move systemd related code to separate modules

checks/shfmt.nix

@@ -3,6 +3,6 @@
 , ...
 }:
 runCommand "check-shfmt" { nativeBuildInputs = [ shfmt ]; } ''
-  shfmt -i 2 -d ${./../scripts}/*.sh
+  shfmt -i 2 -d $(find ${./..} -name '*.sh')
   touch $out
 ''

modules/default.nix

@@ -5,6 +5,7 @@
     ./installer.nix
     ./interop.nix
     ./recovery.nix
+    ./systemd
     ./version.nix
     ./wsl-conf.nix
     ./wsl-distro.nix

modules/recovery.nix

@@ -24,7 +24,7 @@ in
   config = {
     wsl.extraBin = [
       # needs to be a copy, not a symlink, to be executable from outside
-      { src = "${recovery}/bin/nixos-wsl-recovery"; copy = true;}
+      { src = "${recovery}/bin/nixos-wsl-recovery"; copy = true; }
     ];
   };
 

modules/systemd/default.nix

@@ -0,0 +1,50 @@
+{ config, pkgs, lib, ... }:
+with lib; {
+
+  imports = [
+    ./native
+    ./syschdemd
+  ];
+
+  options.wsl = with types; {
+    nativeSystemd = mkOption {
+      type = bool;
+      default = false;
+      description = "Use native WSL systemd support";
+    };
+  };
+
+  config =
+    let
+      cfg = config.wsl;
+    in
+    mkIf (cfg.enable) {
+
+      wsl.binShPkg = if cfg.nativeSystemd then bashWrapper else pkgs.bashInteractive;
+
+      # systemd-oomd requires cgroup pressure info which WSL doesn't have
+      systemd.oomd.enable = false;
+
+      # useful for usbip but adds a dependency on various firmwares which are combined over 300 MB big
+      services.udev.enable = lib.mkDefault false;
+
+      systemd = {
+        # Disable systemd units that don't make sense on WSL
+        services = {
+          firewall.enable = false;
+          systemd-resolved.enable = lib.mkDefault false;
+          # systemd-timesyncd actually works in WSL and without it the clock can drift
+          systemd-timesyncd.unitConfig.ConditionVirtualization = "";
+        };
+
+        # Don't allow emergency mode, because we don't have a console.
+        enableEmergencyMode = false;
+
+        # Link the X11 socket into place. This is a no-op on a normal setup,
+        # but helps if /tmp is a tmpfs or mounted from some other location.
+        tmpfiles.rules = [ "L /tmp/.X11-unix - - - - ${cfg.wslConf.automount.root}/wslg/.X11-unix" ];
+      };
+
+    };
+
+}

modules/systemd/native/default.nix

@@ -0,0 +1,37 @@
+{ config, pkgs, lib, ... }:
+with lib; {
+
+  config =
+    let
+      cfg = config.wsl;
+      nativeUtils = pkgs.callPackage ../scripts/native-utils { };
+    in
+    mkIf (cfg.enable && cfg.nativeSystemd) {
+      wsl.wslConf = {
+        user.default = config.users.users.${cfg.defaultUser}.name;
+        boot.systemd = true;
+      };
+
+      system.activationScripts = {
+        shimSystemd = stringAfter [ ] ''
+          echo "setting up /sbin/init shim..."
+          mkdir -p /sbin
+          ln -sf ${nativeUtils}/bin/systemd-shim /sbin/init
+        '';
+        setupLogin = lib.mkIf cfg.populateBin (stringAfter [ ] ''
+          echo "setting up /bin/login..."
+          mkdir -p /bin
+          ln -sf ${pkgs.shadow}/bin/login /bin/login
+        '');
+      };
+
+      environment = {
+        # preserve $PATH from parent
+        variables.PATH = [ "$PATH" ];
+        extraInit = ''
+          eval $(${nativeUtils}/bin/split-path --automount-root="${cfg.wslConf.automount.root}" ${lib.optionalString cfg.interop.includePath "--include-interop"})
+        '';
+      };
+    };
+
+}

modules/systemd/syschdemd/default.nix

@@ -0,0 +1,37 @@
+{ config, pkgs, lib, ... }:
+with lib; {
+
+  options = { };
+
+  config =
+    let
+      cfg = config.wsl;
+
+      syschdemd = pkgs.callPackage ./syschdemd.nix {
+        automountPath = cfg.wslConf.automount.root;
+        defaultUser = config.users.users.${cfg.defaultUser};
+      };
+    in
+    mkIf (cfg.enable && !cfg.nativeSystemd) {
+
+      users.users.root.shell = "${syschdemd}/bin/syschdemd";
+      security.sudo.extraConfig = ''
+        Defaults env_keep+=INSIDE_NAMESPACE
+      '';
+      wsl.wslConf.user.default = "root";
+
+      # Start a systemd user session when starting a command through runuser
+      security.pam.services.runuser.startSession = true;
+
+      # Include Windows %PATH% in Linux $PATH.
+      environment.extraInit = mkIf cfg.interop.includePath ''PATH="$PATH:$WSLPATH"'';
+      environment.systemPackages = [
+        (pkgs.runCommand "wslpath" { } ''
+          mkdir -p $out/bin
+          ln -s /init $out/bin/wslpath
+        '')
+      ];
+
+    };
+
+}