Summary and Status
Implement the already-specified Breaks package dependency field in dpkg and higher layers.
It is very likely that many problems with installation are traceable to the ubiquitous use of Conflicts where Breaks would be better - eg, deinstallations of ubuntu-desktop - but most of these consequences probably go unreported. Even with the new distro upgrader life will be easier without these. Furthermore, use of Breaks instead would mean less deinstallation and reinstallation of packages and so might reduce or eliminate the impact of other bugs.
Breaks will replace almost every use of Conflicts << without Replaces <<. This will reduce the amount of churn needed during upgrades, as packages will no longer need to be removed. Installation ordering will be less criticial. This will make upgrades less fragile. In particular, it should eliminate most semi-accidental removals implied by the dependency system, and switching routine use to the less-dangerous Breaks will reduce the total amount of damage done by incorrect dependencies. It may also speed up upgrades.
Breezy i386 contains 909 instances of Conflicts << in main; 1921 overall. There is scope for considerable improvement here - Breaks would get rid of nearly all of these.
dpkg and apt and other programs which read/write status files need to be taught about Breaks so as not to mind it.
dpkg needs to be taught how to apply Breaks (usually, deconfigure packages if necessary).
apt does not need to consider Breaks although it might be useful to try to minimise the aforementioned deconfiguration.
Breaks: http://www.dpkg.org/Breaks (currently down unfortunately) http://lists.debian.org/debian-devel/1997/10/msg00643.html
- dpkg proper will have to be taught about Breaks; this involves new code in many of the dependency-handling situations.
- other code in dpkg will need updating a bit too (eg, dpkg-source et al)
- apt should ideally be taught about Breaks; this involves new code in its dependency handling too
- Various other packages will need minor edits.
- If we switch to smartpm we should educate that too, at least as far as we educate apt.
Compatibility and upgrade process
Q and A
Q. Is it really better to allow programs to break than to force stuff to be removed ?
A. The programs will be deconfigured (which is the proper way to avoid bad consequences from them not being properly installed). It is actually wrong to remove them because we don't want them to be removed; really we just want them upgraded (in most of these cases).