sru-release should check that we can still upgrade to later versions with the SRU installed

[Julian Andres Klode]

Bug 1861756 caused broken systems because mesa in focal was older than mesa in releases, but old libgl1-glx-dri required an older library that was not co-installable with newer libraries.

What we need to do to avoid releasing upgrades that cause such breakage is to simulate an upgrade from the release with the SRU installed to all later releases it can upgrade to (e.g. +1, and next LTS for LTS).

This probably needs some work in ubuntu-release-upgrader too, to add a simulation mode. And then we can use that to simulate stuff somehow.

[Julian Andres Klode (juliank)]

Potentially needs to be done server-side in a cron job or something, and sru-release than needs to look at it.

[Iain Lane (laney)]

A cheaper (less good, but probably more practical to implement, not requiring release upgrader work, likely to be faster to run) way to flag this up to to the SRU team would be to check the versions are increasing for all upgrade paths *excluding devel-proposed*.

  n-updates (to be) <= max(n+1-updates,n+1-security) <= ... <= devel

This also implies that in the normal case the SRU bug needs to be fixed in devel *release*, which is probably good practice anyway. It should be possible to override this in the case of urgent SRUs or significant blockers in devel-proposed, after consultation between the SRU team person and the uploader.