Hello Reinhard,

the issue is easy to reproduce: start a Debian:sid container or VM, and
install docker.io.

Output:

```
[...]
Preconfiguring packages ...
Configuring docker.io
---------------------

The /var/lib/docker directory contains Docker images, containers, and
volumes. If you choose this option, all this data will be permanently
removed when the docker.io package is purged.

If you are replacing docker.io with another Docker distribution (like
docker-ce), or if you want to keep your data, you should choose not to
remove it.

Remove all Docker data? [yes/no] yes

Selecting previously unselected package libcom-err2:amd64.
(Reading database ... 4918 files and directories currently installed.)
[...]
```

Since I'm installing the package on a clean system, there is no data,
so why does it ask me to remove data that doesn't exist?

I suppose the code could be gated to the upgrade case only (ie. if [ -z
"$2" ] in the postint).

This bug report is about installing docker.io in a clean environment,
but we can extend it to upgrading systems where docker.io is already
installed.

The postinst basically asks me, when the package is upgraded:

However it doesn't remove any data, it just sets a flag to remove or not
remove data _later_ when (if ever) the package is purged. I think the
wording is confusing.

Another thing that is confusing, from this message:

I am not replacing anything, I am just upgrading my system. But wait.

So I'm supposed to decide _now_ what to do with the data? How can I take
a decision for an event that didn't happen yet? Maybe next year, I will
remove the package, or maybe I will switch from docker.io to docker-ce.
So it's the answer that I gave 1 year ago that will decide what will
happen to my data? I think this approach is not going to work.

I've had a brief read at #1021914, and I read it as a bug report from a
user that configured APT to always purge data, and was then surprised to
see that APT purged its data... I think this APT config flag is
dangerous, and users shouldn't set it on their system. Maybe the APT
documentation should be more explicit that setting this flag is not a
great idea.

If ever docker's data is so important that it warrants an exception, and
should leave data behind even when it's purged, then maybe ask the
question about what to do _at the moment_, ie. when the package is
purged?

However packaging best practice say that "prompting in any case can only
be in the configure stage of the postinst script", cf.
<https://www.debian.org/doc/manuals/developers-reference/best-pkging-practices.en.html#best-practices-for-maintainer-scripts>

I don't have the right answer here, I'm not really familiar with
debconf. I'd be tempted to drop all the debconf thing here. Either purge
data, or don't purge data, no need for more granularity.

What does podman do in this case? What about incus? Maybe just align
with those packages.

Hope that helps.

Best,

Arnaud

Hello,

Bug #1140309 in docker.io reported by you has been fixed in the
Git repository and is awaiting an upload. You can see the commit
message below and you can check the diff of the fix at:

https://salsa.debian.org/go-team/packages/docker/-/commit/22f453f0e399a7093856b10ea60bdaa9d734d241
------------------------------------------------------------------------
Update changelog for debconf prompt clarification

Closes: #1140309, #1137581
------------------------------------------------------------------------

(this message was generated automatically)
-- 
Greetings

https://bugs.debian.org/1140309

We believe that the bug you reported is fixed in the latest version of
docker.io, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1140309@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Mathias Gibbens <gibmat@debian.org> (supplier of updated docker.io package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Tue, 11 Aug 2026 15:35:37 +0000
Source: docker.io
Architecture: source
Version: 28.5.2+dfsg4-4
Distribution: unstable
Urgency: medium
Maintainer: Debian Go Packaging Team <team+pkg-go@tracker.debian.org>
Changed-By: Mathias Gibbens <gibmat@debian.org>
Closes: 1137581 1140309
Changes:
 docker.io (28.5.2+dfsg4-4) unstable; urgency=medium
 .
   * Team upload
 .
   [ Mathias Gibbens ]
   * Include go.mod files in the -dev package
 .
   [ Reinhard Tartler ]
   * Clarify debconf prompt regarding Docker data purge vs removal.
     (Closes: #1140309, #1137581)
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