Re: [PATCH 2/2] t5004: avoid using tar for checking emptiness of archive

> Test 2 of t5004 checks if a supposedly empty tar archive really
> contains no files.  24676f02 (t5004: fix issue with empty archive test
> and bsdtar) removed our commit hash to make it work with bsdtar, but
> the test still fails on NetBSD and OpenBSD, which use their own tar
> that considers a tar file containing only NULs as broken.
>
> Here's what the different archivers do when asked to create a tar
> file without entries:
>
>         $ uname -v
>         NetBSD 6.0.1 (GENERIC)
>         $ gtar --version | head -1
>         tar (GNU tar) 1.26
>         $ bsdtar --version
>         bsdtar 2.8.4 - libarchive 2.8.4
>
>         $ : >zero.tar
>         $ perl -e 'print "\0" x 10240' >tenk.tar
>         $ sha1 zero.tar tenk.tar
>         SHA1 (zero.tar) = da39a3ee5e6b4b0d3255bfef95601890afd80709
>         SHA1 (tenk.tar) = 34e163be8e43c5631d8b92e9c43ab0bf0fa62b9c
>
>         $ : | tar cf - -T - | sha1
>         da39a3ee5e6b4b0d3255bfef95601890afd80709
>         $ : | gtar cf - -T - | sha1
>         34e163be8e43c5631d8b92e9c43ab0bf0fa62b9c
>         $ : | bsdtar cf - -T - | sha1
>         34e163be8e43c5631d8b92e9c43ab0bf0fa62b9c
>
> So NetBSD's native tar creates an empty file, while GNU tar and bsdtar
> both give us 10KB of NULs -- just like git archive with an empty tree.
> Now let's see how the archivers handle these two kinds of empty tar
> files:
>
>         $ tar tf zero.tar; echo $?
>         tar: Unexpected EOF on archive file
>         1
>         $ gtar tf zero.tar; echo $?
>         gtar: This does not look like a tar archive
>         gtar: Exiting with failure status due to previous errors
>         2
>         $ bsdtar tf zero.tar; echo $?
>         0
>
>         $ tar tf tenk.tar; echo $?
>         tar: Cannot identify format. Searching...
>         tar: End of archive volume 1 reached
>         tar: Sorry, unable to determine archive format.

>         $ gtar tf tenk.tar; echo $?
>         0
>         $ bsdtar tf tenk.tar; echo $?
>         0
>
> NetBSD's tar complains about both, bsdtar happily accepts any of them
> and GNU tar doesn't like zero-length archive files.  So the safest
> course of action is to stay with our block-of-NULs format which is
> compatible with GNU tar and bsdtar, as we can't make NetBSD's native
> tar happy anyway.
>
> We can simplify our test, however, by taking tar out of the picture.
> Instead of extracting the archive and checking for the non-presence of
> files, check if the file has a size of 10KB and contains only NULs.
> This makes t5004 pass on NetBSD and OpenBSD.
>
> Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>