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-rw-r--r--nixos/modules/system/activation/activation-script.nix272
-rw-r--r--nixos/modules/system/activation/no-clone.nix8
-rwxr-xr-xnixos/modules/system/activation/switch-to-configuration.pl856
-rw-r--r--nixos/modules/system/activation/top-level.nix355
4 files changed, 1491 insertions, 0 deletions
diff --git a/nixos/modules/system/activation/activation-script.nix b/nixos/modules/system/activation/activation-script.nix
new file mode 100644
index 00000000000..c04d0fc16b2
--- /dev/null
+++ b/nixos/modules/system/activation/activation-script.nix
@@ -0,0 +1,272 @@
+# generate the script used to activate the configuration.
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  addAttributeName = mapAttrs (a: v: v // {
+    text = ''
+      #### Activation script snippet ${a}:
+      _localstatus=0
+      ${v.text}
+
+      if (( _localstatus > 0 )); then
+        printf "Activation script snippet '%s' failed (%s)\n" "${a}" "$_localstatus"
+      fi
+    '';
+  });
+
+  systemActivationScript = set: onlyDry: let
+    set' = mapAttrs (_: v: if isString v then (noDepEntry v) // { supportsDryActivation = false; } else v) set;
+    withHeadlines = addAttributeName set';
+    # When building a dry activation script, this replaces all activation scripts
+    # that do not support dry mode with a comment that does nothing. Filtering these
+    # activation scripts out so they don't get generated into the dry activation script
+    # does not work because when an activation script that supports dry mode depends on
+    # an activation script that does not, the dependency cannot be resolved and the eval
+    # fails.
+    withDrySnippets = mapAttrs (a: v: if onlyDry && !v.supportsDryActivation then v // {
+      text = "#### Activation script snippet ${a} does not support dry activation.";
+    } else v) withHeadlines;
+  in
+    ''
+      #!${pkgs.runtimeShell}
+
+      systemConfig='@out@'
+
+      export PATH=/empty
+      for i in ${toString path}; do
+          PATH=$PATH:$i/bin:$i/sbin
+      done
+
+      _status=0
+      trap "_status=1 _localstatus=\$?" ERR
+
+      # Ensure a consistent umask.
+      umask 0022
+
+      ${textClosureMap id (withDrySnippets) (attrNames withDrySnippets)}
+
+    '' + optionalString (!onlyDry) ''
+      # Make this configuration the current configuration.
+      # The readlink is there to ensure that when $systemConfig = /system
+      # (which is a symlink to the store), /run/current-system is still
+      # used as a garbage collection root.
+      ln -sfn "$(readlink -f "$systemConfig")" /run/current-system
+
+      # Prevent the current configuration from being garbage-collected.
+      mkdir -p /nix/var/nix/gcroots
+      ln -sfn /run/current-system /nix/var/nix/gcroots/current-system
+
+      exit $_status
+    '';
+
+  path = with pkgs; map getBin
+    [ coreutils
+      gnugrep
+      findutils
+      getent
+      stdenv.cc.libc # nscd in update-users-groups.pl
+      shadow
+      nettools # needed for hostname
+      util-linux # needed for mount and mountpoint
+    ];
+
+  scriptType = withDry: with types;
+    let scriptOptions =
+      { deps = mkOption
+          { type = types.listOf types.str;
+            default = [ ];
+            description = "List of dependencies. The script will run after these.";
+          };
+        text = mkOption
+          { type = types.lines;
+            description = "The content of the script.";
+          };
+      } // optionalAttrs withDry {
+        supportsDryActivation = mkOption
+          { type = types.bool;
+            default = false;
+            description = ''
+              Whether this activation script supports being dry-activated.
+              These activation scripts will also be executed on dry-activate
+              activations with the environment variable
+              <literal>NIXOS_ACTION</literal> being set to <literal>dry-activate
+              </literal>.  it's important that these activation scripts  don't
+              modify anything about the system when the variable is set.
+            '';
+          };
+      };
+    in either str (submodule { options = scriptOptions; });
+
+in
+
+{
+
+  ###### interface
+
+  options = {
+
+    system.activationScripts = mkOption {
+      default = {};
+
+      example = literalExpression ''
+        { stdio.text =
+          '''
+            # Needed by some programs.
+            ln -sfn /proc/self/fd /dev/fd
+            ln -sfn /proc/self/fd/0 /dev/stdin
+            ln -sfn /proc/self/fd/1 /dev/stdout
+            ln -sfn /proc/self/fd/2 /dev/stderr
+          ''';
+        }
+      '';
+
+      description = ''
+        A set of shell script fragments that are executed when a NixOS
+        system configuration is activated.  Examples are updating
+        /etc, creating accounts, and so on.  Since these are executed
+        every time you boot the system or run
+        <command>nixos-rebuild</command>, it's important that they are
+        idempotent and fast.
+      '';
+
+      type = types.attrsOf (scriptType true);
+      apply = set: set // {
+        script = systemActivationScript set false;
+      };
+    };
+
+    system.dryActivationScript = mkOption {
+      description = "The shell script that is to be run when dry-activating a system.";
+      readOnly = true;
+      internal = true;
+      default = systemActivationScript (removeAttrs config.system.activationScripts [ "script" ]) true;
+      defaultText = literalDocBook "generated activation script";
+    };
+
+    system.userActivationScripts = mkOption {
+      default = {};
+
+      example = literalExpression ''
+        { plasmaSetup = {
+            text = '''
+              ''${pkgs.libsForQt5.kservice}/bin/kbuildsycoca5"
+            ''';
+            deps = [];
+          };
+        }
+      '';
+
+      description = ''
+        A set of shell script fragments that are executed by a systemd user
+        service when a NixOS system configuration is activated. Examples are
+        rebuilding the .desktop file cache for showing applications in the menu.
+        Since these are executed every time you run
+        <command>nixos-rebuild</command>, it's important that they are
+        idempotent and fast.
+      '';
+
+      type = with types; attrsOf (scriptType false);
+
+      apply = set: {
+        script = ''
+          unset PATH
+          for i in ${toString path}; do
+            PATH=$PATH:$i/bin:$i/sbin
+          done
+
+          _status=0
+          trap "_status=1 _localstatus=\$?" ERR
+
+          ${
+            let
+              set' = mapAttrs (n: v: if isString v then noDepEntry v else v) set;
+              withHeadlines = addAttributeName set';
+            in textClosureMap id (withHeadlines) (attrNames withHeadlines)
+          }
+
+          exit $_status
+        '';
+      };
+
+    };
+
+    environment.usrbinenv = mkOption {
+      default = "${pkgs.coreutils}/bin/env";
+      defaultText = literalExpression ''"''${pkgs.coreutils}/bin/env"'';
+      example = literalExpression ''"''${pkgs.busybox}/bin/env"'';
+      type = types.nullOr types.path;
+      visible = false;
+      description = ''
+        The env(1) executable that is linked system-wide to
+        <literal>/usr/bin/env</literal>.
+      '';
+    };
+  };
+
+
+  ###### implementation
+
+  config = {
+
+    system.activationScripts.stdio = ""; # obsolete
+
+    system.activationScripts.var =
+      ''
+        # Various log/runtime directories.
+
+        mkdir -m 1777 -p /var/tmp
+
+        # Empty, immutable home directory of many system accounts.
