core: check files are not touched by more than one package

core: check files are not touched by more than one package

Currently, we do nothing about packages that touch the same file: given
a specific configuration, the result is reproducible (even though it
might not be what the user expected) because the build order is
guaranteed.

However, when we later introduce top-level parallel build, we will no
longer be able to guarantee a build order, by the mere way of it being
parallel. Reconciliating all those modified files will be impossible to
do automatically. The only way will be to refuse such situations.

As a preliminary step, introduce a helper script that detects files that
are being moified by two or more packages, and reports them and the
impacted packages, at the end of the build.

The list being reported at the end of the build will make it prominently
visible in autobuilder results, so we can assess the problem, if any.

Later on, calling that helper script can be done right after the package
installation step, to bail out early.

Thanks Arnout for the pythonist way to write default dictionaries! ;-)

Note: doing it in python rather than a shell script is impressively
faster: where the shell script takes ~1.2s on a minimalist build, the
python script only takes ~0.015s, that is about 80 times faster.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Peter Seiderer <ps.report@gmx.net>
[Thomas: rename script without .py extension.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>