[Buildroot] [PATCH 1/2] package/systemd: set machine id file instalation as optional choice

[Thomas Petazzoni <thomas.petazzoni@bootlin.com>]

Hello,

On Sun, 17 Nov 2019 17:00:05 +0100
Arnout Vandecappelle <arnout@mind.be> wrote:

> > Could something like that be enough ? can we trust "remount RW" ?
> > maybe "remount RW" should be renamed "create a RW filesystem" and enable various
> > tweaks related to RO vs RW  
> 
>  As written above: no.
> 
>  The problem is: we're not a distro.

Agreed.

> We leave too much freedom for the user to
> integrate things in various ways to be able to make assumptions about what is
> the right way to do things. So, the only thing we can do is to give a decent
> out-of-the-box experience, and let the user figure out how to tweak things -
> possibly adding a config option for a common situation that is easily handled in
> a generic way. The other thing we can do is to provide documentation about the
> proper way to integrate things in different scenarios.
> 
>  I'm starting to agree that this option is maybe not that great.

But I would in fact not come to the same conclusion. Having this empty
machine-id file is useless and causes problems when the filesystem is
R/W. So for the sake of supporting the R/O case (for which we create
this empty machine-id file), we make the R/W experience less good.

So I'd say that the right approach is to not do too much integration by
precisely having the option proposed by Bartosz, with many the tweak
that it should default y if rootfs is really, and default disable
otherwise, but while still being an option that the user can tweak,
because as you rightfully explained, the RW/RO remount option is just a
clue, not a definitive answer on whether /etc is writable or not.

To me, having this option matches the Buildroot way: we are not a
distro, we don't enforce how the system should work, so we provide the
appropriate options, while making sure the option has the most sensible
default values.

Generally speaking, Buildroot kind of supports "out of the box" two use
cases:

 (1) The root filesystem is completely read-write.

 (2) The root filesystem is completely read-only, and all files that need
     to be written are stored in tmpfs, and therefore are volatile.

I.e, we do not have any explicit support for what is I guess a much
more common use case than (2):

 (3) The root filesystem is completely read-only, but there is another
     read-write partition somewhere that stores the information that can
     change but needs to be persistent (user configuration, etc.)

Since we don't have explicit support for (3), there is no way we can
properly support machine-id and ConditionFirstBoot in the case of (2),
because there's nowhere we can store /etc/machine-id.

So the best we can do is in the case of (2), default to creating an
empty /etc/machine-id, while giving the possibility for the user
implementing (3) in its own way, to disable the creation of the empty
/etc/machine-id.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com