tar excludes files too late to stop hardlink detection

[Denys Vlasenko]

Applied, thank you.

On Sat, Jun 26, 2021 at 1:46 AM Harald van Dijk <harald at gigawatt.nl> wrote:
>
> Hi,
>
> tar --exclude results in bad archives when hardlinks are used. Consider
> the following:
>
>    $ mkdir tartest
>    $ echo hello > tartest/a
>    $ ln tartest/a tartest/b
>    $ busybox tar cf - tartest | tar tvf -
>    drwxr-xr-x harald/harald     0 2021-06-26 00:25 tartest/
>    -rw-r--r-- harald/harald     6 2021-06-26 00:25 tartest/b
>    hrw-r--r-- harald/harald     0 2021-06-26 00:25 tartest/a link to
> tartest/b
>
> This is okay. tar may either pick up a first and then detect b as a
> hardlink to a, or pick up b first and then detect a as a hardlink to b.
> On my system, it picks up b first. You can adjust the below accordingly
> if on your system a is picked up first. Now, exclude b:
>
>    $ busybox tar cf - --exclude=b tartest | tar tvf -
>    drwxr-xr-x harald/harald     0 2021-06-26 00:25 tartest/
>    hrw-r--r-- harald/harald     0 2021-06-26 00:25 tartest/a link to
> tartest/b
>
> This resulted in an archive where the contents of tartest/a are missing.
> Extracting the archive results in an attempt to hardlink tartest/b,
> which may or may not exist in the target directory. GNU tar does not do
> this, it stores the contents of the file instead, which seems like a
> better idea to me. Can busybox be modified to do that as well?
>
> Tested with busybox 1.33.1.
>
> Cheers,
> Harald van Dijk
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