+        mkdir -p /var/empty
+        # Make sure it's really empty
+        ${pkgs.e2fsprogs}/bin/chattr -f -i /var/empty || true
+        find /var/empty -mindepth 1 -delete
+        chmod 0555 /var/empty
+        chown root:root /var/empty
+        ${pkgs.e2fsprogs}/bin/chattr -f +i /var/empty || true
+      '';
+
+    system.activationScripts.usrbinenv = if config.environment.usrbinenv != null
+      then ''
+        mkdir -m 0755 -p /usr/bin
+        ln -sfn ${config.environment.usrbinenv} /usr/bin/.env.tmp
+        mv /usr/bin/.env.tmp /usr/bin/env # atomically replace /usr/bin/env
+      ''
+      else ''
+        rm -f /usr/bin/env
+        rmdir --ignore-fail-on-non-empty /usr/bin /usr
+      '';
+
+    system.activationScripts.specialfs =
+      ''
+        specialMount() {
+          local device="$1"
+          local mountPoint="$2"
+          local options="$3"
+          local fsType="$4"
+
+          if mountpoint -q "$mountPoint"; then
+            local options="remount,$options"
+          else
+            mkdir -m 0755 -p "$mountPoint"
+          fi
+          mount -t "$fsType" -o "$options" "$device" "$mountPoint"
+        }
+        source ${config.system.build.earlyMountScript}
+      '';
+
+    systemd.user = {
+      services.nixos-activation = {
+        description = "Run user-specific NixOS activation";
+        script = config.system.userActivationScripts.script;
+        unitConfig.ConditionUser = "!@system";
+        serviceConfig.Type = "oneshot";
+        wantedBy = [ "default.target" ];
+      };
+    };
+  };
+
+}
diff --git a/nixos/modules/system/activation/no-clone.nix b/nixos/modules/system/activation/no-clone.nix
new file mode 100644
index 00000000000..912420347dc
--- /dev/null
+++ b/nixos/modules/system/activation/no-clone.nix
@@ -0,0 +1,8 @@
+{ lib, ... }:
+
+with lib;
+
+{
+  boot.loader.grub.device = mkOverride 0 "nodev";
+  specialisation = mkOverride 0 {};
+}
diff --git a/nixos/modules/system/activation/switch-to-configuration.pl b/nixos/modules/system/activation/switch-to-configuration.pl
new file mode 100755
index 00000000000..459d09faa53
--- /dev/null
+++ b/nixos/modules/system/activation/switch-to-configuration.pl
@@ -0,0 +1,856 @@
+#! @perl@/bin/perl
+
+use strict;
+use warnings;
+use Config::IniFiles;
+use File::Path qw(make_path);
+use File::Basename;
+use File::Slurp qw(read_file write_file edit_file);
+use Net::DBus;
+use Sys::Syslog qw(:standard :macros);
+use Cwd 'abs_path';
+
+## no critic(CodeLayout::ProhibitParensWithBuiltins)
+
+my $out = "@out@";
+
+my $curSystemd = abs_path("/run/current-system/sw/bin");
+
+# To be robust against interruption, record what units need to be started etc.
+my $startListFile = "/run/nixos/start-list";
+my $restartListFile = "/run/nixos/restart-list";
+my $reloadListFile = "/run/nixos/reload-list";
+
+# Parse restart/reload requests by the activation script.
+# Activation scripts may write newline-separated units to the restart
+# file and switch-to-configuration will handle them. While
+# `stopIfChanged = true` is ignored, switch-to-configuration will
+# handle `restartIfChanged = false` and `reloadIfChanged = true`.
+# This is the same as specifying a restart trigger in the NixOS module.
+#
+# The reload file asks the script to reload a unit. This is the same as
+# specifying a reload trigger in the NixOS module and can be ignored if
+# the unit is restarted in this activation.
+my $restartByActivationFile = "/run/nixos/activation-restart-list";
+my $reloadByActivationFile = "/run/nixos/activation-reload-list";
+my $dryRestartByActivationFile = "/run/nixos/dry-activation-restart-list";
+my $dryReloadByActivationFile = "/run/nixos/dry-activation-reload-list";
+
+make_path("/run/nixos", { mode => oct(755) });
+
+my $action = shift(@ARGV);
+
+if ("@localeArchive@" ne "") {
+    $ENV{LOCALE_ARCHIVE} = "@localeArchive@";
+}
+
+if (!defined($action) || ($action ne "switch" && $action ne "boot" && $action ne "test" && $action ne "dry-activate")) {
+    print STDERR <<EOF;
+Usage: $0 [switch|boot|test]
+
+switch:       make the configuration the boot default and activate now
+boot:         make the configuration the boot default
+test:         activate the configuration, but don\'t make it the boot default
+dry-activate: show what would be done if this configuration were activated
+EOF
+    exit(1);
+}
+
+$ENV{NIXOS_ACTION} = $action;
+
+# This is a NixOS installation if it has /etc/NIXOS or a proper
+# /etc/os-release.
+die("This is not a NixOS installation!\n") unless
+    -f "/etc/NIXOS" || (read_file("/etc/os-release", err_mode => 'quiet') // "") =~ /ID="?nixos"?/s;
+
+openlog("nixos", "", LOG_USER);
+
+# Install or update the bootloader.
+if ($action eq "switch" || $action eq "boot") {
+    chomp(my $installBootLoader = <<'EOFBOOTLOADER');
+@installBootLoader@
+EOFBOOTLOADER
+    system("$installBootLoader $out") == 0 or exit 1;
+}
+
+# Just in case the new configuration hangs the system, do a sync now.
+system("@coreutils@/bin/sync", "-f", "/nix/store") unless ($ENV{"NIXOS_NO_SYNC"} // "") eq "1";
+
+exit(0) if $action eq "boot";
+
+# Check if we can activate the new configuration.
+my $oldVersion = read_file("/run/current-system/init-interface-version", err_mode => 'quiet') // "";
+my $newVersion = read_file("$out/init-interface-version");
+
+if ($newVersion ne $oldVersion) {
+    print STDERR <<EOF;
+Warning: the new NixOS configuration has an ‘init’ that is
+incompatible with the current configuration.  The new configuration
+won\'t take effect until you reboot the system.
+EOF
+    exit(100);
+}
+
+# Ignore SIGHUP so that we're not killed if we're running on (say)
+# virtual console 1 and we restart the "tty1" unit.
+$SIG{PIPE} = "IGNORE";
+
+sub getActiveUnits {
+    my $mgr = Net::DBus->system->get_service("org.freedesktop.systemd1")->get_object("/org/freedesktop/systemd1");
+    my $units = $mgr->ListUnitsByPatterns([], []);
+    my $res = {};
+    for my $item (@$units) {
+        my ($id, $description, $load_state, $active_state, $sub_state,
+            $following, $unit_path, $job_id, $job_type, $job_path) = @$item;
+        next unless $following eq '';
+        next if $job_id == 0 and $active_state eq 'inactive';
+        $res->{$id} = { load => $load_state, state => $active_state, substate => $sub_state };
+    }
+    return $res;
+}
+
+# Returns whether a systemd unit is active
+sub unit_is_active {
+    my ($unit_name) = @_;
+
+    my $mgr = Net::DBus->system->get_service('org.freedesktop.systemd1')->get_object('/org/freedesktop/systemd1');
+    my $units = $mgr->ListUnitsByNames([$unit_name]);
+    if (scalar(@{$units}) == 0) {
+        return 0;
+    }
+    my $active_state = $units->[0]->[3]; ## no critic (ValuesAndExpressions::ProhibitMagicNumbers)
+    return $active_state eq 'active' || $active_state eq 'activating';
+}
+
+sub parseFstab {
+    my ($filename) = @_;
+    my ($fss, $swaps);
+    foreach my $line (read_file($filename, err_mode => 'quiet')) {
+        chomp($line);
+        $line =~ s/^\s*#.*//;
+        next if $line =~ /^\s*$/;
+        my @xs = split(/ /, $line);
+        if ($xs[2] eq "swap") {
+            $swaps->{$xs[0]} = { options => $xs[3] // "" };
+        } else {
+            $fss->{$xs[1]} = { device => $xs[0], fsType => $xs[2], options => $xs[3] // "" };
+        }
+    }
+    return ($fss, $swaps);
+}
+
+# This subroutine takes a single ini file that specified systemd configuration
+# like unit configuration and parses it into a hash where the keys are the sections
+# of the unit file and the values are hashes themselves. These hashes have the unit file
+# keys as their keys (left side of =) and an array of all values that were set as their
+# values. If a value is empty (for example `ExecStart=`), then all current definitions are
+# removed.
