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Re: supporting merged-/usr-via-aliased-dirs in dpkg

Hi Guillem,

On Fri, Nov 11, 2022 at 01:21:32PM +0100, Guillem Jover wrote:
> I'm doing a shallow reply over this, can expand further during the
> weekend probably if necessary.

Thank you for taking the time to reply.

> > https://lists.debian.org/20181223030614.GA8788@gaara.hadrons.org and
> > https://git.hadrons.org/cgit/debian/dpkg/dpkg.git/commit/?h=pu/query-map-pathnames&id=b3f56ff6f3eaed17f534d544a3b6f8cc952e49c6
> > are starting points towards solving the problems arising from aliasing.
> Not really, the first lists things that are *not* proper solutions, the
> branch includes a deadend which I discarded long time ago. Those two are at
> least workarounds, and are definitely not on the path to proper solutions.

I fear that the perfect is the enemy of the good here. I appreciate that
you look for proper solutions, but I think there is another aspect to
this that makes considering less than ideal solutions relevant.

Throughout this discussion, you've been helpfully agreeing that the goal
of having the actual files below /usr is something that can be done in
principle. The disagreement arises from the way to get there. Whether we
like it or not, we will have to deal with the consequences of the
aliasing approach for quite some time. If we could somehow perform the
actual move of files from / to /usr in binary packages, we'd mitigate
much (not all) of the practical downsides of this approach. However, we
currently cannot perform that move, because doing it with dpkg is
unsafe. So while a more complete solution would certainly be desirable,
a partial solution that allows us to perform these moves would get us a
long way in eliminating much of the practical issues.

So yeah, I do want you to consider this workaround, because I believe
that it is achievable in a more timely manner and mitigating the bad
effects is something we need sooner rather than later.

> > [dpkg-statoverride]
> It's probably missing because I run out of steam, and making the page
> more accurate has seemed pointless, TBH. But sure, go ahead.


> > In reading all of the above, I had the impression that you spent much thought
> > on explaining why merged-/usr-via-aliased-dirs is bad and how alternatives may
> > look like. Unfortunately, this is the implementation strategy that we're
> > heading for.
> I've also mentioned what an hypothetical solution might be founded on.
> On filesystem metadata tracking. But again I'm not even convinced this
> can either solve the issues in a non-interface-breaking way either. :/

Given what you wrote, I'm fairly convinced that there is no solution
that retains each and every interface assumption. And while building on
filesystem metadata tracking sounds really nice, it is not something
that we can rely on soon given that it entails database and package
format changes. As such, I believe that we need to consider partial
solutions to the worst of problems and also accept some form of
interface breakage.

> > Then we have this proof of concept patch by uau at https://0x0.st/oNFG.diff
> > (and an earlier versions https://0x0.st/-7ev.diff and
> > https://0x0.st/-7vq.diff). Evidently, this was discussed on IRC (presumably
> > #debian-dpkg) and categorized as "conceptually broken". While I identified the
> > lack of separation of policy and mechanism, you appear to take more issues with
> > this approach. As I looked through all of this, I failed to identify what other
> > issues you see. It sure is an irreversible operation on the dpkg database. Once
> > performed however, a number of the problems arising from the aliasing
> > disappear.
> This would break interfaces, as it introduces change at a distance (as
> packages can expect their paths to match what's shipped from what's on
> the db, as packages are internally coherent).

In a strict sense, I think we already broke interfaces (in allowing the
aliasing to proceed). We now look into mitigating the most pressing
issues. At the same time, this kind of breakage affects a minority of
packages. We're talking about less than 2000 packages in unstable. I do
see how retaining backwards compatibility is important. I don't think
we'll be able to do that here.

> Even not updating the db and remapping on the fly or outputting both
> pathnames would be a breaking change.

While that is true, I think it is a change we can live with whereas not
being able to move files from / to /usr and never being able to finish
this transition would be rather bad for the morale of Debian as a whole.

> Both of these approaches do not really solve the problem, they just shift
> it elsewhere.

I agree. However, they shift the problem sufficiently far away that we
can start moving files and thus allow finishing the transition. When I
say finish, I mean that packages stop shipping files in aliased
directories. Once this state is achieved, many problems loose practical

> Old package shipping stuff in both aliased directories would also
> still not be installable, even though their deps could be satisfied.

I agree that this problem remains unsolved. At the same time, the only
technical measure I've seen addressing this has been versioned Breaks
issued by usrmerge. Do you see any better solution to this problem? My
understanding is that this would remain unsolved even with a "proper

> > [dpkg --add-alias proposal]
> This is equivalent (although perhaps slightly better as instead of a config
> this is stored in the db so it can be used by commands that do not parse config
> files) to the deadend approach from the above branch. This is trying to encode
> filesystem knowledge that is supposed to be shipped in .deb into an option,
> that can get out of sync, and still does not cover the change at a distance
> issues.

I am aware of these limitations. Maybe I should have spelled them out
more explicitly in my previous mail. I still believe that this is a
viable way of addressing the worst of practical effects.

> u-a does not interact with the dpkg fsys database.

That statement is not as obvious as it seems initially. u-a has its own
directory inside the admindir. It would become a consumer of the
recorded aliases, so it would increase that interaction.

> See above, I think this is the wrong way to go.

Yes, I do see that you very much disagree with this route. What I fail
to see is a "right way". As much as I trust your expertise, any way that
starts with "roll back the aliasing" is something I would reject on
practicality grounds, but that seems to be your only notion of "right
way". We both agree that this transition has been improperly planned and
executed. However bad that may be, that's where we are. So the question
no longer is "How do we get from unmerged to merged in a proper way?".
The question unfortunately has become "How do we mitigate the worst of
issues resulting from aliasing?". You appear to be rejecting that change
of question.

With a all that being said, the projected --add-alias approach still
seems to be the best available trade-off in the solution space to me.
>From my point of view, the most important property is finding a way to
actually move files from / to /usr in a reasonably safe way. I think
this is important, because most of the aliasing problems are only
relevant to packages shipping files on aliased paths and moving files
thus reduces the amount of affected packages. If you want to make moves
safe while retaining the chosen aliasing approach, it becomes fairly
obvious that dpkg needs to know about such aliases in some way. That
train of thought appears to leave little room for alternatives, but
maybe I'm missing something important.

Thank you for considering


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