dfvfs unit test fails with libfvde experimental-20180108

[ant1]

Hi,

With version experimental-20180108, 1 regression test from dfvfs fails:

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ERROR: testScanFVDE (helpers.source_scanner.SourceScannerTest)
Test the Scan function on FVDE.
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Traceback (most recent call last):
  File "/wrkdirs/dfvfs-20171230/tests/helpers/source_scanner.py", line 182, in testScanFVDE
    self._source_scanner.Scan(scan_context)
  File "./dfvfs/helpers/source_scanner.py", line 565, in Scan
    self._ScanNode(scan_context, scan_node, auto_recurse=auto_recurse)
  File "./dfvfs/helpers/source_scanner.py", line 440, in _ScanNode
    scan_context, sub_scan_node, auto_recurse=auto_recurse)
  File "./dfvfs/helpers/source_scanner.py", line 450, in _ScanNode
    file_object.close()
  File "./dfvfs/file_io/file_io.py", line 98, in close
    self._Close()
  File "./dfvfs/file_io/file_object_io.py", line 35, in _Close
    self._file_object.close()
IOError: pyfvde_volume_close: unable to close volume. libfvde_volume_close: invalid volume - missing file IO handle.

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[joachimmetz]

This looks more than a dormant bug in dfvfs than regression in libfvde. Since the pyfvde volume seems to be closed multiple times.

[ant1]

A workaround was added and the issue was not in libfvde itself.