backuptar: Export tar header parsing functions

Separates out Security Descriptor (SDDL), Extended Attribute (EA) and Reparse tag encoding
code into individual functions so that those functions can be reused in other code (like
CimFS layer writing)

Signed-off-by: Amit Barve <ambarve@microsoft.com>

backuptar/tar.go

@@ -109,6 +109,69 @@ func BasicInfoHeader(name string, size int64, fileInfo *winio.FileBasicInfo) *ta
 	return hdr
 }
 
+// SecurityDescriptorFromTarHeader reads the SDDL associated with the header of the current file
+// from the tar header and returns the security descriptor into a byte slice.
+func SecurityDescriptorFromTarHeader(hdr *tar.Header) ([]byte, error) {
+	// Maintaining old SDDL-based behavior for backward
+	// compatibility.  All new tar headers written by this library
+	// will have raw binary for the security descriptor.
+	var sd []byte
+	var err error
+	if sddl, ok := hdr.PAXRecords[hdrSecurityDescriptor]; ok {
+		sd, err = winio.SddlToSecurityDescriptor(sddl)
+		if err != nil {
+			return nil, err
+		}
+	}
+	if sdraw, ok := hdr.PAXRecords[hdrRawSecurityDescriptor]; ok {
+		sd, err = base64.StdEncoding.DecodeString(sdraw)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return sd, nil
+}
+
+// ExtendedAttributesFromTarHeader reads the EAs associated with the header of the
+// current file from the tar header and returns it as a byte slice.
+func ExtendedAttributesFromTarHeader(hdr *tar.Header) ([]byte, error) {
+	var eas []winio.ExtendedAttribute
+	var eadata []byte
+	var err error
+	for k, v := range hdr.PAXRecords {
+		if !strings.HasPrefix(k, hdrEaPrefix) {
+			continue
+		}
+		data, err := base64.StdEncoding.DecodeString(v)
+		if err != nil {
+			return nil, err
+		}
+		eas = append(eas, winio.ExtendedAttribute{
+			Name:  k[len(hdrEaPrefix):],
+			Value: data,
+		})
+	}
+	if len(eas) != 0 {
+		eadata, err = winio.EncodeExtendedAttributes(eas)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return eadata, nil
+}
+
+// EncodeReparsePointFromTarHeader reads the ReparsePoint structure from the tar header
+// and encodes it into a byte slice. The file for which this function is called must be a
+// symlink.
+func EncodeReparsePointFromTarHeader(hdr *tar.Header) []byte {
+	_, isMountPoint := hdr.PAXRecords[hdrMountPoint]
+	rp := winio.ReparsePoint{
+		Target:       filepath.FromSlash(hdr.Linkname),
+		IsMountPoint: isMountPoint,
+	}
+	return winio.EncodeReparsePoint(&rp)
+}
+
 // WriteTarFileFromBackupStream writes a file to a tar writer using data from a Win32 backup stream.
 //
 // This encodes Win32 metadata as tar pax vendor extensions starting with MSWINDOWS.
@@ -330,21 +393,10 @@ func FileInfoFromHeader(hdr *tar.Header) (name string, size int64, fileInfo *win
 // tar file that was not processed, or io.EOF is there are no more.
 func WriteBackupStreamFromTarFile(w io.Writer, t *tar.Reader, hdr *tar.Header) (*tar.Header, error) {
 	bw := winio.NewBackupStreamWriter(w)
-	var sd []byte
-	var err error
-	// Maintaining old SDDL-based behavior for backward compatibility.  All new tar headers written
-	// by this library will have raw binary for the security descriptor.
-	if sddl, ok := hdr.PAXRecords[hdrSecurityDescriptor]; ok {
-		sd, err = winio.SddlToSecurityDescriptor(sddl)
-		if err != nil {
-			return nil, err
-		}
-	}
-	if sdraw, ok := hdr.PAXRecords[hdrRawSecurityDescriptor]; ok {
-		sd, err = base64.StdEncoding.DecodeString(sdraw)
-		if err != nil {
-			return nil, err
-		}
+
+	sd, err := SecurityDescriptorFromTarHeader(hdr)
+	if err != nil {
+		return nil, err
 	}
 	if len(sd) != 0 {
 		bhdr := winio.BackupHeader{
@@ -360,25 +412,12 @@ func WriteBackupStreamFromTarFile(w io.Writer, t *tar.Reader, hdr *tar.Header) (
 			return nil, err
 		}
 	}
-	var eas []winio.ExtendedAttribute
-	for k, v := range hdr.PAXRecords {
-		if !strings.HasPrefix(k, hdrEaPrefix) {
-			continue
-		}
-		data, err := base64.StdEncoding.DecodeString(v)
-		if err != nil {
-			return nil, err
-		}
-		eas = append(eas, winio.ExtendedAttribute{
-			Name:  k[len(hdrEaPrefix):],
-			Value: data,
-		})
+
+	eadata, err := ExtendedAttributesFromTarHeader(hdr)
+	if err != nil {
+		return nil, err
 	}
-	if len(eas) != 0 {
-		eadata, err := winio.EncodeExtendedAttributes(eas)
-		if err != nil {
-			return nil, err
-		}
+	if len(eadata) != 0 {
 		bhdr := winio.BackupHeader{
 			Id:   winio.BackupEaData,
 			Size: int64(len(eadata)),
@@ -392,13 +431,9 @@ func WriteBackupStreamFromTarFile(w io.Writer, t *tar.Reader, hdr *tar.Header) (
 			return nil, err
 		}
 	}
+
 	if hdr.Typeflag == tar.TypeSymlink {
-		_, isMountPoint := hdr.PAXRecords[hdrMountPoint]
-		rp := winio.ReparsePoint{
-			Target:       filepath.FromSlash(hdr.Linkname),
-			IsMountPoint: isMountPoint,
-		}
-		reparse := winio.EncodeReparsePoint(&rp)
+		reparse := EncodeReparsePointFromTarHeader(hdr)
 		bhdr := winio.BackupHeader{
 			Id:   winio.BackupReparseData,
 			Size: int64(len(reparse)),
@@ -411,7 +446,9 @@ func WriteBackupStreamFromTarFile(w io.Writer, t *tar.Reader, hdr *tar.Header) (
 		if err != nil {
 			return nil, err
 		}
+
 	}
+
 	if hdr.Typeflag == tar.TypeReg || hdr.Typeflag == tar.TypeRegA {
 		bhdr := winio.BackupHeader{
 			Id:   winio.BackupData,