Bug#1108193: apt: Ordering issue with libc6:i386 on amd64-m-a system breaks partial upgrade ("just apt and dpkg") from bookworm to trixie

[Aurelien Jarno]

Hi,

On 2025-07-12 15:03, Helmut Grohne wrote:
> Hello,
> 
> On Wed, Jun 25, 2025 at 03:13:23PM +0200, Helmut Grohne wrote:
> > I talked to Aurelien on IRC. This mail gives some context and summarizes
> > our discussion.
> 
> I worked out and tested a patch somewhat and I also talked to Paul and 
> Ivo. Summarizing results here.
> 
> > The other side is what I discussed with Aurelien and much of what
> > follows is due to him (thank you).  systemd needs to be restarted when
> > upgrading libc6, because it uses nss modules. Failing to do so results
> > in e.g. #993821. The way to restart systemd has changed over time. For
> > instance, restarting user managers and other daemons such as resolved
> > has become a thing. Due to these changes, the responsibility of
> > restarting was eventually transferred to systemd via means of a dpkg
> > trigger (see #1074607) and the Breaks ensures that systemd can handle
> > the trigger. The Breaks declaration shall ensure that systemd ends up
> > being restarted, but what also does is require that systemd.postinst is
> > run before libc6.postinst. We observe that we do not expect to bump the
> > Breaks version in forky as it is an artifact of transitioning to dpkg
> > triggers, so in later dist-upgrades the cycle at hand is expected to
> > disappear. If we are trying to relax it, we'd have to re-add code for
> > restarting systemd to libc6.postinst:
> > 
> >     if the installed systemd does not support triggers; then
> >         # Expect a bookworm systemd:
> > 	#  * If it were older, we'd be doing a skip upgrade.
> > 	#  * If it is newer (e.g. backports), it supports triggers.
> > 	restart systemd the bookworm way
> > 	# https://sources.debian.org/src/systemd/252.36-1~deb12u1/debian/systemd.postinst/#L77
> >     fi
> > 
> > I note that this is the code that Aurelien spent effort on to get
> > removed from libc6 and moved into glibc. He expressed that he does not
> > like this approach (for obvious reasons).
> 
> I have prepared patch for glibc that drops the Breaks and adds he init 
> script. Tests performed:
>  * https://debusine.debian.net/debian/developers/work-request/103866/
>  * Attempted to reproduce the problem with Breaks dropped, but I could
>    not reproduce it then. apt would simply upgrade libc6 earlier and not
>    do stupid things.
>  * Upgraded just libc6 in a VM (not container) and observed that systemd 
>    would properly be restarted when expected.
> 
> I hope that others agree with this level of testing striking a good 
> balance on effort and risk.
> 
> Talking to the release team, Ivo and Paul were not super happy with the 
> maintainer script changes due to the risk involved in added code. They 
> pointed out that the lack of that addition would result in a risk of 
> people doing a partial upgrade (i.e. libc6, but not systemd) would not 
> get their systemd restarted. According to them, the scenario is quite 
> unlikely in a release upgrade and that people should not be doing 
> partial upgrades. If systemd is upgraded, it restarts itself. If libc 
> happens to be upgraded after systemd has done this restart, the trigger 
> added in trixie still will work. The one case the Breaks help is when 
> you upgrade libc6 but not systemd. Therefore, they suggested merely 
> dropping the Breaks without adding to the maintainer script.
> 
> I am fine either way. Dropping the breaks is what makes package 
> installation no longer fail.
> 
> Aurelien, are you happy with doing an upload with or without maintainer 
> script changes?
> 
> Release team, can you vaguely pre-approve the attached patch with or 
> without maintainer script changes?

Thanks Helmut for working on that. The patch looks good to me, and I 
would definitely prefer to include the maintainer script changes, in 
order to avoid bugs and mails during the lifetime of trixie. I have 
looked at the patch, and it's essentially the code that was there up to 
2.38-13, limited to systemd restart and with #1074607 fixed, so the 
risks are quite low.

I plan to submit an unblock pre-approval with this change with an update 
to the latest upstream stable branch (only small changes to the test 
infrastructure and sparc32). Please shout quickly if you disagree.

Regards
Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien at aurel32.net                     http://aurel32.net
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