lib/model: Allow empty subdirs in scan request (fixes #3829)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3833

lib/model/model.go

@@ -1706,8 +1706,16 @@ func (m *Model) ScanFolderSubdirs(folder string, subs []string) error {
 }
 
 func (m *Model) internalScanFolderSubdirs(folder string, subDirs []string) error {
-	for i, sub := range subDirs {
+	for i := 0; i < len(subDirs); i++ {
-		sub = osutil.NativeFilename(sub)
+		sub := osutil.NativeFilename(subDirs[i])
+
+		if sub == "" {
+			// A blank subdirs means to scan the entire folder. We can trim
+			// the subDirs list and go on our way.
+			subDirs = nil
+			break
+		}
+
 		// We test each path by joining with "root". What we join with is
 		// not relevant, we just want the dotdot escape detection here. For
 		// historical reasons we may get paths that end in a slash. We

lib/model/model_test.go

@@ -2181,6 +2181,21 @@ func TestIssue3804(t *testing.T) {
 	}
 }
 
+func TestIssue3829(t *testing.T) {
+	dbi := db.OpenMemory()
+	m := NewModel(defaultConfig, protocol.LocalDeviceID, "device", "syncthing", "dev", dbi, nil)
+	m.AddFolder(defaultFolderConfig)
+	m.StartFolder("default")
+	m.ServeBackground()
+	defer m.Stop()
+
+	// Empty subdirs should be accepted
+
+	if err := m.ScanFolderSubdirs("default", []string{""}); err != nil {
+		t.Error("Unexpected error:", err)
+	}
+}
+
 func TestRootedJoinedPath(t *testing.T) {
 	type testcase struct {
 		root   string