Re: Stateful system directories

["Ludovic Courtès" <ludo@gnu.org>]

Howdy!

Efraim Flashner <efraim@flashner.co.il> skribis:

> On Sat, Oct 19, 2019 at 11:08:57PM +0200, Ludovic Courtès wrote:
>> Hello Efraim,
>> 
>> Efraim Flashner <efraim@flashner.co.il> skribis:
>> 
>> > Ignoring the directories in users' home directories, /var/lib/gdm has
>> > been a source of pain on GNOME upgrades, and we still have some problems
>> > with /var/cache/fontconfig and I believe there is something else with
>> > permissions if you switch between ntp and openntpd. I actually have the
>> > following snippet in my OS-config:
>> >
>> > ;; This directory shouldn't exist
>> > (file-system
>> >   (device "none")
>> >   (mount-point "/var/cache/fontconfig")
>> >   (type "tmpfs")
>> >   (flags '(read-only))
>> >   (check? #f))
>> 
>> I think that would work, or we could even make it a writable tmpfs?
>
> I got angry with it and wanted to see if I could generate any error
> messages. :) So far nothing. Of course there isn't a compelling reason
> to really make it read-only if we recreate it each time, and it should
> cut down on bugs for other directories.

Yup, let’s do that.

>> (Somehow, I do have /var/cache/fontconfig, but never hard any problems
>> with it.  It hasn’t been written to in months, and it’s only writable by
>> root anyway.  Does that mean that people run into problem when they run
>> GUIs as root?)
>
> I have it too, not sure from what. I'm guessing some of the packages
> which have fontconfig as an input get a dbus-something to create the
> directory if it's missing.

Heh, these dbus things doing stuff behind our back.  :-)

>> > While we work on fixing these does it make sense to modify some of these
>> > services to unconditionally recreate their home directories on
>> > boot/activation?
>> 
>> Like /var/lib/gdm?  Maybe.  Or maybe ‘gdm-service-type’ could extend
>> ‘file-system-service-type’ with a tmpfs for /var/lib/gdm?
>> 
>
> Sounds like a good idea. Would that also cause the directory to be
> removed if gdm is removed? It should create a tmpfs and mount it over an
> existing /var/lib/gdm, right?

Yes.  So the directory won’t be removed if gdm is removed, but that’s
fine, it’ll just be an empty directory sitting there.

>> I suppose that might increase startup time a bit since it’d be
>> rebuilding its cache every time.  Perhaps we’d also lose bits of state,
>> no?
>
> The increase in startup time should be negligible, and according to
> rekado, who seems to run into GDM issues the most, removing /var/lib/gdm
> is one of the first steps when upgrading gnome or debugging gdm issues.

Yeah, it’s a tradeoff, but we should try it on the bare metal to get a
feel.  There’s quite a bit of data in there that we’d be recreating at
each boot:

--8<---------------cut here---------------start------------->8---
$ sudo ls -l /var/lib/gdm/.cache
totalo 16
drwxr-xr-x  2 gdm gdm 4096 Sep 19 08:45 fontconfig
drwxr-xr-x  3 gdm gdm 4096 Apr 11  2019 ibus
drwx------  2 gdm gdm 4096 Apr  1  2019 libgweather
drwxr-xr-x 97 gdm gdm 4096 Sep 19 08:45 mesa_shader_cache
--8<---------------cut here---------------end--------------->8---

If you give it a spin, let us know how it goes!

Ludo’.