Bug#919915: grub2-common: fails to upgrade from 'stretch-backports' - trying to overwrite /etc/kernel/postinst.d/zz-update-grub from grub-cloud-amd64 0.0.4~bpo9+1

[Colin Watson]

Control: reassign -1 grub-cloud-amd64 0.4.4~bpo9+1
Control: affects -1 grub2-common

On Sun, Jan 20, 2019 at 06:03:36PM +0100, Andreas Beckmann wrote:
> during a test with piuparts I noticed your package fails to upgrade from
> 'stretch'.
> It installed fine in 'stretch', then the upgrade to 'buster' fails
> because it tries to overwrite other packages files without declaring a
> Breaks+Replaces relation.
> 
> See policy 7.6 at
> https://www.debian.org/doc/debian-policy/ch-relationships.html#overwriting-files-and-replacing-packages-replaces
> 
> >From the attached log (scroll to the bottom...):
> 
>   Unpacking grub2-common (2.02+dfsg1-10) over (2.02~beta3-5+deb9u1) ...
>   dpkg: error processing archive /tmp/apt-dpkg-install-lSZYhO/12-grub2-common_2.02+dfsg1-10_amd64.deb (--unpack):
>    trying to overwrite '/etc/kernel/postinst.d/zz-update-grub', which is also in package grub-cloud-amd64 0.0.4~bpo9+1
>   dpkg-deb: error: paste subprocess was killed by signal (Broken pipe)
> 
> Interestingly I could only trigger this error so far on a stretch->stetch-backports->buster
> upgrade path. Dependencies seem to be tight enough to prevent this within buster itself.

It isn't me who needs the policy lecture here - this is grub2-common's
configuration file, and the version of grub-cloud-amd64 in
stretch-backports seems to have decided that it's OK to overwrite it
unilaterally.  This is not OK.  Bastian, if you need the update-grub
snippets to be modified for some reason, then you need to coordinate
with us to get the change into grub2-common rather than just shipping
your own version.

I'm happy to add Breaks/Replaces, but first the version of
grub-cloud-amd64 in stretch-backports needs to be fixed to stop
hijacking grub2-common's conffiles.  After that has been done, I would
be happy to add Breaks/Replaces on grub-cloud-amd64 (<< fixed-version).

-- 
Colin Watson                                       [cjwatson at debian.org]