When opening a tarfile for writing and adding several
files, some files end up being a hardlink to a
previously added tar member instead of being a proper
file member.

I attach a demo that demonstrates the problem. It
basically does:

tarfile.open('tarfile-bug.tar', 'w')
tar.add('tarfile-bug-f1')
tar.add('tarfile-bug-f2')
tar.close()

in the resulting tar, "tarfile-bug-f2" is a hard link
to tarfile-bug-f1, although both entries should be
proper files.

It works when the tarfile is close()d and opened again
in append mode between the two add()s, but that slows
down the process dramatically and is certainly not the
intended way.

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This is a feature ;-)
tarfile.py records the inode and device number (st_ino,
st_dev) for each added file in a list (TarFile.inodes). When
a new file is added and its inode and device number is found
in this list, it will be added as a hardlink member,
otherwise as a regular file.
Because your test script adds and immediately removes each
file, both files are assigned the same inode number. If you
had another process creating a file in the meantime, the
problem would not occur, because it would take over the
inode number before the second file has the chance.

Your problem shows that the way tarfile.py handles hardlinks
is too sloppy. It must take the stat.st_nlink field into
account. I will create a fix for this.

As a workaround you have several options:
- Do not remove the files after adding them, but after the
TarFile is closed.
- Set TarFile.dereference to False before adding files, so
files with several links would always be added as regular
files (see the Documentation). Disadvantage: symbolic links
would be added as regular files as well.
- Tamper with the source code. Edit TarFile.gettarinfo().
Change the line that says "if inode in self.inodes and not
self.dereference:" to "if statres.st_nlink > 1 and inode in
self.inodes and not self.dereference:".
- Empy the TarFile.inodes list after each file. Ugh!

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I just submitted patch #1331635 which ought to fix your
problem. Thank you for your report.

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Martin, I have checked in Lars' patch.  If this does not fix
your problem, please re-open this bug report.

Checked in as:
 * Lib/tarfile.py 1.34 and 1.21.2.6
 * Lib/test/test_tarfile.py 1.20 and 1.16.2.2

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Thanks for the quick reply!

Unfortunately, not removing the files after adding them to
the tarfile is not really an option. I want to create a
really huge tar file and put compressed files into it. For
that purpose I create a temporary gzip file, put that into
the tarfile, and remove the temporary file again. First,
keeping track of all temp files would be cumbersome, and
second it could quickly lead to disk space exhaustion.

I'll try your patch now.

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Confirmed, works perfectly now. Thank you very much! Will
this also be fixed in a stable point release? Or just in 2.5?

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It will be fixed in 2.4.3 when released (that's the branch
tags below, ie the second RCS rev number after each file).