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 . lolcat if package lolcat broke since nixos-24.05. This uses git bisect run to run the test until the culprit is found, while the --first-parent flag selects merge commits, which in nixpkgs should be cached in Hydra.
• nix-bisect: smartly pick bisect bad/skip in automated bisects and give nicer outputs
• hydrasect: prioritize cached commits in nixpkgs bisects - unfortunately still lacks nix package/flake
  • after each checkout run git checkout $(hydrasect-search | head -1)
• nixpkgs-staging-bisecter: like hydrasect but minimize how many derivations you will build even in staging

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-prefetch-url, nix-hash, jq and sed, this process can be scripted.