Merge pull request #177012 from hercules-ci/nixos-modular-system

`flake.lib.nixosSystem`: Allow `nixpkgs.system` to be set modularly; improve error message

flake.nix

@@ -20,13 +20,20 @@
         nixos = import ./nixos/lib { lib = final; };
 
         nixosSystem = args:
-          import ./nixos/lib/eval-config.nix (args // {
-            modules = args.modules ++ [ {
-              system.nixos.versionSuffix =
-                ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
-              system.nixos.revision = final.mkIf (self ? rev) self.rev;
-            } ];
-          });
+          import ./nixos/lib/eval-config.nix (
+            args // {
+              modules = args.modules ++ [{
+                system.nixos.versionSuffix =
+                  ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
+                system.nixos.revision = final.mkIf (self ? rev) self.rev;
+              }];
+            } // lib.optionalAttrs (! args?system) {
+              # Allow system to be set modularly in nixpkgs.system.
+              # We set it to null, to remove the "legacy" entrypoint's
+              # non-hermetic default.
+              system = null;
+            }
+          );
       });
 
       checks.x86_64-linux.tarball = jobs.tarball;

nixos/lib/eval-config.nix

@@ -9,7 +9,9 @@
 # expressions are ever made modular at the top level) can just use
 # types.submodule instead of using eval-config.nix
 evalConfigArgs@
-{ # !!! system can be set modularly, would be nice to remove
+{ # !!! system can be set modularly, would be nice to remove,
+  #     however, removing or changing this default is too much
+  #     of a breaking change. To set it modularly, pass `null`.
   system ? builtins.currentSystem
 , # !!! is this argument needed any more? The pkgs argument can
   # be set modularly anyway.
@@ -48,7 +50,7 @@ let
       # 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;
+      nixpkgs.system = lib.mkIf (system != null) (lib.mkDefault system);
 
       _module.args.pkgs = lib.mkIf (pkgs_ != null) (lib.mkForce pkgs_);
     };

nixos/modules/misc/nixpkgs.nix

@@ -244,6 +244,14 @@ in
       defaultText = literalExpression
         ''(import "''${nixos}/../lib").lib.systems.examples.aarch64-multiplatform'';
       description = ''
+        Systems with a recently generated <literal>hardware-configuration.nix</literal>
+        do not need to specify this option, unless cross-compiling, in which case
+        you should set <emphasis>only</emphasis> <option>nixpkgs.buildPlatform</option>.
+
+        If this is somehow not feasible, you may fall back to removing the
+        <option>nixpkgs.hostPlatform</option> line from the generated config and
+        use the old options.
+
         Specifies the platform on which NixOS should be built. When
         <code>nixpkgs.crossSystem</code> is unset, it also specifies
         the platform <emphasis>for</emphasis> which NixOS should be
@@ -265,6 +273,10 @@ in
       default = null;
       example = { system = "aarch64-linux"; config = "aarch64-unknown-linux-gnu"; };
       description = ''
+        Systems with a recently generated <literal>hardware-configuration.nix</literal>
+        may instead specify <emphasis>only</emphasis> <option>nixpkgs.buildPlatform</option>,
+        or fall back to removing the <option>nixpkgs.hostPlatform</option> line from the generated config.
+
         Specifies the platform for which NixOS should be
         built. Specify this only if it is different from
         <code>nixpkgs.localSystem</code>, the platform
@@ -280,7 +292,29 @@ in
     system = mkOption {
       type = types.str;
       example = "i686-linux";
+      default =
+        if opt.hostPlatform.isDefined
+        then
+          throw ''
+            Neither ${opt.system} nor any other option in nixpkgs.* is meant
+            to be read by modules and configurations.
+            Use pkgs.stdenv.hostPlatform instead.
+          ''
+        else
+          throw ''
+            Neither ${opt.hostPlatform} nor or the legacy option ${opt.system} has been set.
+            You can set ${opt.hostPlatform} in hardware-configuration.nix by re-running
+            a recent version of nixos-generate-config.
+            The option ${opt.system} is still fully supported for NixOS 22.05 interoperability,
+            but will be deprecated in the future, so we recommend to set ${opt.hostPlatform}.
+          '';
+      defaultText = lib.literalMD ''
+        Traditionally `builtins.currentSystem`, but unset when invoking NixOS through `lib.nixosSystem`.
+      '';
       description = ''
+        This option does not need to be specified for NixOS configurations
+        with a recently generated <literal>hardware-configuration.nix</literal>.
+
         Specifies the Nix platform type on which NixOS should be built.
         It is better to specify <code>nixpkgs.localSystem</code> instead.
         <programlisting>

nixos/modules/services/misc/dysnomia.nix

@@ -186,7 +186,7 @@ in
 
     dysnomia.properties = {
       hostname = config.networking.hostName;
-      inherit (config.nixpkgs.localSystem) system;
+      inherit (pkgs.stdenv.hostPlatform) system;
 
       supportedTypes = [
         "echo"

nixos/modules/virtualisation/nixos-containers.nix

@@ -132,7 +132,7 @@ let
 
       # If the host is 64-bit and the container is 32-bit, add a
       # --personality flag.
-      ${optionalString (config.nixpkgs.localSystem.system == "x86_64-linux") ''
+      ${optionalString (pkgs.stdenv.hostPlatform.system == "x86_64-linux") ''
         if [ "$(< ''${SYSTEM_PATH:-/nix/var/nix/profiles/per-container/$INSTANCE/system}/system)" = i686-linux ]; then
           extraFlags+=" --personality=x86"
         fi