+#
+# Instead of returning the hash, this subroutine takes a hashref to return the data in. This
+# allows calling the subroutine multiple times with the same hash to parse override files.
+sub parseSystemdIni {
+    my ($unitContents, $path) = @_;
+    # Tie the ini file to a hash for easier access
+    tie(my %fileContents, 'Config::IniFiles', (-file => $path, -allowempty => 1, -allowcontinue => 1)); ## no critic(Miscellanea::ProhibitTies)
+
+    # Copy over all sections
+    foreach my $sectionName (keys(%fileContents)) {
+        if ($sectionName eq "Install") {
+            # Skip the [Install] section because it has no relevant keys for us
+            next;
+        }
+        # Copy over all keys
+        foreach my $iniKey (keys(%{$fileContents{$sectionName}})) {
+            # Ensure the value is an array so it's easier to work with
+            my $iniValue = $fileContents{$sectionName}{$iniKey};
+            my @iniValues;
+            if (ref($iniValue) eq "ARRAY") {
+                @iniValues = @{$iniValue};
+            } else {
+                @iniValues = $iniValue;
+            }
+            # Go over all values
+            for my $iniValue (@iniValues) {
+                # If a value is empty, it's an override that tells us to clean the value
+                if ($iniValue eq "") {
+                    delete $unitContents->{$sectionName}->{$iniKey};
+                    next;
+                }
+                push(@{$unitContents->{$sectionName}->{$iniKey}}, $iniValue);
+            }
+        }
+    }
+    return;
+}
+
+# This subroutine takes the path to a systemd configuration file (like a unit configuration),
+# parses it, and returns a hash that contains the contents. The contents of this hash are
+# explained in the `parseSystemdIni` subroutine. Neither the sections nor the keys inside
+# the sections are consistently sorted.
+#
+# If a directory with the same basename ending in .d exists next to the unit file, it will be
+# assumed to contain override files which will be parsed as well and handled properly.
+sub parse_unit {
+    my ($unit_path) = @_;
+
+    # Parse the main unit and all overrides
+    my %unit_data;
+    # Replace \ with \\ so glob() still works with units that have a \ in them
+    # Valid characters in unit names are ASCII letters, digits, ":", "-", "_", ".", and "\"
+    $unit_path =~ s/\\/\\\\/gmsx;
+    foreach (glob("${unit_path}{,.d/*.conf}")) {
+        parseSystemdIni(\%unit_data, "$_")
+    }
+    return %unit_data;
+}
+
+# Checks whether a specified boolean in a systemd unit is true
+# or false, with a default that is applied when the value is not set.
+sub parseSystemdBool {
+    my ($unitConfig, $sectionName, $boolName, $default) = @_;
+
+    my @values = @{$unitConfig->{$sectionName}{$boolName} // []};
+    # Return default if value is not set
+    if (scalar(@values) lt 1 || not defined($values[-1])) {
+        return $default;
+    }
+    # If value is defined multiple times, use the last definition
+    my $last = $values[-1];
+    # These are valid values as of systemd.syntax(7)
+    return $last eq "1" || $last eq "yes" || $last eq "true" || $last eq "on";
+}
+
+sub recordUnit {
+    my ($fn, $unit) = @_;
+    write_file($fn, { append => 1 }, "$unit\n") if $action ne "dry-activate";
+}
+
+# The opposite of recordUnit, removes a unit name from a file
+sub unrecord_unit {
+    my ($fn, $unit) = @_;
+    edit_file(sub { s/^$unit\n//msx }, $fn) if $action ne "dry-activate";
+}
+
+# Compare the contents of two unit files and return whether the unit
+# needs to be restarted or reloaded. If the units differ, the service
+# is restarted unless the only difference is `X-Reload-Triggers` in the
+# `Unit` section. If this is the only modification, the unit is reloaded
+# instead of restarted.
+# Returns:
+# - 0 if the units are equal
+# - 1 if the units are different and a restart action is required
+# - 2 if the units are different and a reload action is required
+sub compare_units {
+    my ($old_unit, $new_unit) = @_;
+    my $ret = 0;
+    # Keys to ignore in the [Unit] section
+    my %unit_section_ignores = map { $_ => 1 } qw(
+        X-Reload-Triggers
+        Description Documentation
+        OnFailure OnSuccess OnFailureJobMode
+        IgnoreOnIsolate StopWhenUnneeded
+        RefuseManualStart RefuseManualStop
+        AllowIsolate CollectMode
+        SourcePath
+    );
+
+    my $comp_array = sub {
+      my ($a, $b) = @_;
+      return join("\0", @{$a}) eq join("\0", @{$b});
+    };
+
+    # Comparison hash for the sections
+    my %section_cmp = map { $_ => 1 } keys(%{$new_unit});
+    # Iterate over the sections
+    foreach my $section_name (keys(%{$old_unit})) {
+        # Missing section in the new unit?
+        if (not exists($section_cmp{$section_name})) {
+            # If the [Unit] section was removed, make sure that only keys
+            # were in it that are ignored
+            if ($section_name eq 'Unit') {
+                foreach my $ini_key (keys(%{$old_unit->{'Unit'}})) {
+                    if (not defined($unit_section_ignores{$ini_key})) {
+                        return 1;
+                    }
+                }
+                next; # check the next section
+            } else {
+                return 1;
+            }
+            if ($section_name eq 'Unit' and %{$old_unit->{'Unit'}} == 1 and defined(%{$old_unit->{'Unit'}}{'X-Reload-Triggers'})) {
+                # If a new [Unit] section was removed that only contained X-Reload-Triggers,
+                # do nothing.
+                next;
+            } else {
+                return 1;
+            }
+        }
+        delete $section_cmp{$section_name};
+        # Comparison hash for the section contents
+        my %ini_cmp = map { $_ => 1 } keys(%{$new_unit->{$section_name}});
+        # Iterate over the keys of the section
+        foreach my $ini_key (keys(%{$old_unit->{$section_name}})) {
+            delete $ini_cmp{$ini_key};
+            my @old_value = @{$old_unit->{$section_name}{$ini_key}};
+            # If the key is missing in the new unit, they are different...
