nixos-rebuild: Support `system.nix` by default #333788

infinisil · 2024-08-10T22:40:20Z

Description of changes

(in addition to various minor improvements)

Previously, nixos-rebuild -A foo would build the default.nix in the current directory, while nixos-rebuild without -A would not use file-based building at all.

This difference compared to what nix-build would do is confusing. Furthermore, using default.nix is not great, because there's many of those files all around, and they're almost never for NixOS.

This PR fixes those two issues by:

Removing support for using default.nix as a default when using --attr, this is a breaking change, but support for this has not made it to a stable branch
Using the <nixos-system> entry in NIX_PATH as the default if it exists
Otherwise, use the system.nix file in the current directory or any of its parents, if it exists, as a default
Otherwise, use /etc/nixos/system.nix as a default

While system.nix could be conflated with the system Nixpkgs argument, there's no practical way for this to cause problems, because system is never specified in a file.

Having this finally allows using nixos-rebuild without any arguments and without any NIX_PATH entries, effectively a way to declaratively use NixOS without nix-channel!

I think it's important to get this for the next release, so that we don't bake in stability for the default.nix behavior.

Unfortunately I'm a bit low on time, but if @amozeo (who pioneered the file-based building with #320462) could take this PR to its conclusion I'd be very grateful!

Things done

Tests
Documentation

Add a 👍 reaction to pull requests you find important.

In bash it's convention for "" to represent false, and "1" true. The `buildingAttribute` variable didn't honor that before.

Otherwise we'd run into trouble when a filename contained e.g. a space

To more accurately represent what it does

Opens the file that would be built from

Previously, `nixos-rebuild -A foo` would build the `default.nix` in the current directory, while `nixos-rebuild` without `-A` would not use file-based building at all. This difference compared to what `nix-build` would do is confusing. Furthermore, using `default.nix` is not great, because there's many of those files all around, and they're almost never for NixOS. This commit fixes those two issues by: - Removing support for using `default.nix` as a default when using `--attr` - If it exists, using the `<nixos-system>` entry in NIX_PATH as the default - Otherwise, use the `system.nix` file in the current directory or any of its parents, if it exists, as a default - Otherwise, use `/etc/nixos/system.nix` as a default While `system.nix` could be conflated with the `system` Nixpkgs argument, there's no practical way for this to cause problems, because `system` is never specified in a file. Having this finally allows using `nixos-rebuild` without any arguments and without any `NIX_PATH` entries, effectively a way to declaratively use NixOS from just /etc/nixos!

amozeo · 2024-08-22T15:46:33Z

hey @infinisil, I wrote tests for configuration detection. I think you might be interested looking into it
it is available here: permalink, latest, permalink diff, latest diff

infinisil · 2024-08-25T01:29:25Z

@amozeo Neat idea, it seems to work well with system.nix! Were you thinking of merging that idea into this PR? I'd prefer having smaller separate PRs that only focus on fixing one problem :)

amozeo · 2024-08-25T01:32:34Z

@infinisil how I should split it then?

infinisil · 2024-08-25T01:40:00Z

Honestly #327740 seemed fine to me. I guess it would be best to have this PR first though. I'm unfortunately a bit low on time right now, so I'd appreciate if you could help out!

In particular I think this PR should get done before the release in ~2 months, because I don't think the current behavior regarding default.nix is great, and we'd be locking that into stable 😓

nyabinary · 2024-10-11T15:56:45Z

We should probably get this moving if we want to make it before next release right?

amozeo · 2024-10-12T00:56:16Z

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

+            # Hardcoded to /etc/nixos/system.nix
+            if [[ -f "/etc/nixos/system.nix" ]]; then
+                buildFile=/etc/nixos/system.nix
+            fi


I don't think this path should be hardcoded in nixos-rebuild. Setup with channels doesn't have /etc/nixos/configuration.nix hardcoded and uses nixos-config in NIX_PATH instead to find its configuration there. If the /etc/nixos/system.nix must be hardcoded, it should be in NixOS modules instead, similar to channel setup.

flake path /etc/nixos/flake.nix is hardcoded in nixos-rebuild, I don't think the issue that I brought up has any significance anymore

amozeo · 2024-10-12T07:59:03Z

@infinisil I have force-updated the branch on my fork that I linked before.
permalink, latest, permalink diff, latest diff.
I want to make sure before I do something. Are you sure that I should make 4 separate pull requests for each feature that this batch of commits brings?
These pull requests would be named:

nixos-{rebuild, install}: Support system.nix by default and improve configuration detection
lib/version-info-fixup: create
nixos-generate-config: Introduce templates
nixos-rebuild: Add test for configuration detection

nyabinary · 2024-10-15T00:07:58Z

@infinisil I have force-updated the branch on my fork that I linked before. permalink, latest, permalink diff, latest diff. I want to make sure before I do something. Are you sure that I should make 4 separate pull requests for each feature that this batch of commits brings? These pull requests would be named:
* nixos-{rebuild, install}: Support `system.nix` by default and improve configuration detection

* lib/version-info-fixup: create

* nixos-generate-config: Introduce templates

* nixos-rebuild: Add test for configuration detection

Doesn't hurt to open the PR right now as all of them together, and then you can split them later if required.

nyabinary · 2024-10-28T00:27:06Z

Bump :P 24.11 is rapidly approaching

emilazy · 2024-10-28T00:31:09Z

We’re in feature freeze, so this would need @RossComputerGuy’s sign‐off even if it wasn’t a draft.

