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_test.go

@@ -0,0 +1,75 @@
+// 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"
+	"testing"
+
+	"github.com/d4l3k/messagediff"
+)
+
+// TestEmptyDirs models the following .stversions structure:
+// .stversions/
+// ├── keep1
+// │   └── file1
+// ├── keep2
+// │   └── keep21
+// │       └── keep22
+// │           └── file1
+// ├── remove1
+// └── remove2
+//     └── remove21
+//         └── remove22
+func TestEmptyDirs(t *testing.T) {
+	var paths = []struct {
+		path   string
+		isFile bool
+	}{
+		{".", false},
+		{"keep1", false},
+		{"keep1/file1", true},
+		{"keep2", false},
+		{"keep2/keep21", false},
+		{"keep2/keep21/keep22", false},
+		{"keep2/keep21/keep22/file1", true},
+		{"remove1", false},
+		{"remove2", false},
+		{"remove2/remove21", false},
+		{"remove2/remove21/remove22", false},
+	}
+
+	var expected = []string{
+		"remove2/remove21/remove22",
+		"remove2/remove21",
+		"remove2",
+		"remove1",
+	}
+
+	// For compatibility with Windows
+	for i, p := range paths {
+		paths[i].path = filepath.FromSlash(p.path)
+	}
+
+	for i, p := range expected {
+		expected[i] = filepath.FromSlash(p)
+	}
+
+	dirTracker := make(emptyDirTracker)
+	for _, p := range paths {
+		if p.isFile {
+			dirTracker.addFile(p.path)
+		} else {
+			dirTracker.addDir(p.path)
+		}
+	}
+
+	result := dirTracker.emptyDirs()
+	if diff, equal := messagediff.PrettyDiff(expected, result); !equal {
+		t.Errorf("Incorrect empty directories list; got %v, expected %v\n%v", result, expected, diff)
+	}
+}