Add storage/disabled

The default storage will create empty files on torrent open, which is undesirable in some circumstances. This storage implementation is explicit about not storing anything.

storage/disabled/disabled.go

@@ -0,0 +1,58 @@
+package disabled
+
+import (
+	"errors"
+
+	"github.com/anacrolix/torrent/metainfo"
+	"github.com/anacrolix/torrent/storage"
+)
+
+type Client struct{}
+
+var capacity int64
+
+func (c Client) OpenTorrent(info *metainfo.Info, infoHash metainfo.Hash) (storage.TorrentImpl, error) {
+	capFunc := func() *int64 {
+		return &capacity
+	}
+	return storage.TorrentImpl{
+		Piece: func(piece metainfo.Piece) storage.PieceImpl {
+			return Piece{}
+		},
+		Close: func() error {
+			return nil
+		},
+		Capacity: &capFunc,
+	}, nil
+}
+
+func (c Client) capacity() *int64 {
+	return &capacity
+}
+
+type Piece struct{}
+
+func (Piece) ReadAt(p []byte, off int64) (n int, err error) {
+	err = errors.New("disabled")
+	return
+}
+
+func (Piece) WriteAt(p []byte, off int64) (n int, err error) {
+	err = errors.New("disabled")
+	return
+}
+
+func (Piece) MarkComplete() error {
+	return errors.New("disabled")
+}
+
+func (Piece) MarkNotComplete() error {
+	return errors.New("disabled")
+}
+
+func (Piece) Completion() storage.Completion {
+	return storage.Completion{
+		Complete: false,
+		Ok:       true,
+	}
+}