+            if (not $new_unit->{$section_name}{$ini_key}) {
+                # ... unless the key that is now missing is one of the ignored keys
+                if ($section_name eq 'Unit' and defined($unit_section_ignores{$ini_key})) {
+                    next;
+                }
+                return 1;
+            }
+            my @new_value = @{$new_unit->{$section_name}{$ini_key}};
+            # If the contents are different, the units are different
+            if (not $comp_array->(\@old_value, \@new_value)) {
+                # Check if only the reload triggers changed or one of the ignored keys
+                if ($section_name eq 'Unit') {
+                    if ($ini_key eq 'X-Reload-Triggers') {
+                        $ret = 2;
+                        next;
+                    } elsif (defined($unit_section_ignores{$ini_key})) {
+                        next;
+                    }
+                }
+                return 1;
+            }
+        }
+        # A key was introduced that was missing in the old unit
+        if (%ini_cmp) {
+            if ($section_name eq 'Unit') {
+                foreach my $ini_key (keys(%ini_cmp)) {
+                    if ($ini_key eq 'X-Reload-Triggers') {
+                        $ret = 2;
+                    } elsif (defined($unit_section_ignores{$ini_key})) {
+                        next;
+                    } else {
+                        return 1;
+                    }
+                }
+            } else {
+                return 1;
+            }
+        };
+    }
+    # A section was introduced that was missing in the old unit
+    if (%section_cmp) {
+        if (%section_cmp == 1 and defined($section_cmp{'Unit'})) {
+            foreach my $ini_key (keys(%{$new_unit->{'Unit'}})) {
+                if (not defined($unit_section_ignores{$ini_key})) {
+                    return 1;
+                } elsif ($ini_key eq 'X-Reload-Triggers') {
+                    $ret = 2;
+                }
+            }
+        } else {
+            return 1;
+        }
+    }
+
+    return $ret;
+}
+
+sub handleModifiedUnit {
+    my ($unit, $baseName, $newUnitFile, $newUnitInfo, $activePrev, $unitsToStop, $unitsToStart, $unitsToReload, $unitsToRestart, $unitsToSkip) = @_;
+
+    if ($unit eq "sysinit.target" || $unit eq "basic.target" || $unit eq "multi-user.target" || $unit eq "graphical.target" || $unit =~ /\.path$/ || $unit =~ /\.slice$/) {
+        # Do nothing.  These cannot be restarted directly.
+
+        # Slices and Paths don't have to be restarted since
+        # properties (resource limits and inotify watches)
+        # seem to get applied on daemon-reload.
+    } elsif ($unit =~ /\.mount$/) {
+        # Reload the changed mount unit to force a remount.
+        # FIXME: only reload when Options= changed, restart otherwise
+        $unitsToReload->{$unit} = 1;
+        recordUnit($reloadListFile, $unit);
+    } elsif ($unit =~ /\.socket$/) {
+        # FIXME: do something?
+        # Attempt to fix this: https://github.com/NixOS/nixpkgs/pull/141192
+        # Revert of the attempt: https://github.com/NixOS/nixpkgs/pull/147609
+        # More details: https://github.com/NixOS/nixpkgs/issues/74899#issuecomment-981142430
+    } else {
+        my %unitInfo = $newUnitInfo ? %{$newUnitInfo} : parse_unit($newUnitFile);
+        if (parseSystemdBool(\%unitInfo, "Service", "X-ReloadIfChanged", 0) and not $unitsToRestart->{$unit} and not $unitsToStop->{$unit}) {
+            $unitsToReload->{$unit} = 1;
+            recordUnit($reloadListFile, $unit);
+        }
+        elsif (!parseSystemdBool(\%unitInfo, "Service", "X-RestartIfChanged", 1) || parseSystemdBool(\%unitInfo, "Unit", "RefuseManualStop", 0) || parseSystemdBool(\%unitInfo, "Unit", "X-OnlyManualStart", 0)) {
+            $unitsToSkip->{$unit} = 1;
+        } else {
+            # It doesn't make sense to stop and start non-services because
+            # they can't have ExecStop=
+            if (!parseSystemdBool(\%unitInfo, "Service", "X-StopIfChanged", 1) || $unit !~ /\.service$/) {
+                # This unit should be restarted instead of
+                # stopped and started.
+                $unitsToRestart->{$unit} = 1;
+                recordUnit($restartListFile, $unit);
+                # Remove from units to reload so we don't restart and reload
+                if ($unitsToReload->{$unit}) {
+                    delete $unitsToReload->{$unit};
+                    unrecord_unit($reloadListFile, $unit);
+                }
+            } else {
+                # If this unit is socket-activated, then stop the
+                # socket unit(s) as well, and restart the
+                # socket(s) instead of the service.
+                my $socket_activated = 0;
+                if ($unit =~ /\.service$/) {
+                    my @sockets = split(/ /, join(" ", @{$unitInfo{Service}{Sockets} // []}));
+                    if (scalar(@sockets) == 0) {
+                        @sockets = ("$baseName.socket");
+                    }
+                    foreach my $socket (@sockets) {
+                        if (defined($activePrev->{$socket})) {
+                            # We can now be sure this is a socket-activate unit
+
+                            $unitsToStop->{$socket} = 1;
+                            # Only restart sockets that actually
+                            # exist in new configuration:
+                            if (-e "$out/etc/systemd/system/$socket") {
+                                $unitsToStart->{$socket} = 1;
+                                if ($unitsToStart eq $unitsToRestart) {
+                                    recordUnit($restartListFile, $socket);
+                                } else {
+                                    recordUnit($startListFile, $socket);
+                                }
+                                $socket_activated = 1;
+                            }
+                            # Remove from units to reload so we don't restart and reload
+                            if ($unitsToReload->{$unit}) {
+                                delete $unitsToReload->{$unit};
+                                unrecord_unit($reloadListFile, $unit);
+                            }
+                        }
+                    }
+                }
+
+                # If the unit is not socket-activated, record
+                # that this unit needs to be started below.
+                # We write this to a file to ensure that the
+                # service gets restarted if we're interrupted.
+                if (!$socket_activated) {
+                    $unitsToStart->{$unit} = 1;
+                    if ($unitsToStart eq $unitsToRestart) {
+                        recordUnit($restartListFile, $unit);
+                    } else {
+                        recordUnit($startListFile, $unit);
+                    }
+                }
+
+                $unitsToStop->{$unit} = 1;
+                # Remove from units to reload so we don't restart and reload
+                if ($unitsToReload->{$unit}) {
+                    delete $unitsToReload->{$unit};
+                    unrecord_unit($reloadListFile, $unit);
+                }
+            }
+        }
+    }
+}
+
+# Figure out what units need to be stopped, started, restarted or reloaded.
+my (%unitsToStop, %unitsToSkip, %unitsToStart, %unitsToRestart, %unitsToReload);
+
+my %unitsToFilter; # units not shown
+
+$unitsToStart{$_} = 1 foreach
+    split('\n', read_file($startListFile, err_mode => 'quiet') // "");
+
+$unitsToRestart{$_} = 1 foreach
+    split('\n', read_file($restartListFile, err_mode => 'quiet') // "");
+
+$unitsToReload{$_} = 1 foreach
+    split('\n', read_file($reloadListFile, err_mode => 'quiet') // "");
+
+my $activePrev = getActiveUnits();
+while (my ($unit, $state) = each(%{$activePrev})) {
+    my $baseUnit = $unit;
+
+    my $prevUnitFile = "/etc/systemd/system/$baseUnit";
+    my $newUnitFile = "$out/etc/systemd/system/$baseUnit";
+
+    # Detect template instances.
