Many packages have some kind of flag indicating whether or not to build with
systemd support. Most of these default to `stdenv.isLinux`, but systemd does
not build on (and is marked `broken` for) `isStatic`. Only a few packages have
the needed `&& !isStatic` in the default value for their parameter.
This commit moves the logic for the default value of these flags into
`systemd.meta.{platforms,badPlatforms}` and evaluates those conditions using
`lib.meta.availableOn`.
This provides three benefits:
1. The default values are set correctly (i.e. including `&& isStatic`)
2. The default values are set consistently
3. The way is paved for any future non-Linux systemd platforms (FreeBSD is
reported to have experimental systemd support)
checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
This commit fixes a papercut in nixos-rebuild where people wanting to
switch to a specialisation (or test one) were forced to manually figure
out the specialisation's path and run its activation script - since now,
there's a dedicated option to do just that.
This is a backwards-compatible change which doesn't affect the existing
behavior, which - to be fair - might still be considered sus by some
people, the painful scenario here being:
- you boot into specialisation `foo`,
- you run `nixos-rebuild switch`,
- whoops, you're no longer at specialisation `foo`, but you're rather
brought back to the base system.
(it's especially painful for cases where specialisation is used to load
extra drivers, e.g. Nvidia, since then launching `nixos-rebuild switch`,
while forgetting that you're inside a specialisation, can cause some
parts of your system to get accidentally unloaded.)
I've tried to mitigate that by improving specialisations so that they
create a dedicated file somewhere in `/run/current-system` containing
the specialisation's name (which `nixos-rebuild` could then use as the
default value for `--specialisation`), but I haven't been able to come
up with anything working (plus it would be a breaking change then).
Closes https://github.com/NixOS/nixpkgs/issues/174065
