From 9f9723b179961e7235d8e808c4ee8eaf52e05086 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Thu, 17 May 2018 18:53:13 -0400
Subject: [PATCH] nixpkgs module: Fix defaulting of `localSystem` and `system`

Take two of #40708 (4fe289860888668956b7e79e24efeb101c2f51d1).

That PR attempted to bidirectionally default `config.nixpkgs.system` and
`config.nixpkgs.localSystem.system` to each be updated by the other. But
this is not possible with the way the module system works. Divergence in
certain cases in inevitable.

This PR is more conservative and just has `system` default `localSystem`
and `localSystem` make the final call as-is. This solves a number of
issues.

 - `localSystem` completely overrides `system`, just like with nixpkgs
 proper. There is no need to specify `localSystem.system` to clobber the
 old system.

 - `config.nixpkgs.localSystem` is exactly what is passed to nixpkgs. No
 spooky steps.

 - `config.nixpkgs.localSystem` is elaborated just as nixpkgs would so
 that all attributes are available, not just the ones the user
 specified.

The remaining issue is just that `config.nixpkgs.system` doesn't update
based on `config.nixpkgs.localSystem.system`. It should never be
referred to lest it is a bogus stale value because
`config.nixpkgs.localSystem` overwrites it.

Fixes #46320
---
 nixos/lib/eval-config.nix      |  6 +++++-
 nixos/modules/misc/nixpkgs.nix | 12 ++++++++----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/nixos/lib/eval-config.nix b/nixos/lib/eval-config.nix
index 97c79487df4..ef685949ae1 100644
--- a/nixos/lib/eval-config.nix
+++ b/nixos/lib/eval-config.nix
@@ -36,7 +36,11 @@ let
     _file = ./eval-config.nix;
     key = _file;
     config = {
-      nixpkgs.localSystem = lib.mkDefault { inherit system; };
+      # Explicit `nixpkgs.system` or `nixpkgs.localSystem` should override
+      # this.  Since the latter defaults to the former, the former should
+      # default to the argument. That way this new default could propagate all
+      # they way through, but has the last priority behind everything else.
+      nixpkgs.system = lib.mkDefault system;
       _module.args.pkgs = lib.mkIf (pkgs_ != null) (lib.mkForce pkgs_);
     };
   };
diff --git a/nixos/modules/misc/nixpkgs.nix b/nixos/modules/misc/nixpkgs.nix
index 8fbe218b232..7f9833e184a 100644
--- a/nixos/modules/misc/nixpkgs.nix
+++ b/nixos/modules/misc/nixpkgs.nix
@@ -62,12 +62,11 @@ in
     pkgs = mkOption {
       defaultText = literalExample
         ''import "''${nixos}/.." {
-            inherit (config.nixpkgs) config overlays localSystem crossSystem;
+            inherit (cfg) config overlays localSystem crossSystem;
           }
         '';
       default = import ../../.. {
-        localSystem = { inherit (cfg) system; } // cfg.localSystem;
-        inherit (cfg) config overlays crossSystem;
+        inherit (cfg) config overlays localSystem crossSystem;
       };
       type = pkgsType;
       example = literalExample ''import <nixpkgs> {}'';
@@ -140,8 +139,11 @@ in
 
     localSystem = mkOption {
       type = types.attrs; # TODO utilize lib.systems.parsedPlatform
-      default = { system = builtins.currentSystem; };
+      default = { inherit (cfg) system; };
       example = { system = "aarch64-linux"; config = "aarch64-unknown-linux-gnu"; };
+      # Make sure that the final value has all fields for sake of other modules
+      # referring to this. TODO make `lib.systems` itself use the module system.
+      apply = lib.systems.elaborate;
       defaultText = literalExample
         ''(import "''${nixos}/../lib").lib.systems.examples.aarch64-multiplatform'';
       description = ''
@@ -180,6 +182,7 @@ in
     system = mkOption {
       type = types.str;
       example = "i686-linux";
+      default = { system = builtins.currentSystem; };
       description = ''
         Specifies the Nix platform type on which NixOS should be built.
         It is better to specify <code>nixpkgs.localSystem</code> instead.
@@ -196,6 +199,7 @@ in
         </programlisting>
         See <code>nixpkgs.localSystem</code> for more information.
 
+        Ignored when <code>nixpkgs.localSystem</code> is set.
         Ignored when <code>nixpkgs.pkgs</code> is set.
       '';
     };