treewide: rewrite stateVersion docs (again), clean up some stateVersion usages (again)

nixos/maintainers/scripts/azure-new/examples/basic/system.nix

@@ -21,7 +21,6 @@ in
 
   virtualisation.azureImage.diskSize = 2500;
 
-  system.stateVersion = "20.03";
   boot.kernelPackages = pkgs.linuxPackages_latest;
 
   # test user doesn't have a password

nixos/maintainers/scripts/lxd/lxd-container-image-inner.nix

@@ -2,19 +2,19 @@
 # your system.  Help is available in the configuration.nix(5) man page
 # and in the NixOS manual (accessible by running ‘nixos-help’).
 
-{ config, pkgs, lib, ... }:
+{ config, pkgs, lib, modulesPath, ... }:
 
 {
   imports =
     [
       # Include the default lxd configuration.
-      ../../../modules/virtualisation/lxc-container.nix
+      "${modulesPath}/modules/virtualisation/lxc-container.nix"
       # Include the container-specific autogenerated configuration.
       ./lxd.nix
     ];
 
   networking.useDHCP = false;
   networking.interfaces.eth0.useDHCP = true;
 
-  system.stateVersion = "21.05"; # Did you read the comment?
+  system.stateVersion = "@stateVersion@"; # Did you read the comment?
 }

nixos/maintainers/scripts/lxd/lxd-container-image.nix

@@ -13,11 +13,15 @@
   };
 
   # copy the config for nixos-rebuild
-  system.activationScripts.config = ''
+  system.activationScripts.config = let
+    config = pkgs.substituteAll {
+      src = ./lxd-container-image-inner.nix;
+      stateVersion = lib.trivial.release;
+    };
+  in ''
     if [ ! -e /etc/nixos/configuration.nix ]; then
       mkdir -p /etc/nixos
-      cat ${./lxd-container-image-inner.nix} > /etc/nixos/configuration.nix
-      ${lib.getExe pkgs.gnused} 's|../../../modules/virtualisation/lxc-container.nix|<nixpkgs/nixos/modules/virtualisation/lxc-container.nix>|g' -i /etc/nixos/configuration.nix
+      cp ${config} /etc/nixos/configuration.nix
     fi
   '';
 

nixos/maintainers/scripts/lxd/lxd-virtual-machine-image-inner.nix

@@ -2,19 +2,19 @@
 # your system.  Help is available in the configuration.nix(5) man page
 # and in the NixOS manual (accessible by running ‘nixos-help’).
 
-{ config, pkgs, lib, ... }:
+{ config, pkgs, lib, modulesPath, ... }:
 
 {
   imports =
     [
       # Include the default lxd configuration.
-      ../../../modules/virtualisation/lxd-virtual-machine.nix
+      "${modulesPath}/virtualisation/lxd-virtual-machine.nix"
       # Include the container-specific autogenerated configuration.
       ./lxd.nix
     ];
 
   networking.useDHCP = false;
   networking.interfaces.eth0.useDHCP = true;
 
-  system.stateVersion = "23.05"; # Did you read the comment?
+  system.stateVersion = "@stateVersion@"; # Did you read the comment?
 }

nixos/maintainers/scripts/lxd/lxd-virtual-machine-image.nix

@@ -13,11 +13,15 @@
   };
 
   # copy the config for nixos-rebuild
-  system.activationScripts.config = ''
+  system.activationScripts.config = let
+    config = pkgs.substituteAll {
+      src = ./lxd-virtual-machine-image-inner.nix;
+      stateVersion = lib.trivial.release;
+    };
+  in ''
     if [ ! -e /etc/nixos/configuration.nix ]; then
       mkdir -p /etc/nixos
-      cat ${./lxd-virtual-machine-image-inner.nix} > /etc/nixos/configuration.nix
-      ${lib.getExe pkgs.gnused} 's|../../../modules/virtualisation/lxd-virtual-machine.nix|<nixpkgs/nixos/modules/virtualisation/lxd-virtual-machine.nix>|g' -i /etc/nixos/configuration.nix
+      cp ${config} /etc/nixos/configuration.nix
     fi
   '';
 

nixos/modules/installer/tools/tools.nix

@@ -224,12 +224,22 @@ in
         # accidentally delete configuration.nix.
         # system.copySystemConfiguration = true;
 
-        # This value determines the NixOS release from which the default
-        # settings for stateful data, like file locations and database versions
-        # on your system were taken. It's perfectly fine and recommended to leave
-        # this value at the release version of the first install of this system.
-        # Before changing this value read the documentation for this option
-        # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
+        # This option defines the first version of NixOS you have installed on this particular machine,
+        # and is used to maintain compatibility with application data (e.g. databases) created on older NixOS versions.
+        #
+        # Most users should NEVER change this value after the initial install, for any reason,
+        # even if you've upgraded your system to a new NixOS release.
+        #
+        # This value does NOT affect the Nixpkgs version your packages and OS are pulled from,
+        # so changing it will NOT upgrade your system.
+        #
+        # This value being lower than the current NixOS release does NOT mean your system is
+        # out of date, out of support, or vulnerable.
+        #
+        # Do NOT change this value unless you have manually inspected all the changes it would make to your configuration,
+        # and migrated your data accordingly.
+        #
+        # For more information, see `man configuration.nix` or https://nixos.org/manual/nixos/stable/options#opt-system.stateVersion .
         system.stateVersion = "${config.system.nixos.release}"; # Did you read the comment?
 
