For a bit of context,
I already had seen some tar: Ignoring unknown extended header keyword XXXXXXXXX'` in the past but I didn't recall it to be an issue at the time (everything was building fine in the end).
I found the issue while working on patchelf-pypi when tests started to fail when running locally on my laptop (most likely after a macOS update). This time around there were no tar warnings printed out and thus tracking down the root cause took a bit of time.

GNU tar was extracting some of the files produced by BSD tar as "sparse" (I don't know much about this):

/project/src/patchelf-upstream/tests/no-rpath-prebuild/GNUSparseFile.0/no-rpath-pie-powerpc

vs

/project/src/patchelf-upstream/tests/no-rpath-prebuild/no-rpath-pie-powerpc

Using --format pax does not change the outcome.
Using --format ustar allows to build/test patchelf-pypi but there are some : Pathname too long warnings printed out along the way.

I converted to draft because I need to run some more checks (the image is alpine in my case so: does it only impact musllinux & shall something be done in there ?).

So in fact it's not an issue between BSD tar & GNU tar that I'm seeing but with BusyBox tar used in musllinux images.
Using GNU tar inside musllinux images, I'm getting a successful build with BSD tar on host side with a bunch of tar: Ignoring unknown extended header keyword warnings printed out.

It seems there's also a --format gnutar that seems to work in all cases (but is that limited to certain versions of macOS) ? maybe that's the way to go here ?

I'll probably add GNU tar to musllinux images in any cases.

Gotcha, thanks for the info.

Using --format pax does not change the outcome.

Ah, my apologies, I was not clear, I meant to use the pax tool, which can compress/decompress tar archives (and I believe was created in response to differing tar implementations).

(Please take these comments only as ideas, I'm happy with the current change as-is.)

Oops, our replies overlapped there.

So in fact it's not an issue between BSD tar & GNU tar that I'm seeing but with BusyBox tar used in musllinux images. Using GNU tar inside musllinux images, I'm getting a successful build with BSD tar on host side with a bunch of tar: Ignoring unknown extended header keyword warnings printed out.

Interesting! I suppose we still don't know what that 'unknown extended header keyword' is, it might not be relevant/critical, though it's kinda a smell.

It seems there's also a --format gnutar that seems to work in all cases (but is that limited to certain versions of macOS) ? maybe that's the way to go here ?

I don't have that option on my machine, running macOS 13.2.1...

$ /usr/bin/tar --help       
tar(bsdtar): manipulate archive files
First option must be a mode specifier:
  -c Create  -r Add/Replace  -t List  -u Update  -x Extract
Common Options:
  -b #  Use # 512-byte records per I/O block
  -f <filename>  Location of archive
  -v    Verbose
  -w    Interactive
Create: tar -c [options] [<file> | <dir> | @<archive> | -C <dir> ]
  <file>, <dir>  add these items to archive
  -z, -j, -J, --lzma  Compress archive with gzip/bzip2/xz/lzma
  --format {ustar|pax|cpio|shar}  Select archive format
  --exclude <pattern>  Skip files that match pattern
  -C <dir>  Change to <dir> before processing remaining files
  @<archive>  Add entries from <archive> to output
List: tar -t [options] [<patterns>]
  <patterns>  If specified, list only entries that match
Extract: tar -x [options] [<patterns>]
  <patterns>  If specified, extract only entries that match
  -k    Keep (don't overwrite) existing files
  -m    Don't restore modification times
  -O    Write entries to stdout, don't restore to disk
  -p    Restore permissions (including ACLs, owner, file flags)
bsdtar 3.5.3 - libarchive 3.5.3 zlib/1.2.11 liblzma/5.0.5 bz2lib/1.0.8 

I don't have that option on my machine, running macOS 13.2.1...

It's not listed in --help, if you do man tar, it redirects you to man libarchive-formats

it might not be relevant/critical, though it's kinda a smell.

If/When GNU tar is added to musllinux, agreed. Otherwise, that still leave to debug issues on musllinux (where I didn't see those warnings).

With GNU tar added in musllinux images, I'm closing this PR.