Include hard-linked files as hard-links in the tarstream

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

internal/wclayer/baselayerreader.go

@@ -186,6 +186,18 @@ func (r *baseLayerReader) Next() (path string, size int64, fileInfo *winio.FileB
 	return
 }
 
+func (r *baseLayerReader) LinkInfo() (uint32, *winio.FileIDInfo, error) {
+	fileStandardInfo, err := winio.GetFileStandardInfo(r.currentFile)
+	if err != nil {
+		return 0, nil, err
+	}
+	fileIDInfo, err := winio.GetFileID(r.currentFile)
+	if err != nil {
+		return 0, nil, err
+	}
+	return fileStandardInfo.NumberOfLinks, fileIDInfo, nil
+}
+
 func (r *baseLayerReader) Read(b []byte) (int, error) {
 	if r.backupReader == nil {
 		if r.currentFile == nil {

internal/wclayer/exportlayer.go

@@ -46,6 +46,8 @@ func ExportLayer(ctx context.Context, path string, exportFolderPath string, pare
 type LayerReader interface {
 	// Next advances to the next file and returns the name, size, and file info
 	Next() (string, int64, *winio.FileBasicInfo, error)
+	// LinkInfo returns the number of links and the file identifier for the current file.
+	LinkInfo() (uint32, *winio.FileIDInfo, error)
 	// Read reads data from the current file, in the format of a Win32 backup stream, and
 	// returns the number of bytes read.
 	Read(b []byte) (int, error)

internal/wclayer/legacy.go

@@ -295,6 +295,18 @@ func (r *legacyLayerReader) Next() (path string, size int64, fileInfo *winio.Fil
 	return
 }
 
+func (r *legacyLayerReader) LinkInfo() (uint32, *winio.FileIDInfo, error) {
+	fileStandardInfo, err := winio.GetFileStandardInfo(r.currentFile)
+	if err != nil {
+		return 0, nil, err
+	}
+	fileIDInfo, err := winio.GetFileID(r.currentFile)
+	if err != nil {
+		return 0, nil, err
+	}
+	return fileStandardInfo.NumberOfLinks, fileIDInfo, nil
+}
+
 func (r *legacyLayerReader) Read(b []byte) (int, error) {
 	if r.backupReader == nil {
 		if r.currentFile == nil {

pkg/ociwclayer/export.go

@@ -53,6 +53,8 @@ func ExportLayerToTar(ctx context.Context, w io.Writer, path string, parentLayer
 }
 
 func writeTarFromLayer(ctx context.Context, r hcsshim.LayerReader, w io.Writer) error {
+	linkRecords := make(map[[16]byte]string)
+
 	t := tar.NewWriter(w)
 	for {
 		select {
@@ -78,6 +80,27 @@ func writeTarFromLayer(ctx context.Context, r hcsshim.LayerReader, w io.Writer)
 				return err
 			}
 		} else {
+			numberOfLinks, fileIDInfo, err := r.LinkInfo()
+			if err != nil {
+				return err
+			}
+			if numberOfLinks > 1 {
+				if linkName, ok := linkRecords[fileIDInfo.FileID]; ok {
+					// We've seen this file before, by another name, so put a hardlink in the tar stream.
+					hdr := backuptar.BasicInfoHeader(name, 0, fileInfo)
+					hdr.Mode = 0644
+					hdr.Typeflag = tar.TypeLink
+					hdr.Linkname = linkName
+					if err := t.WriteHeader(hdr); err != nil {
+						return err
+					}
+					continue
+				}
+
+				// All subsequent names for this file will be hard-linked to this name
+				linkRecords[fileIDInfo.FileID] = filepath.ToSlash(name)
+			}
+
 			err = backuptar.WriteTarFileFromBackupStream(t, r, name, size, fileInfo)
 			if err != nil {
 				return err