       }

nixos/modules/installer/virtualbox-demo.nix

@@ -21,7 +21,7 @@ with lib;
   services.xserver.videoDrivers = mkOverride 40 [ "virtualbox" "vmware" "cirrus" "vesa" "modesetting" ];
 
   powerManagement.enable = false;
-  system.stateVersion = mkDefault "18.03";
+  system.stateVersion = lib.mkDefault lib.trivial.release;
 
   installer.cloneConfigExtra = ''
   # Let demo build as a trusted user.

nixos/modules/misc/version.nix

@@ -121,22 +121,32 @@ in
       default = cfg.release;
       defaultText = literalExpression "config.${opt.release}";
       description = lib.mdDoc ''
-        Every once in a while, a new NixOS release may change
-        configuration defaults in a way incompatible with stateful
-        data. For instance, if the default version of PostgreSQL
-        changes, the new version will probably be unable to read your
-        existing databases. To prevent such breakage, you should set the
-        value of this option to the NixOS release with which you want
-        to be compatible. The effect is that NixOS will use
-        defaults corresponding to the specified release (such as using
-        an older version of PostgreSQL).
-        It’s perfectly fine and recommended to leave this value at the
-        release version of the first install of this system.
-        Changing this option will not upgrade your system. In fact it
-        is meant to stay constant exactly when you upgrade your system.
-        You should only bump this option, if you are sure that you can
-        or have migrated all state on your system which is affected
-        by this option.
+        This option defines the first version of NixOS you have installed on this particular machine,
+        and is used to maintain compatibility with application data (e.g. databases) created on older NixOS versions.
+
+        For example, if NixOS version XX.YY ships with AwesomeDB version N by default, and is then
+        upgraded to version XX.YY+1, which ships AwesomeDB version N+1, the existing databases
+        may no longer be compatible, causing applications to fail, or even leading to data loss.
+
+        The `stateVersion` mechanism avoids this situation by making the default version of such packages
+        conditional on the first version of NixOS you've installed (encoded in `stateVersion`), instead of
+        simply always using the latest one.
+
+        Note that this generally only affects applications that can't upgrade their data automatically -
+        applications and services supporting automatic migrations will remain on latest versions when
+        you upgrade.
+
+        Most users should **never** change this value after the initial install, for any reason,
+        even if you've upgraded your system to a new NixOS release.
+
+        This value does **not** affect the Nixpkgs version your packages and OS are pulled from,
+        so changing it will **not** upgrade your system.
+
+        This value being lower than the current NixOS release does **not** mean your system is
+        out of date, out of support, or vulnerable.
+
+        Do **not** change this value unless you have manually inspected all the changes it would
+        make to your configuration, and migrated your data accordingly.
       '';
     };
 

nixos/modules/virtualisation/nixos-containers.nix

@@ -754,7 +754,7 @@ in
                   { services.postgresql.enable = true;
                     services.postgresql.package = pkgs.postgresql_14;
 
-                    system.stateVersion = "21.05";
+                    system.stateVersion = "${lib.trivial.release}";
                   };
               };
           }
@@ -906,4 +906,6 @@ in
       "tun"
     ];
   });
+
+  meta.buildDocsInSandbox = false;
 }

nixos/release.nix

@@ -398,7 +398,7 @@ in rec {
         modules = singleton ({ ... }:
           { fileSystems."/".device  = mkDefault "/dev/sda1";
             boot.loader.grub.device = mkDefault "/dev/sda";
-            system.stateVersion = mkDefault "18.03";
+            system.stateVersion = mkDefault lib.trivial.release;
           });
       }).config.system.build.toplevel;
       preferLocalBuild = true;

nixos/tests/containers-imperative.nix

@@ -21,9 +21,7 @@ import ./make-test-python.nix ({ pkgs, lib, ... }: {
           modules = lib.singleton {
             nixpkgs = { inherit (config.nixpkgs) localSystem; };
 
-            containers.foo.config = {
-              system.stateVersion = "18.03";
-            };
+            containers.foo.config = {};
           };
 
           # The system is inherited from the host above.

nixos/tests/nextcloud/basic.nix

@@ -37,8 +37,6 @@ in {
         "d /var/lib/nextcloud-data 0750 nextcloud nginx - -"
       ];
 
-      system.stateVersion = "22.11"; # stateVersion >=21.11 to make sure that we use OpenSSL3
-
       services.nextcloud = {
         enable = true;
         datadir = "/var/lib/nextcloud-data";