nixos module: support NixOS user packages install

When using the NixOS module we cannot guarantee that the Nix store
will be writable during startup. Installing the user packages through
`nix-env -i` will fail in these cases.

This commit adds a NixOS option `home-manager.useUserPackages` that,
when enabled, installs user packages through the NixOS

    users.users.<name?>.packages

option.

Note, when submodule support and external package install is enabled
then the installed packages are not available in `~/.nix-profile`. We
therefore set `home.profileDirectory` directly to the HM profile
packages.

modules/home-environment.nix

@@ -269,7 +269,11 @@ in
     home.username = mkDefault (builtins.getEnv "USER");
     home.homeDirectory = mkDefault (builtins.getEnv "HOME");
 
-    home.profileDirectory = cfg.homeDirectory + "/.nix-profile";
+    home.profileDirectory =
+      if config.submoduleSupport.enable
+        && config.submoduleSupport.externalPackageInstall
+      then config.home.path
+      else cfg.homeDirectory + "/.nix-profile";
 
     home.sessionVariables =
       let
@@ -307,9 +311,33 @@ in
     home.activation.writeBoundary = dag.entryAnywhere "";
 
     # Install packages to the user environment.
-    home.activation.installPackages = dag.entryAfter ["writeBoundary"] ''
-      $DRY_RUN_CMD nix-env -i ${cfg.path}
-    '';
+    #
+    # Note, sometimes our target may not allow modification of the Nix
+    # store and then we cannot rely on `nix-env -i`. This is the case,
+    # for example, if we are running as a NixOS module and building a
+    # virtual machine. Then we must instead rely on an external
+    # mechanism for installing packages, which in NixOS is provided by
+    # the `users.users.<name?>.packages` option. The activation
+    # command is still needed since some modules need to run their
+    # activation commands after the packages are guaranteed to be
+    # installed.
+    #
+    # In case the user has moved from a user-install of Home Manager
+    # to a submodule managed one we attempt to uninstall the
+    # `home-manager-path` package if it is installed.
+    home.activation.installPackages = dag.entryAfter ["writeBoundary"] (
+      if config.submoduleSupport.externalPackageInstall
+      then
+        ''
+          if nix-env -q | grep '^home-manager-path$'; then
+            $DRY_RUN_CMD nix-env -e home-manager-path
+          fi
+        ''
+      else
+        ''
+          $DRY_RUN_CMD nix-env -i ${cfg.path}
+        ''
+    );
 
     home.activationPackage =
       let

modules/misc/news.nix

@@ -966,6 +966,24 @@ in
           as an Emacs daemon.
         '';
       }
+
+      {
+        time = "2019-02-16T20:33:56+00:00";
+        condition = hostPlatform.isLinux;
+        message = ''
+          When using Home Manager as a NixOS submodule it is now
+          possible to install packages using the NixOS
+
+              users.users.<name?>.packages
+
+          option. This is enabled by adding
+
+              home-manager.useUserPackages = true;
+
+          to your NixOS system configuration. This mode of operation
+          is necessary if you want to use 'nixos-rebuild build-vm'.
+        '';
+      }
     ];
   };
 }

modules/misc/submodule-support.nix

@@ -15,5 +15,18 @@ with lib;
         in, for example, NixOS or nix-darwin.
       '';
     };
+
+    externalPackageInstall = mkOption {
+      type = types.bool;
+      default = false;
+      internal = true;
+      description = ''
+        Whether the packages of <option>home.packages</option> are
+        installed separately from the Home Manager activation script.
+        In NixOS, for example, this may be accomplished by installing
+        the packages through
+        <option>users.users.&lt;name?&gt;.packages</option>.
+      '';
+    };
   };
 }

nixos/default.nix

@@ -11,6 +11,7 @@ let
 
     config = {
       submoduleSupport.enable = true;
+      submoduleSupport.externalPackageInstall = cfg.useUserPackages;
       home.username = config.users.users.${name}.name;
       home.homeDirectory = config.users.users.${name}.home;
     };
@@ -20,16 +21,29 @@ in
 
 {
   options = {
-    home-manager.users = mkOption {
-      type = types.attrsOf hmModule;
-      default = {};
-      description = ''
-        Per-user Home Manager configuration.
+    home-manager = {
+      useUserPackages = mkEnableOption ''
+        installation of user packages through the
+        <option>users.users.&lt;name?&gt;.packages</option> option.
       '';
+
+      users = mkOption {
+        type = types.attrsOf hmModule;
+        default = {};
+        description = ''
+          Per-user Home Manager configuration.
+        '';
+      };
     };
   };
 
   config = mkIf (cfg.users != {}) {
+    users.users = mkIf cfg.useUserPackages (
+      mapAttrs (username: usercfg: {
+        packages = usercfg.home.packages;
+      }) cfg.users
+    );
+
     systemd.services = mapAttrs' (username: usercfg:
       nameValuePair ("home-manager-${utils.escapeSystemdPath username}") {
         description = "Home Manager environment for ${username}";