Re: RFC: git-commit based mtime-reproducible tarballs

[Bruno Haible]

Paul Eggert wrote:
> some users want to "trust but verify" and a reproducible 
> tarball is easier to audit than a non-reproducible one, so for these 
> users it can be a win to omit the irrelevant data from the tarball.

Reproducibility can be implemented in different ways:
  - by omitting irrelevant data from the tarball,
  - by having a customized comparison program 'diff', such that
    "diff --ignore-irrelevant-metadata contents1 contents2"
    would ignore the irrelevant parts.

> when I do an 'ls 
> -l' of a source directory that I got from a distribution tarball, it's 
> useful to see the last time the contents of each source file was changed 
> upstream.

OK, now we're discussing different ways to make a tarball reproducible.
That's nice, because Simon's proposal was to make all timestamps equal,
and that puts me off.
In binutils-2.40.tar.bz2 all files are from 2023-01-14.
In android-studio-2021.3.1.17-linux.tar.gz all files are from 2010-01-01.
It gives me as a user no idea whether this tarball is 13 years old,
2 years old, or from yesterday.

I much prefer Paul's approach, since it still conveys meaningful
timestamps:

> For TZDB, where users have long wanted reproducibility, I use something 
> like this in a Makefile recipe for each source file $$file:
> 
>             time=`git log -1 --format='tformat:%ct' $$file` &&
>             touch -cmd @$$time $$file

That's good for the files that are under version control.

> 2. What about platform-independent files that are automatically created 
> from source files from the repository, and that are shipped in the 
> release tarball?

For these, you could unpack the tarball, see in which order the timestamps
are, and then assign artificial timestamps, in the same order but exactly
2 seconds apart. For example, if the tarball contains
under version control:
  hello.c         2023-01-14 13:28:14
  configure.ac    2023-01-01 14:03:07
and not under version control:
  configure       2023-01-15 04:09:10
  config.h.in     2023-01-15 04:05:19
then you would determine the
  max_timestamp_under_vc = max { 2023-01-14 13:28:14, 2023-01-01 14:03:07 }
                         = 2023-01-14 13:28:14
and then, since config.h.in is older than configure:
  touch -m (max_timestamp_under_vc + 2 seconds) config.h.in
  touch -m (max_timestamp_under_vc + 4 seconds) configure

You can do this without knowing the Makefile rules or scripts which created
config.h.in and configure.

The increment of 2 seconds is, of course, for VFAT file systems, which have
only 2 seconds of resolution for file modification times.

> GNUTARFLAGS uses delete=atime,delete=ctime so that atime and 
> ctime do not leak into the tarball and make it less reproducible

I agree, it's pointless to have atime and ctime in a tarball.

Bruno