Hi Andreas,

Thanks for reviewing! 

Summary: among all points you raised, the least-negative answer/option seems to be, indeed, this is another case of changes to nova.conf (i.e., there are precedents; 6 versions of the file in the 21 package versions since focal-release).

...

Details:

> For non-juju installations of nova-common (which ships /etc/nova/nova.conf), if there was a local change in nova.conf, this update will trigger a dpkg conf prompt:

Nice catch; thanks for the careful review, as always!

> Usually this is not desirable, specially if a new default value won't change. [snip]

Agreed, it's not desirable. 

> [snip] Is there another way to fix this? 

- If 'this' is nova.conf, probably not in an elegant way, as it's automatically generated at build-time (so it's likely a hack in `debian/rules`).

- Now, if 'this' is the technical approach, probably not, because an opt-in is probably the best compromise between stability (no behavior change/less regression risk) and functionality (fix), in this case.

> [snip] Is there a precedent for such a change in the history of updates of this stack of packages? [snip]

There _are_ 5 precedents of nova.conf changes since the version in focal-release (total of 6 unique md5sums; see shell commands below).

> [snip] Or is it very unlikely that nova.conf will have been changed locally?

It's very _likely_ that nova.conf will have been changed.
The nova docs for installation [1, 2] change nova.conf immediately after package install:

  1. Install the packages:
  2. Edit the /etc/nova/nova.conf file and complete the following actions:

Thanks again,
Mauricio

...

$ curl -s 'https://launchpad.net/ubuntu/focal/+source/nova' \
  | grep -o '/ubuntu/+source/nova/[^"]\+' | sort -Vu \
  | sed -n '/2:21.0.0~b3~git2020041013.57ff308d6d-0ubuntu2/,$ p' \
  | cut -d/ -f5- | cut -d: -f2- \
  | while read VERSION; do \
      wget "https://launchpad.net/ubuntu/+archive/primary/+files/nova-common_${VERSION}_all.deb"; \
    done

$ for deb in *.deb; do dir=${deb%_*}; dpkg-deb -x $deb $dir; done

$ md5sum nova-common_*/etc/nova/nova.conf | sort
0c12bc3f6253cb10b1e25fc43bec7edb  nova-common_21.0.0-0ubuntu0.20.04.1/etc/nova/nova.conf
0c12bc3f6253cb10b1e25fc43bec7edb  nova-common_21.0.0-0ubuntu0.20.04.2/etc/nova/nova.conf
0c12bc3f6253cb10b1e25fc43bec7edb  nova-common_21.0.0-0ubuntu0.20.04.3/etc/nova/nova.conf
34e79bb12168a44ce1bd8d824a309788  nova-common_21.1.0-0ubuntu1/etc/nova/nova.conf
34e79bb12168a44ce1bd8d824a309788  nova-common_21.1.0-0ubuntu2/etc/nova/nova.conf
34e79bb12168a44ce1bd8d824a309788  nova-common_21.1.1-0ubuntu1/etc/nova/nova.conf
34e79bb12168a44ce1bd8d824a309788  nova-common_21.1.1-0ubuntu2/etc/nova/nova.conf
5b99773742952b3633791b641a36aa5e  nova-common_21.2.2-0ubuntu1/etc/nova/nova.conf
5b99773742952b3633791b641a36aa5e  nova-common_21.2.3-0ubuntu1/etc/nova/nova.conf
5b99773742952b3633791b641a36aa5e  nova-common_21.2.4-0ubuntu1/etc/nova/nova.conf
5b99773742952b3633791b641a36aa5e  nova-common_21.2.4-0ubuntu2/etc/nova/nova.conf
79b5bb5ba349227ec756340b673b8e4c  nova-common_21.2.4-0ubuntu2.1/etc/nova/nova.conf
79b5bb5ba349227ec756340b673b8e4c  nova-common_21.2.4-0ubuntu2.2/etc/nova/nova.conf
79b5bb5ba349227ec756340b673b8e4c  nova-common_21.2.4-0ubuntu2.3/etc/nova/nova.conf
79b5bb5ba349227ec756340b673b8e4c  nova-common_21.2.4-0ubuntu2.4/etc/nova/nova.conf
79b5bb5ba349227ec756340b673b8e4c  nova-common_21.2.4-0ubuntu2.5/etc/nova/nova.conf
b89558162c52dcf9c4d3eda3f9764664  nova-common_21.0.0~b3~git2020041013.57ff308d6d-0ubuntu2/etc/nova/nova.conf
dd548356b4cc6bd3de2e3bd493fb8f2f  nova-common_21.1.2-0ubuntu1/etc/nova/nova.conf
dd548356b4cc6bd3de2e3bd493fb8f2f  nova-common_21.1.2-0ubuntu2/etc/nova/nova.conf
dd548356b4cc6bd3de2e3bd493fb8f2f  nova-common_21.2.0-0ubuntu1/etc/nova/nova.conf
dd548356b4cc6bd3de2e3bd493fb8f2f  nova-common_21.2.1-0ubuntu1/etc/nova/nova.conf

$ md5sum nova-common_*/etc/nova/nova.conf | sort | wc -l
21

$ md5sum nova-common_*/etc/nova/nova.conf | sort -k1,1 -u | wc -l
6

[1] https://docs.openstack.org/nova/ussuri/install/compute-install-ubuntu.html#install-and-configure-components
[2] https://docs.openstack.org/nova/ussuri/install/controller-install-ubuntu.html#install-and-configure-components