I have assembled a patch that adds some features from
my own development path of tarfile.py
(http://www.gustaebel.de/lars/tarfile/) and fixes
#1471427 which made some restructuring necessary. The
patch affects Lib/tarfile.py, Lib/test/test_tarfile.py
and Doc/lib/libtarfile.tex.

The changes the patch makes are as follows:

Sets the version to 0.8.0.

Support for base256 encoding of number fields (nti()
and itn()). Up to now this was hardcoded for the
filesize field to allow filesizes greater than 8 GB but
it is applicable to all number fields.

TarInfo.tobuf() has a boolean argument "posix" which
controls how number fields are written (base256 is
non-posix).

Both unsigned and signed (Sun and NeXT) checksums are
calculated. Header validation moves from TarFile.next()
to TarInfo.frombuf(). A header is valid only if its
checksum is okay, in the past the checksum was
calculated but ignored.

The TarFile.next() method was rearranged which makes
header processing clearer and more abstract and fixes
bug #1471427. However, this change breaks the interface
for subclassing in order to implement custom member
types but makes it much easier at the same time. The
mapping TYPE_METH was removed.

A new test ReadGNULongTest was added to test_tarfile.py
and testtar.tar was updated to be able to test the GNU
extensions LONGNAME and LONGLINK.

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Here is testtar.tar to replace Lib/test/testtar.tar.

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Thanks for the patch, applied as rev. 45954.

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(1)  Why change the exception style?

When raising an instance, the style guide (PEP-8, http://
www.python.org/dev/peps/pep-0008/) prefers to construct 
that instance; the older form is left over from String 
exceptions and will be removed in Python 3.

I could understand leaving them as they were, but if you're 
going to change them to make them consistent, why not use 
the current format?

(2)  Why get rid of the debug messages (such as the 
checksum check) entirely?  Guarding them in if self.debug, 
I would understand.

(3)  I wouldn't count on str(e) (where e is any ValueError 
instance) being as meaningful as the (current version's) 
ReadError("empty, unreadable or compressed file")

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Jim: I agree with your comments and have committed an
improved version.

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On (1): agreed.

On (2): There is still a debug message emitted for a bad
checksum: In TarInfo.frombuf() at the bottom a ValueError is
raised if they don't match and is passed on to
TarFile.next() where it is put out as a debug message using
the _dbg() method in the except clause. The debug message
where it is now (r46040) is senseless because the try-block
will be left when TarInfo.frombuf() fails .

On (3): You're right, I attached a patch that adds another
Exception HeaderError which is raised in TarInfo.frombuf()
instead of ValueError in case of a bad header. I hope that
is acceptable.

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I would like to emphasize on point (2) of my previous post,
which should be resolved before 2.5 comes out.

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Lars, I don't see the debug (_dbg) call.  Which version are
you using?  I didn't see r46040 in Lib/tafile.py for either
2.5 or 2.4.  Could you point me to file/line/version where
there's a problem (or just make a patch)?  Thanks.

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I attached a tar with two diffs:
only-tarfile.diff only removes the debug message.
complete-with-headererror.diff removes the debug message and
adds the HeaderError exception to TarInfo.frombuf()
mentioned below.

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Ah, I see.  I checked in the patch to remove the debug
message since it's dead code.  We are in feature freeze for
2.5, so I've updated the patch, attached it, set the group
to 2.6.  We can add the patch after 2.5 is out in a month or so.

Committed revision 50503.

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Could someone please backport this to 2.4 branch?

Currently, 2.4.4c0 is broken when you try to extract a file
that happens to have a "/" at index 99 in its name
(tarinfo.name ends with "/" in this case, but this is NOT
the end of the complete (LONG) filename). It gets
misinterpreted as directory in this case.

MoinMoin build process is broken on 2.4.4c0 due to this bug.

What's the status of this patch? Does it still have to be applied somewhere?

Yes, please apply complete-with-headererror.diff to the trunk.

Applied in rev. 53090.

Lars, could you also create a patch to test this?  Thanks!

Created a testcase (which revealed an inconsistency). Thanks for the tip! Attached is a patch against test_tarfile.py and tarfile.py.
File Added: tests-for-1484695.diff

Applied as rev. 53106.