lib/fs: Properly handle Windows deduplicated files (fixes #9120) (#9168)

### Purpose

Deduplicated files are apparently considered 'irregular' under the hood,
this causes them to simply be ignored by Syncthing. This change is more
of a workaround than a proper fix, as the fix should probably happen in
the underlying libraries? - which may take some time. In the meanwhile,
this change should make deduplicated files be treated as regular files
and be indexed and synced as they should.

### Testing

Create some volume where deduplication is turned on (see the relevant
issue for details, including a proper description of how to reproduce
it). Prior to this change, the deduplicated files were simply ignored
(even by the indexer). After this change, the deduplicated files are
being index and synced properly.

lib/fs/basicfs_lstat_windows.go

@@ -18,15 +18,17 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-func isDirectoryJunction(path string) (bool, error) {
+const IO_REPARSE_TAG_DEDUP = 0x80000013
+
+func readReparseTag(path string) (uint32, error) {
 	namep, err := syscall.UTF16PtrFromString(path)
 	if err != nil {
-		return false, fmt.Errorf("syscall.UTF16PtrFromString failed with: %s", err)
+		return 0, fmt.Errorf("syscall.UTF16PtrFromString failed with: %s", err)
 	}
 	attrs := uint32(syscall.FILE_FLAG_BACKUP_SEMANTICS | syscall.FILE_FLAG_OPEN_REPARSE_POINT)
 	h, err := syscall.CreateFile(namep, 0, 0, nil, syscall.OPEN_EXISTING, attrs, 0)
 	if err != nil {
-		return false, fmt.Errorf("syscall.CreateFile failed with: %s", err)
+		return 0, fmt.Errorf("syscall.CreateFile failed with: %s", err)
 	}
 	defer syscall.CloseHandle(h)
 
@@ -47,10 +49,19 @@ func isDirectoryJunction(path string) (bool, error) {
 			// instance to indicate no symlinks are possible.
 			ti.ReparseTag = 0
 		} else {
-			return false, fmt.Errorf("windows.GetFileInformationByHandleEx failed with: %s", err)
+			return 0, fmt.Errorf("windows.GetFileInformationByHandleEx failed with: %s", err)
 		}
 	}
-	return ti.ReparseTag == windows.IO_REPARSE_TAG_MOUNT_POINT, nil
+
+	return ti.ReparseTag, nil
+}
+
+func isDirectoryJunction(reparseTag uint32) bool {
+	return reparseTag == windows.IO_REPARSE_TAG_MOUNT_POINT
+}
+
+func isDeduplicatedFile(reparseTag uint32) bool {
+	return reparseTag == IO_REPARSE_TAG_DEDUP
 }
 
 type dirJunctFileInfo struct {
@@ -67,17 +78,38 @@ func (fi *dirJunctFileInfo) IsDir() bool {
 	return true
 }
 
+type dedupFileInfo struct {
+	os.FileInfo
+}
+
+func (fi *dedupFileInfo) Mode() os.FileMode {
+	// A deduplicated file should be treated as a regular file and not an
+	// irregular file.
+	return fi.FileInfo.Mode() &^ os.ModeIrregular
+}
+
 func (f *BasicFilesystem) underlyingLstat(name string) (os.FileInfo, error) {
 	var fi, err = os.Lstat(name)
 
-	// NTFS directory junctions can be treated as ordinary directories,
-	// see https://forum.syncthing.net/t/option-to-follow-directory-junctions-symbolic-links/14750
-	if err == nil && f.junctionsAsDirs && fi.Mode()&os.ModeSymlink != 0 {
-		var isJunct bool
-		isJunct, err = isDirectoryJunction(name)
-		if err == nil && isJunct {
-			return &dirJunctFileInfo{fi}, nil
+	// There are cases where files are tagged as symlink, but they end up being
+	// something else. Make sure we properly handle those types.
+	if err == nil {
+		// NTFS directory junctions can be treated as ordinary directories,
+		// see https://forum.syncthing.net/t/option-to-follow-directory-junctions-symbolic-links/14750
+		if fi.Mode()&os.ModeSymlink != 0 && f.junctionsAsDirs {
+			if reparseTag, reparseErr := readReparseTag(name); reparseErr == nil && isDirectoryJunction(reparseTag) {
+				return &dirJunctFileInfo{fi}, nil
+			}
+		}
+
+		// Workaround for #9120 till golang properly handles deduplicated files by
+		// considering them regular files.
+		if fi.Mode()&os.ModeIrregular != 0 {
+			if reparseTag, reparseErr := readReparseTag(name); reparseErr == nil && isDeduplicatedFile(reparseTag) {
+				return &dedupFileInfo{fi}, nil
+			}
 		}
 	}
+
 	return fi, err
 }