nixos: add system.switch.enable flag

This flag allows the user to optionally exclude
switch-to-confguration.pl from toplevel.

This is interesting for appliance images where you don't want to re-build
the system. This flag is called `rebuildable` because the standard
interface to do this is `nixos-rebuild` which will not work anymore with
this change.

nixos/modules/module-list.nix

@@ -1392,6 +1392,7 @@
   ./system/activation/activatable-system.nix
   ./system/activation/activation-script.nix
   ./system/activation/specialisation.nix
+  ./system/activation/switchable-system.nix
   ./system/activation/bootspec.nix
   ./system/activation/top-level.nix
   ./system/boot/binfmt.nix

nixos/modules/system/activation/activatable-system.nix

@@ -1,52 +1,16 @@
-{ config, lib, pkgs, ... }:
+{ options, config, lib, pkgs, ... }:
 
 let
   inherit (lib)
     mkOption
-    optionalString
     types
     ;
 
-  perlWrapped = pkgs.perl.withPackages (p: with p; [ ConfigIniFiles FileSlurp ]);
-
   systemBuilderArgs = {
     activationScript = config.system.activationScripts.script;
     dryActivationScript = config.system.dryActivationScript;
   };
 
-  systemBuilderCommands = ''
-    echo "$activationScript" > $out/activate
-    echo "$dryActivationScript" > $out/dry-activate
-    substituteInPlace $out/activate --subst-var-by out ''${!toplevelVar}
-    substituteInPlace $out/dry-activate --subst-var-by out ''${!toplevelVar}
-    chmod u+x $out/activate $out/dry-activate
-    unset activationScript dryActivationScript
-
-    mkdir $out/bin
-    substitute ${./switch-to-configuration.pl} $out/bin/switch-to-configuration \
-      --subst-var out \
-      --subst-var-by toplevel ''${!toplevelVar} \
-      --subst-var-by coreutils "${pkgs.coreutils}" \
-      --subst-var-by distroId ${lib.escapeShellArg config.system.nixos.distroId} \
-      --subst-var-by installBootLoader ${lib.escapeShellArg config.system.build.installBootLoader} \
-      --subst-var-by localeArchive "${config.i18n.glibcLocales}/lib/locale/locale-archive" \
-      --subst-var-by perl "${perlWrapped}" \
-      --subst-var-by shell "${pkgs.bash}/bin/sh" \
-      --subst-var-by su "${pkgs.shadow.su}/bin/su" \
-      --subst-var-by systemd "${config.systemd.package}" \
-      --subst-var-by utillinux "${pkgs.util-linux}" \
-      ;
-
-    chmod +x $out/bin/switch-to-configuration
-    ${optionalString (pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) ''
-      if ! output=$(${perlWrapped}/bin/perl -c $out/bin/switch-to-configuration 2>&1); then
-        echo "switch-to-configuration syntax is not valid:"
-        echo "$output"
-        exit 1
-      fi
-    ''}
-  '';
-
 in
 {
   options = {
@@ -60,6 +24,18 @@ in
         do, but for image based systems, this may not be needed or not be desirable.
       '';
     };
+    system.activatableSystemBuilderCommands = options.system.systemBuilderCommands // {
+      description = lib.mdDoc ''
+        Like `system.systemBuilderCommands`, but only for the commands that are
+        needed *both* when the system is activatable and when it isn't.
+
+        Disclaimer: This option might go away in the future. It might be
+        superseded by separating switch-to-configuration into a separate script
+        which will make this option superfluous. See
+        https://github.com/NixOS/nixpkgs/pull/263462#discussion_r1373104845 for
+        a discussion.
+      '';
+    };
     system.build.separateActivationScript = mkOption {
       type = types.package;
       description = ''
@@ -71,7 +47,18 @@ in
     };
   };
   config = {
-    system.systemBuilderCommands = lib.mkIf config.system.activatable systemBuilderCommands;
+    system.activatableSystemBuilderCommands = ''
+      echo "$activationScript" > $out/activate
+      echo "$dryActivationScript" > $out/dry-activate
+      substituteInPlace $out/activate --subst-var-by out ''${!toplevelVar}
+      substituteInPlace $out/dry-activate --subst-var-by out ''${!toplevelVar}
+      chmod u+x $out/activate $out/dry-activate
+      unset activationScript dryActivationScript
+    '';
+
+    system.systemBuilderCommands = lib.mkIf
+      config.system.activatable
+      config.system.activatableSystemBuilderCommands;
     system.systemBuilderArgs = lib.mkIf config.system.activatable
       (systemBuilderArgs // {
         toplevelVar = "out";
@@ -86,7 +73,7 @@ in
         })
         ''
           mkdir $out
-          ${systemBuilderCommands}
+          ${config.system.activatableSystemBuilderCommands}
         '';
   };
 }

