lib/fs: Handle deduplicated files on NTFS (fixes #1845)

These files always have the symlink bit set, because they are reparse
points. Nonetheless they are not symlinks, and Lstat reports a size for
them. We use this fact to disambiguate, and hope fervently that nothing
else matches this description so it comes back to bite us...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4622

lib/fs/basicfs.go

@@ -322,14 +322,12 @@ type fsFileInfo struct {
 	os.FileInfo
 }
 
-func (e fsFileInfo) Mode() FileMode {
-	return FileMode(e.FileInfo.Mode())
+func (e fsFileInfo) IsSymlink() bool {
+	// Must use fsFileInfo.Mode() because it may apply magic.
+	return e.Mode()&ModeSymlink != 0
 }
 
 func (e fsFileInfo) IsRegular() bool {
-	return e.FileInfo.Mode().IsRegular()
-}
-
-func (e fsFileInfo) IsSymlink() bool {
-	return e.FileInfo.Mode()&os.ModeSymlink == os.ModeSymlink
+	// Must use fsFileInfo.Mode() because it may apply magic.
+	return e.Mode()&ModeType == 0
 }

lib/fs/filesystem.go

@@ -126,6 +126,8 @@ const ModePerm = FileMode(os.ModePerm)
 const ModeSetgid = FileMode(os.ModeSetgid)
 const ModeSetuid = FileMode(os.ModeSetuid)
 const ModeSticky = FileMode(os.ModeSticky)
+const ModeSymlink = FileMode(os.ModeSymlink)
+const ModeType = FileMode(os.ModeType)
 const PathSeparator = os.PathSeparator
 const OptAppend = os.O_APPEND
 const OptCreate = os.O_CREATE

lib/fs/fsfileinfo_unix.go

@@ -0,0 +1,13 @@
+// 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/.
+
+// +build !windows
+
+package fs
+
+func (e fsFileInfo) Mode() FileMode {
+	return FileMode(e.FileInfo.Mode())
+}

lib/fs/fsfileinfo_windows.go

@@ -0,0 +1,21 @@
+// 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 fs
+
+import (
+	"os"
+)
+
+func (e fsFileInfo) Mode() FileMode {
+	m := e.FileInfo.Mode()
+	if m&os.ModeSymlink != 0 && e.Size() > 0 {
+		// "Symlinks" with nonzero size are in fact "hard" links, such as
+		// NTFS deduped files. Remove the symlink bit.
+		m &^= os.ModeSymlink
+	}
+	return FileMode(m)
+}