backuptar: Fix sparse file handling

A recent OS change altered how sparse files are represented in backup
streams. This caused backuptar to no longer work with certain files. The
specific behavior that changed is as follows:
- Empty sparse files (size = 0), previously did not have any data or
  sparse block streams in the backup stream. Now, they will have a
  data stream with size = 0, and no sparse block streams.
- Sparse files with a single allocated range (e.g. a normal file that
  has the sparse attribute set) previously would not show as sparse in
  the backup stream. Now, they will show as sparse.

The old backuptar behavior assumed that if the sparse flag was set on
the data stream, then there would always be a set of sparse blocks
following. These changes break this assumption, and so require special
handling.

It is unsupported to have a data stream, marked sparse, that contains
file content AND a series of sparse block streams following. As far as
I can tell this is not a valid case for backup streams.

This change also cleans up some code and error messages, and expands on
the test coverage for backuptar.

For more information on backup stream format see: https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-bkup/f67950c8-d583-469a-83dd-c4ff4cedf533

Signed-off-by: Kevin Parsons <kevpar@microsoft.com>

backuptar/tar.go

@@ -5,7 +5,6 @@ package backuptar
 import (
 	"archive/tar"
 	"encoding/base64"
-	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -42,19 +41,14 @@ const (
 	hdrCreationTime = "LIBARCHIVE.creationtime"
 )
 
-func writeZeroes(w io.Writer, count int64) error {
-	buf := make([]byte, 8192)
-	c := len(buf)
-	for i := int64(0); i < count; i += int64(c) {
-		if int64(c) > count-i {
-			c = int(count - i)
-		}
-		_, err := w.Write(buf[:c])
-		if err != nil {
-			return err
-		}
+// zeroReader is an io.Reader that always returns 0s.
+type zeroReader struct{}
+
+func (zr zeroReader) Read(b []byte) (int, error) {
+	for i := range b {
+		b[i] = 0
 	}
-	return nil
+	return len(b), nil
 }
 
 func copySparse(t *tar.Writer, br *winio.BackupStreamReader) error {
@@ -71,16 +65,26 @@ func copySparse(t *tar.Writer, br *winio.BackupStreamReader) error {
 			return fmt.Errorf("unexpected stream %d", bhdr.Id)
 		}
 
+		// We can't seek backwards, since we have already written that data to the tar.Writer.
+		if bhdr.Offset < curOffset {
+			return fmt.Errorf("cannot seek back from %d to %d", curOffset, bhdr.Offset)
+		}
 		// archive/tar does not support writing sparse files
 		// so just write zeroes to catch up to the current offset.
-		err = writeZeroes(t, bhdr.Offset-curOffset)
+		if _, err := io.CopyN(t, zeroReader{}, bhdr.Offset-curOffset); err != nil {
+			return fmt.Errorf("seek to offset %d: %s", bhdr.Offset, err)
+		}
 		if bhdr.Size == 0 {
+			// A sparse block with size = 0 is used to mark the end of the sparse blocks.
 			break
 		}
 		n, err := io.Copy(t, br)
 		if err != nil {
 			return err
 		}
+		if n != bhdr.Size {
+			return fmt.Errorf("copied %d bytes instead of %d at offset %d", n, bhdr.Size, bhdr.Offset)
+		}
 		curOffset = bhdr.Offset + n
 	}
 	return nil
@@ -221,20 +225,44 @@ func WriteTarFileFromBackupStream(t *tar.Writer, r io.Reader, name string, size
 		}
 	}
 
+	// The logic for copying file contents is fairly complicated due to the need for handling sparse files,
+	// and the weird ways they are represented by BackupRead. A normal file will always either have a data stream
+	// with size and content, or no data stream at all (if empty). However, for a sparse file, the content can also
+	// be represented using a series of sparse block streams following the data stream. Additionally, the way sparse
+	// files are handled by BackupRead has changed in the OS recently. The specifics of the representation are described
+	// in the list at the bottom of this block comment.
+	//
+	// Sparse files can be represented in four different ways, based on the specifics of the file.
+	// - Size = 0:
+	//     Previously: BackupRead yields no data stream and no sparse block streams.
+	//     Recently: BackupRead yields a data stream with size = 0. There are no following sparse block streams.
+	// - Size > 0, no allocated ranges:
+	//     BackupRead yields a data stream with size = 0. Following is a single sparse block stream with
+	//     size = 0 and offset = <file size>.
+	// - Size > 0, one allocated range:
+	//     BackupRead yields a data stream with size = <file size> containing the file contents. There are no
+	//     sparse block streams. This is the case if you take a normal file with contents and simply set the
+	//     sparse flag on it.
+	// - Size > 0, multiple allocated ranges:
+	//     BackupRead yields a data stream with size = 0. Following are sparse block streams for each allocated
+	//     range of the file containing the range contents. Finally there is a sparse block stream with
+	//     size = 0 and offset = <file size>.
+
 	if dataHdr != nil {
 		// A data stream was found. Copy the data.
-		if (dataHdr.Attributes & winio.StreamSparseAttributes) == 0 {
+		// We assume that we will either have a data stream size > 0 XOR have sparse block streams.
+		if dataHdr.Size > 0 || (dataHdr.Attributes&winio.StreamSparseAttributes) == 0 {
 			if size != dataHdr.Size {
 				return fmt.Errorf("%s: mismatch between file size %d and header size %d", name, size, dataHdr.Size)
 			}
-			_, err = io.Copy(t, br)
-			if err != nil {
-				return err
+			if _, err = io.Copy(t, br); err != nil {
+				return fmt.Errorf("%s: copying contents from data stream: %s", name, err)
 			}
-		} else {
-			err = copySparse(t, br)
-			if err != nil {
-				return err
+		} else if size > 0 {
+			// As of a recent OS change, BackupRead now returns a data stream for empty sparse files.
+			// These files have no sparse block streams, so skip the copySparse call if file size = 0.
+			if err = copySparse(t, br); err != nil {
+				return fmt.Errorf("%s: copying contents from sparse block stream: %s", name, err)
 			}
 		}
 	}
@@ -279,7 +307,7 @@ func WriteTarFileFromBackupStream(t *tar.Writer, r io.Reader, name string, size
 			} else {
 				// Unsupported for now, since the size of the alternate stream is not present
 				// in the backup stream until after the data has been read.
-				return errors.New("tar of sparse alternate data streams is unsupported")
+				return fmt.Errorf("%s: tar of sparse alternate data streams is unsupported", name)
 			}
 		case winio.BackupEaData, winio.BackupLink, winio.BackupPropertyData, winio.BackupObjectId, winio.BackupTxfsData:
 			// ignore these streams