lib/versioner: Restore for all versioners, cross-device support (#5514)

* lib/versioner: Restore for all versioners, cross-device support

Fixes #4631
Fixes #4586
Fixes #1634
Fixes #5338
Fixes #5419

lib/fs/util.go

@@ -103,3 +103,55 @@ func IsParent(path, parent string) bool {
 	}
 	return strings.HasPrefix(path, parent)
 }
+
+func CommonPrefix(first, second string) string {
+	if filepath.IsAbs(first) != filepath.IsAbs(second) {
+		// Whatever
+		return ""
+	}
+
+	firstParts := strings.Split(filepath.Clean(first), string(PathSeparator))
+	secondParts := strings.Split(filepath.Clean(second), string(PathSeparator))
+
+	isAbs := filepath.IsAbs(first) && filepath.IsAbs(second)
+
+	count := len(firstParts)
+	if len(secondParts) < len(firstParts) {
+		count = len(secondParts)
+	}
+
+	common := make([]string, 0, count)
+	for i := 0; i < count; i++ {
+		if firstParts[i] != secondParts[i] {
+			break
+		}
+		common = append(common, firstParts[i])
+	}
+
+	if isAbs {
+		if runtime.GOOS == "windows" && isVolumeNameOnly(common) {
+			// Because strings.Split strips out path separators, if we're at the volume name, we end up without a separator
+			// Wedge an empty element to be joined with.
+			common = append(common, "")
+		} else if len(common) == 1 {
+			// If isAbs on non Windows, first element in both first and second is "", hence joining that returns nothing.
+			return string(PathSeparator)
+		}
+	}
+
+	// This should only be true on Windows when drive letters are different or when paths are relative.
+	// In case of UNC paths we should end up with more than a single element hence joining is fine
+	if len(common) == 0 {
+		return ""
+	}
+
+	// This has to be strings.Join, because filepath.Join([]string{"", "", "?", "C:", "Audrius"}...) returns garbage
+	result := strings.Join(common, string(PathSeparator))
+	return filepath.Clean(result)
+}
+
+func isVolumeNameOnly(parts []string) bool {
+	isNormalVolumeName := len(parts) == 1 && strings.HasSuffix(parts[0], ":")
+	isUNCVolumeName := len(parts) == 4 && strings.HasSuffix(parts[3], ":")
+	return isNormalVolumeName || isUNCVolumeName
+}

lib/fs/util_test.go

@@ -0,0 +1,46 @@
+// Copyright (C) 2019 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 fs
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestCommonPrefix(t *testing.T) {
+	test := func(first, second, expect string) {
+		t.Helper()
+		res := CommonPrefix(first, second)
+		if res != expect {
+			t.Errorf("Expected %s got %s", expect, res)
+		}
+	}
+
+	if runtime.GOOS == "windows" {
+		test(`c:\Audrius\Downloads`, `c:\Audrius\Docs`, `c:\Audrius`)
+		test(`c:\Audrius\Downloads`, `C:\Audrius\Docs`, ``) // Case differences :(
+		test(`C:\Audrius-a\Downloads`, `C:\Audrius-b\Docs`, `C:\`)
+		test(`\\?\C:\Audrius-a\Downloads`, `\\?\C:\Audrius-b\Docs`, `\\?\C:\`)
+		test(`\\?\C:\Audrius\Downloads`, `\\?\C:\Audrius\Docs`, `\\?\C:\Audrius`)
+		test(`Audrius-a\Downloads`, `Audrius-b\Docs`, ``)
+		test(`Audrius\Downloads`, `Audrius\Docs`, `Audrius`)
+		test(`c:\Audrius\Downloads`, `Audrius\Docs`, ``)
+		test(`c:\`, `c:\`, `c:\`)
+		test(`\\?\c:\`, `\\?\c:\`, `\\?\c:\`)
+	} else {
+		test(`/Audrius/Downloads`, `/Audrius/Docs`, `/Audrius`)
+		test(`/Audrius\Downloads`, `/Audrius\Docs`, `/`)
+		test(`/Audrius-a/Downloads`, `/Audrius-b/Docs`, `/`)
+		test(`Audrius\Downloads`, `Audrius\Docs`, ``) // Windows separators
+		test(`Audrius/Downloads`, `Audrius/Docs`, `Audrius`)
+		test(`Audrius-a\Downloads`, `Audrius-b\Docs`, ``)
+		test(`/Audrius/Downloads`, `Audrius/Docs`, ``)
+		test(`/`, `/`, `/`)
+	}
+	test(`Audrius`, `Audrius`, `Audrius`)
+	test(`.`, `.`, `.`)
+}