Having spent some time digging into this (at the request of the SRU
team), I'll summarise my findings:

Firstly, the analysis is correct: py3clean is ultimately the issue;
packages that both use py3clean and have diversions are affected
(diversions are the only circumstances that produce localised output in
dpkg-query -L).

Secondly, the proposed fix upstream is also fine, though personally I'd
also like to see the stdout decode use errors='replace' (there's no good
reason to fail here in the event of dodgy UTF-8 in a translation, for
instance). I'll attach debdiffs for noble and oracular to illustrate my
intent.

Thirdly, the proposed workaround for affected packages: using Breaks on
python3-minimal with a version strictly less than the fixed version also
appears to be sufficient (in all the cases I've tested this causes
python3-minimal to be at least unpacked prior to the prerm script of the
affected package being executed).

So, if the fix for python3-minimal can be uploaded, it simply remains to
determine which packages are affected. This is where things get tricky.

Benjamin's efforts are much appreciated here, and will likely be
decisive in light of the following: determining which packages use
py3clean is relatively simple (pretty much anything that installs a
python module). Determining which packages have diversions turns out to
be extremely difficult.

It's not enough to detect if a package *itself* uses dpkg-divert. Take
the cloud-init case where this was first detected: the postinst calls
dpkg-divert but only to *remove* an old diversion. The diversion that
actually affects this package comes from usr-merge (because cloud-init
still has files under the unmerged paths for various reasons). In
essence, any package (foo) can cause a diversion in another package
(bar) without the affected package (bar) showing any sign of this in
either its source or its binary artefacts. Further, the diversion may or
may not affect the package (bar) as the source of the diversion (foo)
may be optional to install.

Hence, there are two approaches. The thorough, but likely impractical,
approach would be to apply the "Breaks" fix to all packages using
py3clean: i.e. all packages that install python modules. Then there's
(Benjamin's) empirical approach: attempt to install everything from the
former set and see what fails. As noted above, this cannot guarantee
correctness as we cannot be certain that all packages that may divert
files in our target package are installed, but in practice it's probably
(hopefully!) "good enough" given the rarity of diversions and avoids
updates to (presumably) several thousand packages.

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https://bugs.launchpad.net/bugs/2075337

Title:
  py3clean fails when using alternate character set

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