lib/versioner: Purge the empty directories in .stversions (fixes #4406)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4514
LGTM: AudriusButkevicius, imsodin

lib/versioner/empty_dir_tracker.go

@@ -0,0 +1,51 @@
+// Copyright (C) 2017 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package versioner
+
+import (
+	"path/filepath"
+	"sort"
+
+	"github.com/syncthing/syncthing/lib/fs"
+)
+
+type emptyDirTracker map[string]struct{}
+
+func (t emptyDirTracker) addDir(path string) {
+	if path == "." {
+		return
+	}
+	t[path] = struct{}{}
+}
+
+// Remove all dirs from the path to the file
+func (t emptyDirTracker) addFile(path string) {
+	dir := filepath.Dir(path)
+	for dir != "." {
+		delete(t, dir)
+		dir = filepath.Dir(dir)
+	}
+}
+
+func (t emptyDirTracker) emptyDirs() []string {
+	empty := []string{}
+	for dir := range t {
+		empty = append(empty, dir)
+	}
+	sort.Sort(sort.Reverse(sort.StringSlice(empty)))
+	return empty
+}
+
+func (t emptyDirTracker) deleteEmptyDirs(fs fs.Filesystem) {
+	for _, path := range t.emptyDirs() {
+		l.Debugln("Cleaner: deleting empty directory", path)
+		err := fs.Remove(path)
+		if err != nil {
+			l.Warnln("Versioner: can't remove directory", path, err)
+		}
+	}
+}