nixos: introduce system.nixosLabel option and use it where appropriate

Setting nixosVersion to something custom is useful for meaningful GRUB
menus and /nix/store paths, but actuallly changing it rebulids the
whole system path (because of `nixos-version` script and manual
pages). Also, changing it is not a particularly good idea because you
can then be differentitated from other NixOS users by a lot of
programs that read /etc/os-release.

This patch introduces an alternative option that does all you want
from nixosVersion, but rebuilds only the very top system level and
/etc while using your label in the names of system /nix/store paths,
GRUB and other boot loaders' menus, getty greetings and so on.

1 files changed, 14 insertions, 6 deletions

diff --git a/nixos/modules/services/ttys/agetty.nix b/nixos/modules/services/ttys/agetty.nix
index 85ee23c1a3dd..ea7196fc8733 100644
--- a/nixos/modules/services/ttys/agetty.nix
+++ b/nixos/modules/services/ttys/agetty.nix
@@ -2,6 +2,13 @@
 
 with lib;
 
+let
+
+  autologinArg = optionalString (config.services.mingetty.autologinUser != null) "--autologin ${config.services.mingetty.autologinUser}";
+  gettyCmd = extraArgs: "@${pkgs.utillinux}/sbin/agetty agetty --login-program ${pkgs.shadow}/bin/login ${autologinArg} ${extraArgs}";
+
+in
+
 {
 
   ###### interface
@@ -21,9 +28,9 @@ with lib;
 
       greetingLine = mkOption {
         type = types.str;
-        default = ''<<< Welcome to NixOS ${config.system.nixosVersion} (\m) - \l >>>'';
         description = ''
           Welcome line printed by mingetty.
+          The default shows current NixOS version label, machine type and tty.
         '';
       };
 
@@ -55,10 +62,11 @@ with lib;
 
   ###### implementation
 
-  config = let
-    autologinArg = optionalString (config.services.mingetty.autologinUser != null) "--autologin ${config.services.mingetty.autologinUser}";
-    gettyCmd = extraArgs: "@${pkgs.utillinux}/sbin/agetty agetty --login-program ${pkgs.shadow}/bin/login ${autologinArg} ${extraArgs}";
-  in {
+  config = {
+    # Note: this is set here rather than up there so that changing
+    # nixosLabel would not rebuild manual pages
+    services.mingetty.greetingLine = mkDefault ''<<< Welcome to NixOS ${config.system.nixosLabel} (\m) - \l >>>'';
+
     systemd.services."getty@" =
       { serviceConfig.ExecStart = gettyCmd "--noclear --keep-baud %I 115200,38400,9600 $TERM";
         restartIfChanged = false;
@@ -81,7 +89,7 @@ with lib;
       { serviceConfig.ExecStart = gettyCmd "--noclear --keep-baud console 115200,38400,9600 $TERM";
         serviceConfig.Restart = "always";
         restartIfChanged = false;
-	enable = mkDefault config.boot.isContainer;
+        enable = mkDefault config.boot.isContainer;
       };
 
     environment.etc = singleton