I think pam_lastlog is the only thing that writes to these files in
practice on a modern Linux system, so in a configuration that doesn't
use that module, we don't need to create these files.
I used tmpfiles.d instead of activation snippets to create the logs.
It's good enough for upstream and other distros; it's probably good
enough for us.
Nix 2.0 no longer uses these directories.
/run/nix/current-load was moved to /nix/var/nix/current-load in 2017
(Nix commit d7653dfc6dea076ecbe00520c6137977e0fced35). Anyway,
src/build-remote/build-remote.cc will create the current-load directory
if it doesn't exist already.
/run/nix/remote-stores seems to have been deprecated since 2014 (Nix
commit b1af336132cfe8a6e4c54912cc512f8c28d4ebf3) when the documentation
for $NIX_OTHER_STORES was removed, and support for it was dropped
entirely in 2016 (Nix commit 4494000e04122f24558e1436e66d20d89028b4bd).
The default value for journald's Storage option is "auto", which
determines whether to log to /var/log/journal based on whether that
directory already exists. So NixOS has been unconditionally creating
that directory in activation scripts.
However, we can get the same behavior by configuring journald.conf to
set Storage to "persistent" instead. In that case, journald will create
the directory itself if necessary.
Previously, the activation script was responsible for ensuring that
/etc/machine-id exists. However, the only time it could not already
exist is during stage-2-init, not while switching configurations,
because one of the first things systemd does when starting up as PID 1
is to create this file. So I've moved the initialization to
stage-2-init.
Furthermore, since systemd will do the equivalent of
systemd-machine-id-setup if /etc/machine-id doesn't have valid contents,
we don't need to do that ourselves.
We _do_, however, want to ensure that the file at least exists, because
systemd also uses the non-existence of this file to guess that this is a
first-boot situation. In that case, systemd tries to create some
symlinks in /etc/systemd/system according to its presets, which it can't
do because we've already populated /etc according to the current NixOS
configuration.
This is not necessary for any other activation script snippets, so it's
okay to do it after stage-2-init runs the activation script. None of
them declare a dependency on the "systemd" snippet. Also, most of them
only create files or directories in ways that obviously don't need the
machine-id set.
