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

[Simon Josefsson via Gnulib discussion list <bug-gnulib@gnu.org>]

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Bruno Haible <bruno@clisp.org> writes:

> 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.

The problem with a --ignore-irrelevant-metadata approach is that it will
be a judgement call what is irrelevant, and two projects may have
different philosophies that are mutually incompatible.

A devils advocate case: consider a build-system that embeds the
source-code timestamp information in the binary, and the binary sends of
a hash of its executable binary to a remote server for verification
purposes.  In some projects this may be what you want to achieve.  Then
ignoring this particular metadata will be a critical failure for that
project.

I think it is a worthy goal to reach a tarball that is deterministically
and one-way reproducable from git source code [for the same set of tool
versions].

>> 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:

I agree!

I even wonder if the binutils tarball build properly on say HP-UX then?

>> 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.

Clever!

To implement this we would need a dist-hook to do the 'touch -m ...'
dance on all files.

I somewhat fear that the solution here will be more of a problem than
the original problem due to the complexity.

Does anyone see a problem with this approach?  Do you think it is a good
idea?  I like it and don't see any further problems, except for the
complexity but I don't see a way to reduce it.

/Simon

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