The main issue is the handling of security updates within the Nixpkgs ecosystem, which relies on Nix’s CI system, Hydra, to test and build packages. Due to the extensive number of packages in the Nixpkgs repository, the process can be slow, causing delays in the release of updates. As an example, the updated xz 5.4.6 package took nearly 5 days to become available in the unstable branch!
Fundamentally, there needs to be a change in how security fixes are handled in Hydra. As stated in the article, Nix was lucky to be unaffected, but multiple days to push out a security patch of this severity is concerning, even if there was no reason for concern.
Nix & Hydra’s scheduling is super basic. There is room to optimize the builds in many ways. In this case, the fact that xz is in libarchive as well as in input for Nix makes the rebuilds particularly bad.
xz is necessarily in the stdenv. Patching it means rebuilding the world, no matter what you optimise.