+    if (!-e $prevUnitFile && !-e $newUnitFile && $unit =~ /^(.*)@[^\.]*\.(.*)$/) {
+      $baseUnit = "$1\@.$2";
+      $prevUnitFile = "/etc/systemd/system/$baseUnit";
+      $newUnitFile = "$out/etc/systemd/system/$baseUnit";
+    }
+
+    my $baseName = $baseUnit;
+    $baseName =~ s/\.[a-z]*$//;
+
+    if (-e $prevUnitFile && ($state->{state} eq "active" || $state->{state} eq "activating")) {
+        if (! -e $newUnitFile || abs_path($newUnitFile) eq "/dev/null") {
+            my %unitInfo = parse_unit($prevUnitFile);
+            $unitsToStop{$unit} = 1 if parseSystemdBool(\%unitInfo, "Unit", "X-StopOnRemoval", 1);
+        }
+
+        elsif ($unit =~ /\.target$/) {
+            my %unitInfo = parse_unit($newUnitFile);
+
+            # Cause all active target units to be restarted below.
+            # This should start most changed units we stop here as
+            # well as any new dependencies (including new mounts and
+            # swap devices).  FIXME: the suspend target is sometimes
+            # active after the system has resumed, which probably
+            # should not be the case.  Just ignore it.
+            if ($unit ne "suspend.target" && $unit ne "hibernate.target" && $unit ne "hybrid-sleep.target") {
+                unless (parseSystemdBool(\%unitInfo, "Unit", "RefuseManualStart", 0) || parseSystemdBool(\%unitInfo, "Unit", "X-OnlyManualStart", 0)) {
+                    $unitsToStart{$unit} = 1;
+                    recordUnit($startListFile, $unit);
+                    # Don't spam the user with target units that always get started.
+                    $unitsToFilter{$unit} = 1;
+                }
+            }
+
+            # Stop targets that have X-StopOnReconfiguration set.
+            # This is necessary to respect dependency orderings
+            # involving targets: if unit X starts after target Y and
+            # target Y starts after unit Z, then if X and Z have both
+            # changed, then X should be restarted after Z.  However,
+            # if target Y is in the "active" state, X and Z will be
+            # restarted at the same time because X's dependency on Y
+            # is already satisfied.  Thus, we need to stop Y first.
+            # Stopping a target generally has no effect on other units
+            # (unless there is a PartOf dependency), so this is just a
+            # bookkeeping thing to get systemd to do the right thing.
+            if (parseSystemdBool(\%unitInfo, "Unit", "X-StopOnReconfiguration", 0)) {
+                $unitsToStop{$unit} = 1;
+            }
+        }
+
+        else {
+            my %old_unit_info = parse_unit($prevUnitFile);
+            my %new_unit_info = parse_unit($newUnitFile);
+            my $diff = compare_units(\%old_unit_info, \%new_unit_info);
+            if ($diff == 1) {
+                handleModifiedUnit($unit, $baseName, $newUnitFile, \%new_unit_info, $activePrev, \%unitsToStop, \%unitsToStart, \%unitsToReload, \%unitsToRestart, \%unitsToSkip);
+            } elsif ($diff == 2 and not $unitsToRestart{$unit}) {
+                $unitsToReload{$unit} = 1;
+                recordUnit($reloadListFile, $unit);
+            }
+        }
+    }
+}
+
+sub pathToUnitName {
+    my ($path) = @_;
+    # Use current version of systemctl binary before daemon is reexeced.
+    open(my $cmd, "-|", "$curSystemd/systemd-escape", "--suffix=mount", "-p", $path)
+        or die "Unable to escape $path!\n";
+    my $escaped = join("", <$cmd>);
+    chomp($escaped);
+    close($cmd) or die('Unable to close systemd-escape pipe');
+    return $escaped;
+}
+
+# Compare the previous and new fstab to figure out which filesystems
+# need a remount or need to be unmounted.  New filesystems are mounted
+# automatically by starting local-fs.target.  FIXME: might be nicer if
+# we generated units for all mounts; then we could unify this with the
+# unit checking code above.
+my ($prevFss, $prevSwaps) = parseFstab("/etc/fstab");
+my ($newFss, $newSwaps) = parseFstab("$out/etc/fstab");
+foreach my $mountPoint (keys(%$prevFss)) {
+    my $prev = $prevFss->{$mountPoint};
+    my $new = $newFss->{$mountPoint};
+    my $unit = pathToUnitName($mountPoint);
+    if (!defined($new)) {
+        # Filesystem entry disappeared, so unmount it.
+        $unitsToStop{$unit} = 1;
+    } elsif ($prev->{fsType} ne $new->{fsType} || $prev->{device} ne $new->{device}) {
+        # Filesystem type or device changed, so unmount and mount it.
+        $unitsToStop{$unit} = 1;
+        $unitsToStart{$unit} = 1;
+        recordUnit($startListFile, $unit);
+    } elsif ($prev->{options} ne $new->{options}) {
+        # Mount options changes, so remount it.
+        $unitsToReload{$unit} = 1;
+        recordUnit($reloadListFile, $unit);
+    }
+}
+
+# Also handles swap devices.
+foreach my $device (keys(%$prevSwaps)) {
+    my $prev = $prevSwaps->{$device};
+    my $new = $newSwaps->{$device};
+    if (!defined($new)) {
+        # Swap entry disappeared, so turn it off.  Can't use
+        # "systemctl stop" here because systemd has lots of alias
+        # units that prevent a stop from actually calling
+        # "swapoff".
+        if ($action ne "dry-activate") {
+            print STDERR "would stop swap device: $device\n";
+        } else {
+            print STDERR "stopping swap device: $device\n";
+            system("@utillinux@/sbin/swapoff", $device);
+        }
+    }
+    # FIXME: update swap options (i.e. its priority).
+}
+
+
+# Should we have systemd re-exec itself?
+my $prevSystemd = abs_path("/proc/1/exe") // "/unknown";
+my $prevSystemdSystemConfig = abs_path("/etc/systemd/system.conf") // "/unknown";
+my $newSystemd = abs_path("@systemd@/lib/systemd/systemd") or die;
+my $newSystemdSystemConfig = abs_path("$out/etc/systemd/system.conf") // "/unknown";
+
+my $restartSystemd = $prevSystemd ne $newSystemd;
+if ($prevSystemdSystemConfig ne $newSystemdSystemConfig) {
+    $restartSystemd = 1;
+}
+
+
+sub filterUnits {
+    my ($units) = @_;
+    my @res;
+    foreach my $unit (sort(keys(%{$units}))) {
+        push(@res, $unit) if !defined($unitsToFilter{$unit});
+    }
+    return @res;
+}
+
+my @unitsToStopFiltered = filterUnits(\%unitsToStop);
+
+
+# Show dry-run actions.
+if ($action eq "dry-activate") {
+    print STDERR "would stop the following units: ", join(", ", @unitsToStopFiltered), "\n"
+        if scalar(@unitsToStopFiltered) > 0;
+    print STDERR "would NOT stop the following changed units: ", join(", ", sort(keys(%unitsToSkip))), "\n"
+        if scalar(keys(%unitsToSkip)) > 0;
+
+    print STDERR "would activate the configuration...\n";
+    system("$out/dry-activate", "$out");
+
+    # Handle the activation script requesting the restart or reload of a unit.
+    foreach (split('\n', read_file($dryRestartByActivationFile, err_mode => 'quiet') // "")) {
+        my $unit = $_;
+        my $baseUnit = $unit;
+        my $newUnitFile = "$out/etc/systemd/system/$baseUnit";
+
+        # Detect template instances.