Probably just removing the default.nix handling could be spun off into a separate PR.

RossComputerGuy · 2024-10-28T01:47:42Z

Yeah, let's kill the default.nix post release. This should be fine.

nyabinary · 2025-01-23T04:16:59Z

Yeah, let's kill the default.nix post release. This should be fine.

bump :P

justinas · 2025-03-09T16:55:53Z

My 2c as a big fan of "flake-less, channel-less" model: this is a great change.

Would we like to default to building /etc/nixos/system.nix if it exists, over building configuration.nix, similarly how it is done for flakes? This would pretty much non-breaking if the priority remains: build flake.nix if it exists, build system.nix if it exists, else fall back on configuration.nix + <nixpkgs/nixos>. This approach could still break for people who happen to be using /etc/nixos/configuration.nix, and have an unrelated file named /etc/nixos/system.nix. Should be fine if documented in breaking changes?

Are there any plans to make system.nix and the plumbing such as lib/eval-config.nix more of first-class citizens? Right now there's quite a big disconnect in how NixOS is built using flakes (explicit call to lib.nixosSystem - another variation of basically lib/eval-config.nix), and how it is usually built in a flake-less way (/etc/nixos/configuration.nix and <nixpkgs/nixos> inferred from the environment, these are sort of "implementation details" that users often remain ignorant of).

In other words, do we see a future where perhaps nixos-generate-config also generates a /etc/nixos/system.nix with something like:

let
  nixpkgs = <nixpkgs>; # keeping compat with the "channelful" model by default
in
import "${nixpkgs}/nixos/lib/eval-config.nix" {
  system = null;
  modules = [
    ./configuration.nix
  ];
}

and this becomes a supported/recommended approach?

Let me know if this issue is not a place to discuss this, I'll make a new one. I'm just curious and excited about any ways to demystify the NixOS build process and decouple it somewhat from <nixpkgs/nixos> / <nixos-config> assumptions / implementation details in nixos-rebuild.

amozeo · 2025-03-13T12:02:09Z

I created #389487 that addresses issues pointed out in this pull request.
For anyone subscribed to this pull request

infinisil added 5 commits August 10, 2024 23:18

nixos-rebuild: Invert internal buildingAttribute logic

8a00ba4

In bash it's convention for "" to represent false, and "1" true. The `buildingAttribute` variable didn't honor that before.

nixos-rebuild: Quote buildFile and attr variables

aab433a

Otherwise we'd run into trouble when a filename contained e.g. a space

nixos-rebuild: Rename buildingAttribute to buildingFile

e99f301

To more accurately represent what it does

nixos-rebuild: Make edit action with --file work

53bd0ea

Opens the file that would be built from

infinisil added the 1.severity: significant Novel ideas, large API changes, notable refactorings, issues with RFC potential, etc. label Aug 10, 2024

github-actions bot added the 6.topic: nixos Issues or PRs affecting NixOS modules, or package usability issues specific to NixOS label Aug 10, 2024

infinisil mentioned this pull request Aug 10, 2024

nixos-rebuild, nixos-install: add support for --file and --attr flags #320462

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ofborg bot added 10.rebuild-darwin: 1-10 This PR causes between 1 and 10 packages to rebuild on Darwin. 10.rebuild-linux: 1-10 This PR causes between 1 and 10 packages to rebuild on Linux. labels Aug 10, 2024

infinisil added this to the 24.11 milestone Aug 11, 2024

amozeo mentioned this pull request Aug 22, 2024

nixos-generate-config: introduce templates #327740

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amozeo reviewed Oct 12, 2024

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rnhmjoj mentioned this pull request Jan 12, 2025

nixos/nixpkgs: add option to specify Nixpkgs source path #373201

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amozeo mentioned this pull request Mar 13, 2025

nixos-{rebuild; install}: rework logic; use system.nix #389487

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nixos-rebuild: Support `system.nix` by default #333788

nixos-rebuild: Support `system.nix` by default #333788

infinisil commented Aug 10, 2024

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nixos-rebuild: Support system.nix by default #333788

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infinisil commented Aug 10, 2024

Description of changes

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