Comment 6 for bug 581886

The apt backend performs a safe upgrade by default and additionally tries to detect upgrades which would only require the installation of further packages but not the removal of those.

This methods makes it impossible to detect packages which are set on hold on the dpkg level, since apt doesn't provide information why an upgrade was skipped from the safe upgrade.

The aptcc backend doesn't perform a safe upgrade but a full upgrade to detect the updates. Since there isn't any simulate method for a system upgrade it is very dangerous of the aptcc backend to report upgrades as installable without having the possibilty to inform the user about the required removal of packages.

The best fix would be to have a simualte method for the upgrade_system method.