+        if (!-e $newUnitFile && $unit =~ /^(.*)@[^\.]*\.(.*)$/) {
+          $baseUnit = "$1\@.$2";
+          $newUnitFile = "$out/etc/systemd/system/$baseUnit";
+        }
+
+        my $baseName = $baseUnit;
+        $baseName =~ s/\.[a-z]*$//;
+
+        # Start units if they were not active previously
+        if (not defined($activePrev->{$unit})) {
+            $unitsToStart{$unit} = 1;
+            next;
+        }
+
+        handleModifiedUnit($unit, $baseName, $newUnitFile, undef, $activePrev, \%unitsToRestart, \%unitsToRestart, \%unitsToReload, \%unitsToRestart, \%unitsToSkip);
+    }
+    unlink($dryRestartByActivationFile);
+
+    foreach (split('\n', read_file($dryReloadByActivationFile, err_mode => 'quiet') // "")) {
+        my $unit = $_;
+
+        if (defined($activePrev->{$unit}) and not $unitsToRestart{$unit} and not $unitsToStop{$unit}) {
+            $unitsToReload{$unit} = 1;
+            recordUnit($reloadListFile, $unit);
+        }
+    }
+    unlink($dryReloadByActivationFile);
+
+    print STDERR "would restart systemd\n" if $restartSystemd;
+    print STDERR "would reload the following units: ", join(", ", sort(keys(%unitsToReload))), "\n"
+        if scalar(keys(%unitsToReload)) > 0;
+    print STDERR "would restart the following units: ", join(", ", sort(keys(%unitsToRestart))), "\n"
+        if scalar(keys(%unitsToRestart)) > 0;
+    my @unitsToStartFiltered = filterUnits(\%unitsToStart);
+    print STDERR "would start the following units: ", join(", ", @unitsToStartFiltered), "\n"
+        if scalar(@unitsToStartFiltered);
+    exit 0;
+}
+
+
+syslog(LOG_NOTICE, "switching to system configuration $out");
+
+if (scalar(keys(%unitsToStop)) > 0) {
+    print STDERR "stopping the following units: ", join(", ", @unitsToStopFiltered), "\n"
+        if scalar(@unitsToStopFiltered);
+    # Use current version of systemctl binary before daemon is reexeced.
+    system("$curSystemd/systemctl", "stop", "--", sort(keys(%unitsToStop)));
+}
+
+print STDERR "NOT restarting the following changed units: ", join(", ", sort(keys(%unitsToSkip))), "\n"
+    if scalar(keys(%unitsToSkip)) > 0;
+
+# Activate the new configuration (i.e., update /etc, make accounts,
+# and so on).
+my $res = 0;
+print STDERR "activating the configuration...\n";
+system("$out/activate", "$out") == 0 or $res = 2;
+
+# Handle the activation script requesting the restart or reload of a unit.
+foreach (split('\n', read_file($restartByActivationFile, err_mode => 'quiet') // "")) {
+    my $unit = $_;
+    my $baseUnit = $unit;
+    my $newUnitFile = "$out/etc/systemd/system/$baseUnit";
+
+    # Detect template instances.
+    if (!-e $newUnitFile && $unit =~ /^(.*)@[^\.]*\.(.*)$/) {
+      $baseUnit = "$1\@.$2";
+      $newUnitFile = "$out/etc/systemd/system/$baseUnit";
+    }
+
+    my $baseName = $baseUnit;
+    $baseName =~ s/\.[a-z]*$//;
+
+    # Start units if they were not active previously
+    if (not defined($activePrev->{$unit})) {
+        $unitsToStart{$unit} = 1;
+        recordUnit($startListFile, $unit);
+        next;
+    }
+
+    handleModifiedUnit($unit, $baseName, $newUnitFile, undef, $activePrev, \%unitsToRestart, \%unitsToRestart, \%unitsToReload, \%unitsToRestart, \%unitsToSkip);
+}
+# We can remove the file now because it has been propagated to the other restart/reload files
+unlink($restartByActivationFile);
+
+foreach (split('\n', read_file($reloadByActivationFile, err_mode => 'quiet') // "")) {
+    my $unit = $_;
+
+    if (defined($activePrev->{$unit}) and not $unitsToRestart{$unit} and not $unitsToStop{$unit}) {
+        $unitsToReload{$unit} = 1;
+        recordUnit($reloadListFile, $unit);
+    }
+}
+# We can remove the file now because it has been propagated to the other reload file
+unlink($reloadByActivationFile);
+
+# Restart systemd if necessary. Note that this is done using the
+# current version of systemd, just in case the new one has trouble
+# communicating with the running pid 1.
+if ($restartSystemd) {
+    print STDERR "restarting systemd...\n";
+    system("$curSystemd/systemctl", "daemon-reexec") == 0 or $res = 2;
+}
+
+# Forget about previously failed services.
+system("@systemd@/bin/systemctl", "reset-failed");
+
+# Make systemd reload its units.
+system("@systemd@/bin/systemctl", "daemon-reload") == 0 or $res = 3;
+
+# Reload user units
+open(my $listActiveUsers, '-|', '@systemd@/bin/loginctl', 'list-users', '--no-legend');
+while (my $f = <$listActiveUsers>) {
+    next unless $f =~ /^\s*(?<uid>\d+)\s+(?<user>\S+)/;
+    my ($uid, $name) = ($+{uid}, $+{user});
+    print STDERR "reloading user units for $name...\n";
+
+    system("@su@", "-s", "@shell@", "-l", $name, "-c",
+           "export XDG_RUNTIME_DIR=/run/user/$uid; " .
+           "$curSystemd/systemctl --user daemon-reexec; " .
+           "@systemd@/bin/systemctl --user start nixos-activation.service");
+}
+
+close($listActiveUsers);
+
+# Set the new tmpfiles
+print STDERR "setting up tmpfiles\n";
+system("@systemd@/bin/systemd-tmpfiles", "--create", "--remove", "--exclude-prefix=/dev") == 0 or $res = 3;
+
+# Before reloading we need to ensure that the units are still active. They may have been
+# deactivated because one of their requirements got stopped. If they are inactive
+# but should have been reloaded, the user probably expects them to be started.
+if (scalar(keys(%unitsToReload)) > 0) {
+    for my $unit (keys(%unitsToReload)) {
+        if (!unit_is_active($unit)) {
+            # Figure out if we need to start the unit
+            my %unit_info = parse_unit("$out/etc/systemd/system/$unit");
+            if (!(parseSystemdBool(\%unit_info, 'Unit', 'RefuseManualStart', 0) || parseSystemdBool(\%unit_info, 'Unit', 'X-OnlyManualStart', 0))) {
+                $unitsToStart{$unit} = 1;
+                recordUnit($startListFile, $unit);
+            }
+            # Don't reload the unit, reloading would fail
+            delete %unitsToReload{$unit};
+            unrecord_unit($reloadListFile, $unit);
+        }
+    }
+}
+# Reload units that need it. This includes remounting changed mount
+# units.
+if (scalar(keys(%unitsToReload)) > 0) {
+    print STDERR "reloading the following units: ", join(", ", sort(keys(%unitsToReload))), "\n";
+    system("@systemd@/bin/systemctl", "reload", "--", sort(keys(%unitsToReload))) == 0 or $res = 4;
+    unlink($reloadListFile);
+}
+
+# Restart changed services (those that have to be restarted rather
+# than stopped and started).
+if (scalar(keys(%unitsToRestart)) > 0) {
+    print STDERR "restarting the following units: ", join(", ", sort(keys(%unitsToRestart))), "\n";
+    system("@systemd@/bin/systemctl", "restart", "--", sort(keys(%unitsToRestart))) == 0 or $res = 4;
+    unlink($restartListFile);
+}
+
+# Start all active targets, as well as changed units we stopped above.
