depot/third_party/nixpkgs/pkgs/by-name/ni/nixos-rebuild-ng

nixos-rebuild-ng

Work-in-Progress rewrite of nixos-rebuild.

Why the rewrite?

The current state of nixos-rebuild is dare: it is one of the most critical piece of code we have in NixOS, but it has tons of issues:

• The code is written in Bash, and while this by itself is not necessary bad, it means that it is difficult to do refactorings due to the lack of tooling for the language
• The code itself is a hacky mess. Changing even one line of code can cause issues that affects dozens of people
• Lack of proper testing (we do have some integration tests, but no unit tests and coverage is probably pitiful)
• The code predates some of the improvements nix had over the years, e.g.: it builds Flakes inside a temporary directory and read the resulting symlink since the code seems to predate --print-out-paths flag

Given all of those above, improvements in the nixos-rebuild are difficult to do. A full rewrite is probably the easier way to improve the situation since this can be done in a separate package that will not break anyone. So this is an attempt of the rewrite.

Why Python?

• It is the language of choice for many critical things inside nixpkgs, like the NixOSTest and systemd-boot-builder.py activation scripts
• It is a language with great tooling, e.g.: mypy for type checking, ruff for linting, pytest for unit testing
• It is a scripting language that fits well with the scope of this project
• Python's standard library is great and it means we will need a low number of external dependencies for this project. For example, nixos-rebuild currently depends in jq for JSON parsing, while Python has json in standard library

Do's and Don'ts

• Do: be as much of a drop-in replacement as possible
• Do: fix obvious bugs
• Do: improvements that are non-breaking
• Don't: change logic in breaking ways even if this would be an improvement

How to use

{ pkgs, ... }:
{
  environment.systemPackages = [ pkgs.nixos-rebuild-ng ];
}

And use nixos-rebuild-ng instead of nixos-rebuild.

Development

Run:

nix-build -A nixos-rebuild-ng.tests.ci

The command above will run the unit tests and linters, and also check if the code is formatted. However, sometimes is more convenient to run just a few tests to debug, in this case you can run:

nix-shell -A nixos-rebuild-ng.devShell

The command above should automatically put you inside src directory, and you can run:

# run program
python -m nixos_rebuild
# run tests
pytest
# check types
mypy .
# fix lint issues
ruff check --fix .
# format code
ruff format .

Current caveats

• For now we will install it in nixos-rebuild-ng path by default, to avoid conflicting with the current nixos-rebuild. This means you can keep both in your system at the same time, but it also means that a few things like bash completion are broken right now (since it looks at nixos-rebuild binary)
• Bugs in the profile manipulation can cause corruption of your profile that may be difficult to fix, so right now I only recommend using nixos-rebuild-ng if you are testing in a VM or in a filesystem with snapshots like btrfs or ZFS. Those bugs are unlikely to be unfixable but the errors can be difficult to understand. If you want to go anyway, nix-collect-garbage -d and nix store repair are your friends

TODO

• Remote host/builders (via SSH)
• Improve nix arguments handling (e.g.: nixFlags vs copyFlags in the old nixos-rebuild)
• _NIXOS_REBUILD_REEXEC
• Port nixos-rebuild.passthru.tests
• Change module system to allow easier opt-in, like system.switch.enableNg for switch-to-configuration-ng
• Improve documentation
• nixos-rebuild repl
• Generate tab completion via shtab
• Reduce build closure

TODON'T

• Reimplement systemd-run logic: will be moved to the new apply script
• Nix bootstrap: it is only used for non-Flake paths and it is basically useless nowadays. It was created at a time when Nix was changing frequently and there was a need to bootstrap a new version of Nix before evaluating the configuration (otherwise the new Nixpkgs version may have code that is only compatible with a newer version of Nix). Nixpkgs now has a policy to be compatible with Nix 2.3, and even if this is bumped as long we don't do drastic minimum version changes this should not be an issue. Also, the daemon itself always run with the previous version since even we can replace Nix in PATH (so Nix client), but we can't replace the daemon without switching to a new version.