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.
This reverts commit 469f22d717, reversing
changes made to 0078bc5d8f.
Conflicts:
nixos/modules/installer/tools/nixos-generate-config.pl
nixos/modules/system/boot/loader/grub/install-grub.pl
nixos/release.nix
nixos/tests/installer.nix
I tried to keep apparently-safe code in conflicts.
Dmidecode fails in our EFI test with the error "SMBIOS entry point
missing". But we don't need dmidecode because we have already have
systemd-detect-virt.
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.
