Bisecting
Bisecting is a feature of version control systems such as Git and Mercurial to easily pinpoint regressions. Owing to their reproducibility, Nix and NixOS are well-suited to this. As a result, there are a few common use-cases for Nix.
nixpkgs
either:
- directly bisect, say from the root of the nixpkgs repository run
git bisect start --first-parent master nixos-24.05 && git bisect run nix build -f . lolcatif packagelolcatbroke sincenixos-24.05. This uses git bisect run to run the test until the culprit is found, while the--first-parentflag selects merge commits, which in nixpkgs should be cached in Hydra. - nix-bisect: in automated bisects handle picking
bisect bad/bisect skipand give nicer outputs - hydrasect: prioritize hydra-cached commits, nixpkgs only. After each checkout run
git checkout $(hydrasect-search | head -1). - nixpkgs-staging-bisecter: reduce number of derivations to be built,
nix-buildonly.
Flakes
Bisecting changes in a flake itself can just follow the regular bisecting process.
Bisecting regressions in flakes' inputs typically involves updating the dependency's rev and narHash in the flake.lock file. Using tools like nix flake prefetch, jq and sed, this process can be scripted.