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doc: explain where the "pkgs" comes from in a module #107601

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merged 1 commit into from Mar 3, 2021
Merged

doc: explain where the "pkgs" comes from in a module #107601

merged 1 commit into from Mar 3, 2021

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laikq
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@laikq laikq commented Dec 25, 2020

Ahoj! While reading the NixOS manual, I noticed that I didn't really see where "pkgs" is explained. So, I added a link to the "Writing Modules" (which could also be titled "Understanding Modules") chapter, and explained that pkgs is usually taken from the nixpkgs entry of the NIX_PATH environment variable. For me, this was not at all obvious from reading the manual.

Things done
  • Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
  • Built on platform(s)
    • NixOS
    • macOS
    • other Linux distributions
  • Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
  • Tested execution of all binary files (usually in ./result/bin/)
  • Determined the impact on package closure size (by running nix path-info -S before and after)
  • Ensured that relevant documentation is up to date
  • Fits CONTRIBUTING.md.

@laikq laikq requested a review from nbp as a code owner December 25, 2020 22:31
@jtojnar
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jtojnar commented Dec 25, 2020 •
edited

To be precise, it comes from this module:

nixpkgs/nixos/modules/misc/nixpkgs.nix

Lines 228 to 230 in 8d6e8cc

_module.args = {
pkgs = finalPkgs;
};

when not overridden here:

nixpkgs/nixos/lib/eval-config.nix

Line 50 in 8d6e8cc

_module.args.pkgs = lib.mkIf (pkgs_ != null) (lib.mkForce pkgs_);

The former defaults to location relative to the module:

nixpkgs/nixos/modules/misc/nixpkgs.nix

Lines 58 to 60 in 8d6e8cc

defaultPkgs = import ../../.. {
inherit (cfg) config overlays localSystem crossSystem;
};

so it really depends on how the system is built by nixos-rebuild. It does indeed use <nixpkgs/nixos> by default:

nixpkgs/nixos/modules/installer/tools/nixos-rebuild.sh

Line 433 in 8d6e8cc

pathToConfig="$(nixBuild '<nixpkgs/nixos>' --no-out-link -A system "${extraBuildFlags[@]}")"

@laikq
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laikq commented Dec 25, 2020

Thank you for this explanation, I guess I can update my PR based on that.

It also seems like this could benefit from an overview section in the manual, something like "Anatomy of NixOS modules"

@laikq laikq marked this pull request as draft January 3, 2021 12:34
@tomberek
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Manual builds fine. PR needs a squash. @laikq, ready for merge?

@laikq laikq marked this pull request as ready for review March 3, 2021 10:08
@laikq
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laikq commented Mar 3, 2021

Yes! I wanted to prepare a more in-depth explanation based on what @jtojnar wrote, but realistically this won't happen soon. I think this change to the documentation (as it is now) is very small, but I guess it is also not very controversial and some users might benefit from it.

@laikq
doc: explain where pkgs comes from in writing-modules
ea45ec5 
The manual mentions how "[config and pkgs] are explained later". Added a link
to where they are explained, and a hint pointing to the NIX_PATH variable.
@tomberek tomberek merged commit 5c99d24 into NixOS:master Mar 3, 2021
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