[Help-tar] --deterministic option?

[Jérémy Bobbio]

Hi!

We are working in Debian— and I know other free software projects
care— in providing our users with a way to reproduce bit-for-bit
identical binary packages from the source and build enviroment.
See <https://wiki.debian.org/ReproducibleBuilds/About> for some
rationale and further explainations.

In order to do this, we need to make our build processes as
deterministic as possible. As you can imagine, Tar is quite involved in
producing Debian packages. A straightforward call leads to multiple
issues:

 * Order of files in the archive will depend on the filesystem order.
 * User and group names are recorded. This can be seen as a privacy leak
   for the package builder.
 * Permissions are dependent on the builder umask.
 * Last modification times of members of files created during the build
   will be dependent on the build time.
 * Also, if gzip compression is used, a timestamp will be recorded in
   gzip header.

So, we are currently turning calls like:

    tar -zcf archive.tar.gz src

into:

    find src -print0 | LC_ALL=C sort -z |
        GZIP=-9n tar --null -T - --no-recursion \
                --owner=root --group=root --numeric-owner \
                --mode=go=rX,u+rw,a-s \
                --mtime=debian/changelog \
                -zcf archive.tar

It would be great to avoid at least some of the boilerplate. Finding a
generic solution for permissions and modification times might be too
much, but having a `--deterministic` flag for the rest of the issues
would be quite helpful already.

What do you think?

-- 
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