metainfo.Info.BuildFromFilePath: Ensure stable file ordering

Spotted by @axet, with precedent in Transmission.

metainfo/metainfo.go

@@ -11,6 +11,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/anacrolix/missinggo"
+
 	"github.com/anacrolix/torrent/bencode"
 )
 
@@ -83,6 +85,9 @@ func (info *Info) BuildFromFilePath(root string) (err error) {
 	if err != nil {
 		return
 	}
+	missinggo.SortSlice(info.Files, func(l, r FileInfo) bool {
+		return strings.Join(l.Path, "/") < strings.Join(r.Path, "/")
+	})
 	err = info.GeneratePieces(func(fi FileInfo) (io.ReadCloser, error) {
 		return os.Open(filepath.Join(root, strings.Join(fi.Path, string(filepath.Separator))))
 	})

metainfo/metainfo_test.go

@@ -3,7 +3,9 @@ package metainfo
 import (
 	"io"
 	"io/ioutil"
+	"os"
 	"path"
+	"path/filepath"
 	"testing"
 
 	"github.com/anacrolix/missinggo"
@@ -68,3 +70,29 @@ func TestNumPieces(t *testing.T) {
 		assert.EqualValues(t, _case.NumPieces, info.NumPieces())
 	}
 }
+
+func touchFile(path string) (err error) {
+	f, err := os.Create(path)
+	if err != nil {
+		return
+	}
+	err = f.Close()
+	return
+}
+
+func TestBuildFromFilePathOrder(t *testing.T) {
+	td, err := ioutil.TempDir("", "anacrolix")
+	require.NoError(t, err)
+	defer os.RemoveAll(td)
+	require.NoError(t, touchFile(filepath.Join(td, "b")))
+	require.NoError(t, touchFile(filepath.Join(td, "a")))
+	info := Info{
+		PieceLength: 1,
+	}
+	require.NoError(t, info.BuildFromFilePath(td))
+	assert.EqualValues(t, []FileInfo{{
+		Path: []string{"a"},
+	}, {
+		Path: []string{"b"},
+	}}, info.Files)
+}