Commits on Sep 23, 2021

1. 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>

   

   kevpar committed Sep 23, 2021
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