This should be removed after systemd 258 releases, as it is a temporary drop-in fix to circumvent the issue of systemd-oomd failing to detect a valid swap (noticeable in cases of users using `zram` as swap).
I've taken the liberty to clarify which `After=` override should be dropped when, based on the systemd version on Nixpkgs.
Silences 2 warning messages that appear when using the systemd initrd:
1. "System tainted (var-run-bad)": occurs because `/var/run` isn't a
symlink to `/run`. Fixed by making /run and linking /var/run to it.
2. "Failed to make /usr a mountpoint": occurs because ProtectSystem
defaults to true in the initrd, which makes systemd try to remount
`/usr` as read-only, which doesn't exist in the initrd. Fixed by
linking `/usr/bin` and `/usr/sbin` to the initrd bin directories.
Also moves the `/tmp` creation from the initrd module to make-initrd-ng,
to avoid making an unnecessary `/tmp/.keep`, saving a store path and a
few bytes in the initrd image.
When a disposition is not set in a user record, systemd determines user
disposition depending on the range the user's UID falls in. For system
users with UIDs above 1000, this will cause them to be incorrectly
identified as "regular" users.
This will cause `userctl` to report the user as a regular user, and more
importantly, `systemd-homed` will not run the first boot user creation
flow, as regular users are already present on the machine (when they are
really system users).
The most common source of high UID system users will undoubtedly be Nix
build users, so the warning provides additional guidance on how to
remove them or adjust their IDs to be within the system range.
The warning is shown only when userdbd/homed is enabled, and the option
to hide the warning is deliberately hidden, to ensure users will have to
read and acknowledge the warning before proceeding, as otherwise users
could end up deploying an OS with no users and no way of creating one
due to the first boot flow being skipped.
Allow building a systemd initrd with a kernel that does not have
modules support enabled (`CONFIG_MODULES=n`), by removing the
assertion and only include the modulesClosure, kmod and support files
if MODULES is enabled or unset in the kernel.
This ensures the tmpfiles resetup service has permissions
to create suid/sgid files, even if `DefaultRestrictSUIDSGID`
is set in system.conf. This is required, as tmpfiles
are used to e.g. set file permissions on the journal
directory.`DefaultRestrictSUIDSGID` is a new feature
coming in systemd 258 [1].
[1] https://github.com/systemd/systemd/pull/38126
Before this change, systemd-oomd startup was flaky at least with
either systemd-sysusers or userborn enabled. It would restart several
times until users were provisioned, so that it finally succeeded.
An alternative would be to use a DynamicUser which was my first
approach, before I discovered that upstream added the after statement
in Dec 2024[1]. DynamicUsers could have further
implications (sandboxing, etc), so we follow upstream here.
It's not clear to me we why Upstreams "After=systemd-sysusers.service"
doesn't show up on nixos-unstable systems (systemd v257.6).
Userborn is covered, as its unit is aliased to systemd-sysusers.service.
The following test succeeded after this change on x86_64-linux:
nix-build -A nixosTests.systemd-oomd
[1]: 36dd429680
