Key packages, what are they?

In order to run a (minimal) Debian system, there is a set of package that need to be available in a suite. Because we want ourselves and our users to be able to (re)build the software we ship in a given suite, build dependencies are needed in that suite too. Additionally, because we want to be able to install Debian, we also need the installer and its build-dependencies.

Given the above, there's a script (that runs as part of udd) that will calculate the current list of key packages. The script starts with all the binary packages of a given priority (standard, important and required) and a couple of hardcoded source packages and it recursively finds the corresponding source packages, the build-dependencies, the packages build by the source and the dependencies of those two cases until no new packages are found.

How the list of key packages is used

Obviously having a list alone is nice but doesn't bring us anywhere. So, the Release Team uses this list to influence the autoremoval calculation and autoremoval hinting of source packages with Release Critical (RC) bugs. The autoremovals are serving two purposes. One, the state of testing is improving from the point of number of RC bugs. But more importantly, it creates a stronger incentive to solve RC bugs (by the maintainer or maintainers of reverse dependencies). As explained above, autoremoval doesn't work for key packages as it would leave us with a suite that doesn't work (or doesn't meet one of our core values), hence, we use the list to filter key package out from the autoremoval process by default.