 +49,35 @@ func (ft *FileTree) UnmarshalBencode(bytes []byte) (err error) {
 
 var _ bencode.Unmarshaler = (*FileTree)(nil)
 
+func (ft *FileTree) MarshalBencode() (bytes []byte, err error) {
+	if ft.IsDir() {
+		dir := make(map[string]bencode.Bytes, len(ft.Dir))
+		for _, key := range ft.orderedKeys() {
+			if key == FileTreePropertiesKey {
+				continue
+			}
+			sub := g.MapMustGet(ft.Dir, key)
+			subBytes, err := sub.MarshalBencode()
+			if err != nil {
+				return nil, err
+			}
+			dir[key] = subBytes
+		}
+		return bencode.Marshal(dir)
+	} else {
+		fileBytes, err := bencode.Marshal(ft.File)
+		if err != nil {
+			return nil, err
+		}
+		res := map[string]bencode.Bytes{
+			"": fileBytes,
+		}
+		return bencode.Marshal(res)
+	}
+}
+
+var _ bencode.Marshaler = (*FileTree)(nil)
+
 func (ft *FileTree) NumEntries() (num int) {
 	num = len(ft.Dir)
 	if g.MapContains(ft.Dir, FileTreePropertiesKey) {

metainfo/info.go

@@ -13,9 +13,9 @@ import (
 
 // The info dictionary. See BEP 3 and BEP 52.
 type Info struct {
-	PieceLength int64  `bencode:"piece length"` // BEP3
-	Pieces      []byte `bencode:"pieces"`       // BEP3
-	Name        string `bencode:"name"`         // BEP3
+	PieceLength int64  `bencode:"piece length"`     // BEP3
+	Pieces      []byte `bencode:"pieces,omitempty"` // BEP3
+	Name        string `bencode:"name"`             // BEP3
 	NameUtf8    string `bencode:"name.utf-8,omitempty"`
 	Length      int64  `bencode:"length,omitempty"` // BEP3, mutually exclusive with Files
 	ExtendedFileAttrs