+# The latter is necessary because some may not be dependencies of the
+# targets (i.e., they were manually started).  FIXME: detect units
+# that are symlinks to other units.  We shouldn't start both at the
+# same time because we'll get a "Failed to add path to set" error from
+# systemd.
+my @unitsToStartFiltered = filterUnits(\%unitsToStart);
+print STDERR "starting the following units: ", join(", ", @unitsToStartFiltered), "\n"
+    if scalar(@unitsToStartFiltered);
+system("@systemd@/bin/systemctl", "start", "--", sort(keys(%unitsToStart))) == 0 or $res = 4;
+unlink($startListFile);
+
+
+# Print failed and new units.
+my (@failed, @new);
+my $activeNew = getActiveUnits();
+while (my ($unit, $state) = each(%{$activeNew})) {
+    if ($state->{state} eq "failed") {
+        push(@failed, $unit);
+        next;
+    }
+
+    if ($state->{substate} eq "auto-restart") {
+        # A unit in auto-restart substate is a failure *if* it previously failed to start
+        my $main_status = `@systemd@/bin/systemctl show --value --property=ExecMainStatus '$unit'`;
+        chomp($main_status);
+
+        if ($main_status ne "0") {
+            push(@failed, $unit);
+            next;
+        }
+    }
+
+    # Ignore scopes since they are not managed by this script but rather
+    # created and managed by third-party services via the systemd dbus API.
+    # This only lists units that are not failed (including ones that are in auto-restart but have not failed previously)
+    if ($state->{state} ne "failed" && !defined($activePrev->{$unit}) && $unit !~ /\.scope$/msx) {
+        push(@new, $unit);
+    }
+}
+
+if (scalar(@new) > 0) {
+    print STDERR "the following new units were started: ", join(", ", sort(@new)), "\n"
+}
+
+if (scalar(@failed) > 0) {
+    my @failed_sorted = sort(@failed);
+    print STDERR "warning: the following units failed: ", join(", ", @failed_sorted), "\n\n";
+    system("@systemd@/bin/systemctl status --no-pager --full '" . join("' '", @failed_sorted) . "' >&2");
+    $res = 4;
+}
+
+if ($res == 0) {
+    syslog(LOG_NOTICE, "finished switching to system configuration $out");
+} else {
+    syslog(LOG_ERR, "switching to system configuration $out failed (status $res)");
+}
+
+exit($res);
diff --git a/nixos/modules/system/activation/top-level.nix b/nixos/modules/system/activation/top-level.nix
new file mode 100644
index 00000000000..b8aeee8c11b
--- /dev/null
+++ b/nixos/modules/system/activation/top-level.nix
@@ -0,0 +1,355 @@
+{ config, lib, pkgs, extendModules, noUserModules, ... }:
+
+with lib;
+
+let
+
+
+  # This attribute is responsible for creating boot entries for
+  # child configuration. They are only (directly) accessible
+  # when the parent configuration is boot default. For example,
+  # you can provide an easy way to boot the same configuration
+  # as you use, but with another kernel
+  # !!! fix this
+  children =
+    mapAttrs
+      (childName: childConfig: childConfig.configuration.system.build.toplevel)
+      config.specialisation;
+
+  systemBuilder =
+    let
+      kernelPath = "${config.boot.kernelPackages.kernel}/" +
+        "${config.system.boot.loader.kernelFile}";
+      initrdPath = "${config.system.build.initialRamdisk}/" +
+        "${config.system.boot.loader.initrdFile}";
+    in ''
+      mkdir $out
+
+      # Containers don't have their own kernel or initrd.  They boot
+      # directly into stage 2.
+      ${optionalString (!config.boot.isContainer) ''
+        if [ ! -f ${kernelPath} ]; then
+          echo "The bootloader cannot find the proper kernel image."
+          echo "(Expecting ${kernelPath})"
+          false
+        fi
+
+        ln -s ${kernelPath} $out/kernel
+        ln -s ${config.system.modulesTree} $out/kernel-modules
+        ${optionalString (config.hardware.deviceTree.package != null) ''
+          ln -s ${config.hardware.deviceTree.package} $out/dtbs
+        ''}
+
+        echo -n "$kernelParams" > $out/kernel-params
+
+        ln -s ${initrdPath} $out/initrd
+
+        ln -s ${config.system.build.initialRamdiskSecretAppender}/bin/append-initrd-secrets $out
+
+        ln -s ${config.hardware.firmware}/lib/firmware $out/firmware
+      ''}
+
+      echo "$activationScript" > $out/activate
+      echo "$dryActivationScript" > $out/dry-activate
+      substituteInPlace $out/activate --subst-var out
+      substituteInPlace $out/dry-activate --subst-var out
+      chmod u+x $out/activate $out/dry-activate
+      unset activationScript dryActivationScript
+      ${pkgs.stdenv.shellDryRun} $out/activate
+      ${pkgs.stdenv.shellDryRun} $out/dry-activate
+
+      cp ${config.system.build.bootStage2} $out/init
+      substituteInPlace $out/init --subst-var-by systemConfig $out
+
+      ln -s ${config.system.build.etc}/etc $out/etc
+      ln -s ${config.system.path} $out/sw
+      ln -s "$systemd" $out/systemd
+
+      echo -n "$configurationName" > $out/configuration-name
+      echo -n "systemd ${toString config.systemd.package.interfaceVersion}" > $out/init-interface-version
+      echo -n "$nixosLabel" > $out/nixos-version
+      echo -n "${config.boot.kernelPackages.stdenv.hostPlatform.system}" > $out/system
+
+      mkdir $out/specialisation
+      ${concatStringsSep "\n"
+      (mapAttrsToList (name: path: "ln -s ${path} $out/specialisation/${name}") children)}
+
+      mkdir $out/bin
+      export localeArchive="${config.i18n.glibcLocales}/lib/locale/locale-archive"
+      substituteAll ${./switch-to-configuration.pl} $out/bin/switch-to-configuration
+      chmod +x $out/bin/switch-to-configuration
+      ${optionalString (pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) ''
+        if ! output=$($perl/bin/perl -c $out/bin/switch-to-configuration 2>&1); then
+          echo "switch-to-configuration syntax is not valid:"
+          echo "$output"
+          exit 1
+        fi
+      ''}
+
+      echo -n "${toString config.system.extraDependencies}" > $out/extra-dependencies
+
+      ${config.system.extraSystemBuilderCmds}
+    '';
+
+  # Putting it all together.  This builds a store path containing
+  # symlinks to the various parts of the built configuration (the
+  # kernel, systemd units, init scripts, etc.) as well as a script
+  # `switch-to-configuration' that activates the configuration and
+  # makes it bootable.
+  baseSystem = pkgs.stdenvNoCC.mkDerivation {
+    name = "nixos-system-${config.system.name}-${config.system.nixos.label}";
+    preferLocalBuild = true;
+    allowSubstitutes = false;
+    buildCommand = systemBuilder;
+
+    inherit (pkgs) coreutils;
+    systemd = config.systemd.package;
+    shell = "${pkgs.bash}/bin/sh";
+    su = "${pkgs.shadow.su}/bin/su";
+    utillinux = pkgs.util-linux;
+
+    kernelParams = config.boot.kernelParams;
+    installBootLoader = config.system.build.installBootLoader;
+    activationScript = config.system.activationScripts.script;
+    dryActivationScript = config.system.dryActivationScript;
+    nixosLabel = config.system.nixos.label;
+
+    configurationName = config.boot.loader.grub.configurationName;
+
+    # Needed by switch-to-configuration.
