bug#35283: ISO images are not reproducible

["Ludovic Courtès" <ludo@gnu.org>]

Hi,

(Moving discussion to <https://issues.guix.info/issue/35283>, which is
specifically about ISO image reproducibility issues.)

"Thomas Schmitt" <scdbackup@gmx.net> skribis:

> Florian Pelz wrote:
>>  The content is different at the beginning of the ISO image
>> (maybe padding or timestamps in the file system)
>
> That's to expect if not environment SOURCE_DATE_EPOCH is set and exported.
>
> SOURCE_DATE_EPOCH belongs to the specs of reproducible-builds.org. It
> is supposed to be either undefined or to contain a decimal number which
> tells the seconds since january 1st 1970. If it contains a number, then
> it is used for all timestamps and as seed of pseudo-random numbers like
> MBR id or GPT UUIDs.
>
> If all files and directories have the same names and the same content,
> then xorriso runs with the same arguments and the same SOURCE_DATE_EPOCH
> value are supposed to create byte-identical result ISOs.

By mounting the ISO image, I found that some files didn’t have their
timestamp reset: some files in /var/guix (easily fixed), but more
importantly those added by GRUB in /boot and /System.

Files added by ‘grub-mkrescue’ are “out of our control” so we would need
to patch ‘grub-mkrescue’ to honor SOURCE_DATE_EPOCH, for example.

However, after rereading the Xorriso manual, it seemed to me that if we
set SOURCE_DATE_EPOCH and pass:

  -volume_date all_file_dates set_to_mtime

then all the files would have the mtime specified by SOURCE_DATE_EPOCH,
which would solve the problem.

I tried it, but that’s not what happened.  What am I missing, Thomas?

Thanks,
Ludo’.