[Pkg-pascal-devel] fpc and reproducible builds

[Abou Al Montacir]

On Mon, 2015-11-02 at 10:45 +0100, Paul Gevers wrote:
> Hi all,
> 
> You may have noticed that in the last couple of upload of our packages,
> I have been working on getting our tools to build reproducible¹.
That is a great news. Reproducible builds is a so nice feature and we need to
collaborate to that goal.
I assume FPC core team, copied here, may be interested in this topic and could
give advices on that especially on the PPU date discussed topic below.
> I have nearly nailed all the issues that I think should be fixed within
> our own tools (e.g. Latex introducing timestamps should be fixed in
> Latex). However, for the last one I need your help and/or advice.
> 

This will get fixed sooner or later as all packages will need to honor this
goal, isn't it?
> Yesterday, I created the wiki page² about reproducible issues with ppu
> files. The issue is that fpc stores the date of the source file of a ppu
> into the ppu file. Looking at it, however, I am unsure if this timestamp
> is actually really used. You don't need the source to use the ppu,
> rigth? Does anybody know this? Abou? Otherwise, if somebody (Michalis
> maybe) could test if updating the timestamp of the source of an ppu
> (e.g. by using touch) after the ppu is build but before it is used
> causes any issues?
> 

In the old days where the computers where too slow, Borland introduced the time
stamp as a way to avoid recompiling. This was a feature borrowed from make
utility that was considered too complex for common programmers (and is actually
if you look in industrial world).
Later, FPC introduced SHA computation as a more accurate way to check for
modification, especially for the case where only implementation is changed but
not the interface. This avoids recompiling all units depending on the changed
one as only the link needs to be done again.
Now, is that date still used or not, I don't know. That is why I copy here the
FPC core team, in case someone can comment on that.
> If the date in the ppu is not critical, I am going to propose a patch to
> upstream to honor the environment variable SOURCE_DATE_EPOCH³. I will
> include an improved patch for the documentation issues then as well, so
> that we don't need to carry Debian specific patches for long.
> 

That sound to me the right way, but would prefer hear a comment from FPC core
team to avoid divergence in the future.
> Thanks for your help.
> Paul
> P.s. I am working on getting ppudump functionality into diffoscope, the
> tool used by the reproducibility project to view differences.
> 

I was not aware of such tool, will have a look at it.
> ¹
> 
https://reproducible.debian.net/unstable/index_dd-list.html#pkg-pascal-devel@lists.alioth.debian.org
> 
> ² https://wiki.debian.org/ReproducibleBuilds/TimestampsInPPUGeneratedByFPC
> 
> ³ https://reproducible-builds.org/specs/source-date-epoch/
> 
> 
> 

> 
-- 
Cheers,
Abou Al Montacir
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