[OE-core] [RFC] Handling configuration files

[David Vincent]

Following this thread on OE-Core
(http://lists.openembedded.org/pipermail/openembedded-core/2017-April/135812.html),
the subject of handling configuration files for a machine just got
back. It seems there already was a discussion around it but I couldn't
find it. I know that 2.3 has now hit 'feature-freeze' status but I try
to gather some ideas for maybe an implementation in 2.4.

Simply put, the problem is this:
I want to
  - Override a configuration file in a package
  - Make the configuration machine-specific
  - Upgrade the configuration during package upgrade without human interaction

For now, one way to do this is to append the recipe and to replace the
PACKAGE_ARCH with MACHINE_ARCH. The downside of this approach is that
a package is created for each machine on package repositories where
only the configuration is modified.

Another way is to modify the file in ROOTFS_POSTPROCESS_COMMAND. The
downside is that depending on the packager, it would either not
survive the package upgrade or not be upgradable.

The last way would be to use update-alternatives, but IMHO it does
feel weird. Another downside would be the need to modify all recipes
to mark the configurations as alternatives.

My idea was the following. Maybe we could rely on the CONFFILES
variable to create a -conf package (like -dbg/-dev/-staticdev). If
possible, this package only should be marked as MACHINE_ARCH. This
could happen behind the scenes in package.bbclass. The only problem I
see for now is how to identify that the configuration file has been
overridden in a machine-specific recipe (SRC_URI or other variable ?).
Do you think it is feasible or am I missing something ? I could
proceed to implement that kind of behavior but I would like to know
beforehand if there are other things I should take into account.

Thanks,
David