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, we will list a few tips for using tools like this in the Nix context.

Commit selection

There are different ways to tweak git bisect's commit selection to reduce the required builds:

git bisect start flag --first-parent: select merge commits, which depending on the repository can help for caching as well as commit stability.
~~hydrasect: select cached commits cached by Hydra~~ - broken
nixpkgs-staging-bisecter (nix-build only): reduce number of derivations to be built.

Automating bisects

git bisect run: runs the selected command until the culprit is found.
nix-bisect: helps better judge outcome (git bisect's good vs bad skip) and gives nicer outputs than git bisect run.

Flakes

Bisecting changes in a flake itself can just follow the regular bisecting process. Bisecting regressions in flakes' inputs is a bit different in the sense the repositories you are building is not the repository you are bisecting. In this case you can use:

--override-input: nix flag that enables bisecting flakes' inputs
--flake flag on nix operations: allows overriding flake directory so you can use nix flake operations from e.g. git bisect run