nixos-generate-config: add --flake option (#383033)

nixos/modules/installer/tools/manpages/nixos-generate-config.8

@@ -12,6 +12,7 @@
 .Op Fl -force
 .Op Fl -root Ar root
 .Op Fl -dir Ar dir
+.Op Fl -flake
 .
 .
 .
@@ -68,7 +69,14 @@ instead of
 .It Fl -force
 Overwrite
 .Pa /etc/nixos/configuration.nix
-if it already exists.
+(and
+.Pa /etc/nixos/flake.nix
+if --flake is passed) if already present.
+.
+.It Fl -flake
+Also generate
+.Pa /etc/nixos/flake.nix Ns
+\&.
 .
 .It Fl -no-filesystems
 Omit everything concerning file systems and swap devices from the hardware configuration.

nixos/modules/installer/tools/nixos-generate-config.pl

@@ -35,6 +35,7 @@ my $outDir = "/etc/nixos";
 my $rootDir = ""; # = /
 my $force = 0;
 my $noFilesystems = 0;
+my $flake = 0;
 my $showHardwareConfig = 0;
 
 for (my $n = 0; $n < scalar @ARGV; $n++) {
@@ -64,6 +65,9 @@ for (my $n = 0; $n < scalar @ARGV; $n++) {
     elsif ($arg eq "--show-hardware-config") {
         $showHardwareConfig = 1;
     }
+    elsif ($arg eq "--flake") {
+        $flake = 1;
+    }
     else {
         die "$0: unrecognized argument ‘$arg’\n";
     }
@@ -661,6 +665,19 @@ if ($showHardwareConfig) {
     mkpath($outDir, 0, 0755);
     write_file($fn, $hwConfig);
 
+    $fn = "$outDir/flake.nix";
+    if ($flake) {
+        if ($force || ! -e $fn) {
+            print STDERR "writing $fn...\n";
+            mkpath($outDir, 0, 0755);
+            write_file($fn, <<EOF);
+            @flake@
+EOF
+        } else {
+            print STDERR "warning: not overwriting existing $fn\n";
+        }
+    }
+
     # Generate a basic configuration.nix, unless one already exists.
     $fn = "$outDir/configuration.nix";
     if ($force || ! -e $fn) {

nixos/modules/installer/tools/tools.nix

@@ -1,7 +1,7 @@
 # This module generates nixos-install, nixos-rebuild,
 # nixos-generate-config, etc.
 
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, options, ... }:
 
 let
   makeProg = args: pkgs.replaceVarsWith (args // {
@@ -23,7 +23,7 @@ let
       hostPlatformSystem = pkgs.stdenv.hostPlatform.system;
       detectvirt = "${config.systemd.package}/bin/systemd-detect-virt";
       btrfs = "${pkgs.btrfs-progs}/bin/btrfs";
-      inherit (config.system.nixos-generate-config) configuration desktopConfiguration;
+      inherit (config.system.nixos-generate-config) configuration desktopConfiguration flake;
       xserverEnabled = config.services.xserver.enable;
     };
     manPage = ./manpages/nixos-generate-config.8;
@@ -55,6 +55,24 @@ let
     withReexec = true;
   };
 
+  defaultFlakeTemplate = ''
+    {
+      inputs = {
+        # This is pointing to an unstable release.
+        # If you prefer a stable release instead, you can this to the latest number shown here: https://nixos.org/download
+        # i.e. nixos-24.11
+        # Use `nix flake update` to update the flake to the latest revision of the chosen release channel.
+        nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+      };
+      outputs = inputs\@{ self, nixpkgs, ... }: {
+        # NOTE: '${options.networking.hostName.default}' is the default hostname
+        nixosConfigurations.${options.networking.hostName.default} = nixpkgs.lib.nixosSystem {
+          modules = [ ./configuration.nix ];
+        };
+      };
+    }
+  '';
+
   defaultConfigTemplate = ''
     # Edit this configuration file to define what should be installed on
     # your system. Help is available in the configuration.nix(5) man page, on
@@ -176,6 +194,24 @@ let
 in
 {
   options.system.nixos-generate-config = {
+
+    flake = lib.mkOption {
+      internal = true;
+      type = lib.types.str;
+      default = defaultFlakeTemplate;
+      description = ''
+        The NixOS module that `nixos-generate-config`
+        saves to `/etc/nixos/flake.nix` if --flake is set.
+
+        This is an internal option. No backward compatibility is guaranteed.
+        Use at your own risk!
+
+        Note that this string gets spliced into a Perl script. The perl
+        variable `$bootLoaderConfig` can be used to
+        splice in the boot loader configuration.
+      '';
+    };
+
     configuration = lib.mkOption {
       internal = true;
       type = lib.types.str;
@@ -196,7 +232,7 @@ in
     desktopConfiguration = lib.mkOption {
       internal = true;
       type = lib.types.listOf lib.types.lines;
-      default = [];
+      default = [ ];
       description = ''
         Text to preseed the desktop configuration that `nixos-generate-config`
         saves to `/etc/nixos/configuration.nix`.