+    perl = pkgs.perl.withPackages (p: with p; [ ConfigIniFiles FileSlurp NetDBus ]);
+  };
+
+  # Handle assertions and warnings
+
+  failedAssertions = map (x: x.message) (filter (x: !x.assertion) config.assertions);
+
+  baseSystemAssertWarn = if failedAssertions != []
+    then throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
+    else showWarnings config.warnings baseSystem;
+
+  # Replace runtime dependencies
+  system = foldr ({ oldDependency, newDependency }: drv:
+      pkgs.replaceDependency { inherit oldDependency newDependency drv; }
+    ) baseSystemAssertWarn config.system.replaceRuntimeDependencies;
+
+  /* Workaround until https://github.com/NixOS/nixpkgs/pull/156533
+     Call can be replaced by argument when that's merged.
+  */
+  tmpFixupSubmoduleBoundary = subopts:
+    lib.mkOption {
+      type = lib.types.submoduleWith {
+        modules = [ { options = subopts; } ];
+      };
+    };
+
+in
+
+{
+  imports = [
+    ../build.nix
+    (mkRemovedOptionModule [ "nesting" "clone" ] "Use `specialisation.«name» = { inheritParentConfig = true; configuration = { ... }; }` instead.")
+    (mkRemovedOptionModule [ "nesting" "children" ] "Use `specialisation.«name».configuration = { ... }` instead.")
+  ];
+
+  options = {
+
+    specialisation = mkOption {
+      default = {};
+      example = lib.literalExpression "{ fewJobsManyCores.configuration = { nix.settings = { core = 0; max-jobs = 1; }; }";
+      description = ''
+        Additional configurations to build. If
+        <literal>inheritParentConfig</literal> is true, the system
+        will be based on the overall system configuration.
+
+        To switch to a specialised configuration
+        (e.g. <literal>fewJobsManyCores</literal>) at runtime, run:
+
+        <screen>
+        <prompt># </prompt>sudo /run/current-system/specialisation/fewJobsManyCores/bin/switch-to-configuration test
+        </screen>
+      '';
+      type = types.attrsOf (types.submodule (
+        local@{ ... }: let
+          extend = if local.config.inheritParentConfig
+            then extendModules
+            else noUserModules.extendModules;
+        in {
+          options.inheritParentConfig = mkOption {
+            type = types.bool;
+            default = true;
+            description = "Include the entire system's configuration. Set to false to make a completely differently configured system.";
+          };
+
+          options.configuration = mkOption {
+            default = {};
+            description = ''
+              Arbitrary NixOS configuration.
+
+              Anything you can add to a normal NixOS configuration, you can add
+              here, including imports and config values, although nested
+              specialisations will be ignored.
+            '';
+            visible = "shallow";
+            inherit (extend { modules = [ ./no-clone.nix ]; }) type;
+          };
+        })
+      );
+    };
+
+    system.boot.loader.id = mkOption {
+      internal = true;
+      default = "";
+      description = ''
+        Id string of the used bootloader.
+      '';
+    };
+
+    system.boot.loader.kernelFile = mkOption {
+      internal = true;
+      default = pkgs.stdenv.hostPlatform.linux-kernel.target;
+      defaultText = literalExpression "pkgs.stdenv.hostPlatform.linux-kernel.target";
+      type = types.str;
+      description = ''
+        Name of the kernel file to be passed to the bootloader.
+      '';
+    };
+
+    system.boot.loader.initrdFile = mkOption {
+      internal = true;
+      default = "initrd";
+      type = types.str;
+      description = ''
+        Name of the initrd file to be passed to the bootloader.
+      '';
+    };
+
+    system.build = tmpFixupSubmoduleBoundary {
+      installBootLoader = mkOption {
+        internal = true;
+        # "; true" => make the `$out` argument from switch-to-configuration.pl
+        #             go to `true` instead of `echo`, hiding the useless path
+        #             from the log.
+        default = "echo 'Warning: do not know how to make this configuration bootable; please enable a boot loader.' 1>&2; true";
+        description = ''
+          A program that writes a bootloader installation script to the path passed in the first command line argument.
+
+          See <literal>nixos/modules/system/activation/switch-to-configuration.pl</literal>.
+        '';
+        type = types.unique {
+          message = ''
+            Only one bootloader can be enabled at a time. This requirement has not
+            been checked until NixOS 22.05. Earlier versions defaulted to the last
+            definition. Change your configuration to enable only one bootloader.
+          '';
+        } (types.either types.str types.package);
+      };
+
+      toplevel = mkOption {
+        type = types.package;
+        readOnly = true;
+        description = ''
+          This option contains the store path that typically represents a NixOS system.
+
+          You can read this path in a custom deployment tool for example.
+        '';
+      };
+    };
+
+
+    system.copySystemConfiguration = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        If enabled, copies the NixOS configuration file
+        (usually <filename>/etc/nixos/configuration.nix</filename>)
+        and links it from the resulting system
+        (getting to <filename>/run/current-system/configuration.nix</filename>).
+        Note that only this single file is copied, even if it imports others.
+      '';
+    };
+
+    system.extraSystemBuilderCmds = mkOption {
+      type = types.lines;
+      internal = true;
+      default = "";
+      description = ''
+        This code will be added to the builder creating the system store path.
+      '';
+    };
+
+    system.extraDependencies = mkOption {
+      type = types.listOf types.package;
+      default = [];
+      description = ''
+        A list of packages that should be included in the system
+        closure but not otherwise made available to users. This is
+        primarily used by the installation tests.
+      '';
+    };
+
+    system.replaceRuntimeDependencies = mkOption {
+      default = [];
+      example = lib.literalExpression "[ ({ original = pkgs.openssl; replacement = pkgs.callPackage /path/to/openssl { }; }) ]";
+      type = types.listOf (types.submodule (
+        { ... }: {
+          options.original = mkOption {
+            type = types.package;
+            description = "The original package to override.";
+          };
+
+          options.replacement = mkOption {
+            type = types.package;
+            description = "The replacement package.";
+          };
+        })
+      );
+      apply = map ({ original, replacement, ... }: {
+        oldDependency = original;
+        newDependency = replacement;
+      });
+      description = ''
+        List of packages to override without doing a full rebuild.
+        The original derivation and replacement derivation must have the same
+        name length, and ideally should have close-to-identical directory layout.
+      '';
+    };
+
+    system.name = mkOption {
+      type = types.str;
+      default =
+        if config.networking.hostName == ""
+        then "unnamed"
+        else config.networking.hostName;
+      defaultText = literalExpression ''
+        if config.networking.hostName == ""
+        then "unnamed"
+        else config.networking.hostName;
+      '';
+      description = ''
+        The name of the system used in the <option>system.build.toplevel</option> derivation.
+        </para><para>
+        That derivation has the following name:
+        <literal>"nixos-system-''${config.system.name}-''${config.system.nixos.label}"</literal>
+      '';
+    };
+
+  };
+
+
+  config = {
+
+    system.extraSystemBuilderCmds =
+      optionalString
+        config.system.copySystemConfiguration
+        ''ln -s '${import ../../../lib/from-env.nix "NIXOS_CONFIG" <nixos-config>}' \
+            "$out/configuration.nix"
+        '';
+
+    system.build.toplevel = system;
+
+  };
+
+  # uses extendModules to generate a type
+  meta.buildDocsInSandbox = false;
+}