bug#26734: Snippets (even empty ones) of tar sources reset the timestamps of all files

[ludo@gnu.org (Ludovic Courtès)]

Hi!

Clément Lassieur <clement@lassieur.org> skribis:

> I think the code is in guix/packages.scm (patch-and-repack).  Here is my
> understanding of what happens: the presence of the snippet leads to 3
> things:
>
> 1. extraction of the archive,
> 2. modification of some files (because of patches or snippet),
> 3. compression of the archive.
>
> Step 3 sets all timestamps to zero so to avoid non-determinism in the
> archive.  We obviously don't want archives to depend on the time of
> their creation.  But I believe we should only reset the timestamps of
> the files we modified in step 2.  Unmodified files should stay
> unmodified.

I agree.

> What's the point?  Well, while working on the 0ad package, I realized
> that building with 32 cores would always fail with a snippet (even if it
> is empty) and always succeed without a snippet.  It took me a few hours
> to understand this, and I put a comment in the 0ad package.  Maybe it is
> a 0ad bug (some tricky race condition that depends of file timestamps),
> but I believe it is also a Guix bug: an empty snippet should not change
> in any way the binary output of a package.

Yeah I was bitten by the same problem recently.

> I tried to patch 'patch-and-repack', but it triggers a full
> rebuild...  WDYT?

Right, it’s expected to trigger a full rebuild, so this should be fixed
in ‘core-updates’.

I guess we’ll have to collect the timestamps of all non-symlink files¹
in step #1 and to reapply them with ‘set-file-time’ from (guix build
utils) after step #2.

Thoughts?  Would you like to do that?

Thanks,
Ludo’.

¹ Because Guile provides bindings for ‘utime’, which does not support
  setting timestamps on symlinks.