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.
This option requests compatibility with older NixOS releases with
respect to stateful data, in cases where new releases have defaults
that might be incompatible with system state of existing NixOS
deployments. For instance, if we change the default version of
PostgreSQL, existing deployments will break if the new version can't
read databases created by the old version.
So for example, setting
system.stateVersion = "15.07";
requests that options like services.postgresql.package use defaults
corresponding to the 15.07 release branch. Note that
nixos-generate-config emits this option. (In the future, NixOps may
set system.stateVersion to the NixOS release in use when the machine
was created.)
See also #7939 for another motivating example.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
Note that there is a subtle difference in Nix that causes
nixos-rebuild to work and NixOps to fail:
$ nix-instantiate '<nixos>' -A config.system.nixosVersion --eval-only
"13.10pre34915.50f4822"
$ nix-instantiate '<nixos/default.nix>' -A config.system.nixosVersion --eval-only
error: opening file `/nix/var/nix/profiles/per-user/root/channels/nixos/.version': No such file or directory
Fixes NixOS/nixops#145.