nixos/modules/system/activation/switchable-system.nix

@@ -0,0 +1,55 @@
+{ config, lib, pkgs, ... }:
+
+let
+
+  perlWrapped = pkgs.perl.withPackages (p: with p; [ ConfigIniFiles FileSlurp ]);
+
+in
+
+{
+
+  options = {
+    system.switch.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = lib.mdDoc ''
+        Whether to include the capability to switch configurations.
+
+        Disabling this makes the system unable to be reconfigured via `nixos-rebuild`.
+
+        This is good for image based appliances where updates are handled
+        outside the image. Reducing features makes the image lighter and
+        slightly more secure.
+      '';
+    };
+  };
+
+  config = lib.mkIf config.system.switch.enable {
+    system.activatableSystemBuilderCommands = ''
+      mkdir $out/bin
+      substitute ${./switch-to-configuration.pl} $out/bin/switch-to-configuration \
+        --subst-var out \
+        --subst-var-by toplevel ''${!toplevelVar} \
+        --subst-var-by coreutils "${pkgs.coreutils}" \
+        --subst-var-by distroId ${lib.escapeShellArg config.system.nixos.distroId} \
+        --subst-var-by installBootLoader ${lib.escapeShellArg config.system.build.installBootLoader} \
+        --subst-var-by localeArchive "${config.i18n.glibcLocales}/lib/locale/locale-archive" \
+        --subst-var-by perl "${perlWrapped}" \
+        --subst-var-by shell "${pkgs.bash}/bin/sh" \
+        --subst-var-by su "${pkgs.shadow.su}/bin/su" \
+        --subst-var-by systemd "${config.systemd.package}" \
+        --subst-var-by utillinux "${pkgs.util-linux}" \
+        ;
+
+      chmod +x $out/bin/switch-to-configuration
+      ${lib.optionalString (pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) ''
+        if ! output=$(${perlWrapped}/bin/perl -c $out/bin/switch-to-configuration 2>&1); then
+          echo "switch-to-configuration syntax is not valid:"
+          echo "$output"
+          exit 1
+        fi
+      ''}
+    '';
+  };
+
+}

nixos/tests/all-tests.nix

@@ -571,6 +571,7 @@ in {
   node-red = handleTest ./node-red.nix {};
   nomad = handleTest ./nomad.nix {};
   non-default-filesystems = handleTest ./non-default-filesystems.nix {};
+  non-switchable-system = runTest ./non-switchable-system.nix;
   noto-fonts = handleTest ./noto-fonts.nix {};
   noto-fonts-cjk-qt-default-weight = handleTest ./noto-fonts-cjk-qt-default-weight.nix {};
   novacomd = handleTestOn ["x86_64-linux"] ./novacomd.nix {};

nixos/tests/non-switchable-system.nix

@@ -0,0 +1,15 @@
+{ lib, ... }:
+
+{
+  name = "non-switchable-system";
+
+  meta.maintainers = with lib.maintainers; [ nikstur ];
+
+  nodes.machine = {
+    system.switch.enable = false;
+  };
+
+  testScript = ''
+    machine.succeed("test ! -e /run/current-system/bin/switch-to-configuration")
+  '';
+}