cosmic-greeter 1.9.0 panics with RuntimeDirNotSet – greeter never starts (works fine on 1.8.0)

[dennispfueller]

After upgrading cosmic-greeter from 1.8.0 to 1.9.0, the greeter fails to start on every boot. The screen stays black (only a subtle flicker), and cosmic-comp immediately panics:

thread 'main' panicked at 'called Result::unwrap() on an Err value: RuntimeDirNotSet': src/lib.rs:302

greetd then reports check_children: greeter exited without creating a session, retries 5 times, and hits systemd's start-limit (cosmic-greeter.service: Failed with result 'start-limit-hit').

Downgrading cosmic-greeter back to 1.8.0 resolves the issue completely — the greeter starts normally again.

To Reproduce

Run Arch Linux with greetd + cosmic-greeter 1.9.0 and systemd 262.
Boot the system.
Observe: screen stays black, cosmic-greeter.service crash-loops.

Expected behavior

The greeter starts normally and shows the login UI, as it did on 1.9.0's predecessor (1.8.0).

Logs

journalctl -u cosmic-greeter -b:

Sep 27 12:42:06 archlinux greetd[794]: config: Config { ... default_session: ConfigSession { command: "cosmic-greeter-start", user: "cosmic-greeter", service: "greetd-greeter" }, ... }
Sep 27 12:42:06 archlinux cosmic-comp[821]: Unable to become drm master, assuming unprivileged mode
Sep 27 12:42:07 archlinux greetd[794]: error: check_children: greeter exited without creating a session
Sep 27 12:42:07 archlinux systemd[1]: cosmic-greeter.service: Deactivated successfully.
Sep 27 12:42:07 archlinux systemd[1]: cosmic-greeter.service: Scheduled restart job, restart counter is at 1.
[...repeats 5 times, then...]
Sep 27 12:42:16 archlinux systemd[1]: cosmic-greeter.service: Failed with result 'start-limit-hit'.

Manually running sudo -u cosmic-greeter cosmic-greeter-start:

thread 'main' panicked at 'called Result::unwrap() on an Err value: RuntimeDirNotSet': src/lib.rs:302

Confirmed /run/user/ is never created by systemd-logind, even though logind does log a new session for the cosmic-greeter user each attempt (New session 'N' of user 'cosmic-greeter' with class 'greeter' and type 'tty').

System Info

OS: Arch Linux
Kernel: 7.2.7-arch1-1
greetd: 0.10.3-2
cosmic-greeter: 1:1.9.0-1 (broken) / 1:1.8.0-1 (working)
systemd: 262-1
GPU: AMD Ryzen 7 7800X3D (Raphael iGPU) + AMD Radeon RX 6650 XT (dGPU)
/etc/pam.d/greetd uses standard include system-local-login, which includes session optional pam_systemd.so — unmodified, matches pambase 20260616-1

Workaround

Downgrading to cosmic-greeter 1.8.0-1 fixes the issue immediately with no other changes.