Merged revisions 88528 via svnmerge from

Merged revisions 88528 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/branches/py3k

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  r88528 | lars.gustaebel | 2011-02-23 12:42:22 +0100 (Wed, 23 Feb 2011) | 16 lines

  Issue #11224: Improved sparse file read support (r85916) introduced a
  regression in _FileInFile which is used in file-like objects returned
  by TarFile.extractfile(). The inefficient design of the
  _FileInFile.read() method causes various dramatic side-effects and
  errors:

    - The data segment of a file member is read completely into memory
      every(!) time a small block is accessed. This is not only slow
      but may cause unexpected MemoryErrors with very large files.
    - Reading members from compressed tar archives is even slower
      because of the excessive backwards seeking which is done when the
      same data segment is read over and over again.
    - As a backwards seek on a TarFile opened in stream mode is not
      possible, using extractfile() fails with a StreamError.
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