fix(udev): scope SIGUSR2 hotplug signal to ceralive.service main pid (stop killing avahi/mDNS)

[andrescera]

What

The two USB/audio hotplug udev rules now deliver SIGUSR2 to the CeraUI backend via a unit-scoped systemctl kill, instead of a broad pkill -f:

- RUN+="/usr/bin/pkill -o -SIGUSR2 -f ceralive"
+ RUN+="/usr/bin/systemctl kill --kill-whom=main --signal=SIGUSR2 ceralive.service"

• deployment/99-ceralive-check-usb-devices.rules (Elgato/Cam Link, vendor 0fd9)
• deployment/98-ceralive-audio.rules (the single RUN+= under LABEL="signal_ceralive", reached by all 5 GOTO branches + the usbaudio fall-through)

main.ts's SIGUSR2 handler is unchanged — only the delivery mechanism is fixed.

Why

pkill -f ceralive substring-matches the full command line of every process. Once avahi registers a hostname it renames its process title to avahi-daemon: registering [<hostname>.local]; this device's hostname is ceralive.local, so avahi-daemon's title contains ceralive and was caught by the same broad pkill. SIGUSR2's default disposition is terminate, and avahi-daemon.service has no Restart=, so mDNS (ceralive.local resolution) stayed dead until reboot — the "mDNS keeps dying" complaint.

Confirmed live on real hardware via journalctl — both the backend (PID 544) and avahi-daemon (PID 453) received SIGUSR2 at the same udev event:

avahi-daemon.service: Main process exited, code=killed, status=12/USR2
avahi-daemon.service: Failed with result 'signal'.
ceralive[544]: {"level":"error","msg":"SIGUSR2"}

systemctl kill targets the unit's cgroup — zero substring ambiguity — the same idiom already used by ceralive-addon-reconciler.service. --kill-whom=main scopes the signal to the tracked MainPID, preserving the old pkill -o single-process intent. This matters: srtla_send/bcrpt are spawned by the backend and share ceralive.service's cgroup while streaming, and neither handles SIGUSR2 — so the whole-cgroup default (all) would terminate the active stream on every hotplug. systemctl kill is safe from a udev RUN+= rule: it sends the signal via a synchronous PID-1 D-Bus call and creates no systemd job, so the --no-block/deadlock caveats that apply to start/stop/restart do not apply.

How to verify

• cd apps/backend && bun test src/tests/udev-rules-sigusr2-scope.test.ts — new static regression guard (4 tests) over the shipped rule files: no pkill -f ceralive remains, both rules carry the scoped systemctl kill --kill-whom=main --signal=SIGUSR2 ceralive.service idiom, and every audio-rule GOTO path converges on the single scoped RUN+=.
• cd apps/backend && bun tsc --noEmit — clean.
• biome check — clean on changed files.
• On device: plug/unplug a Cam Link or USB-audio device and confirm avahi-daemon stays up (systemctl is-active avahi-daemon) and ceralive.local still resolves, while the backend still re-scans devices.

Risks

• Low. The rules only change how the existing SIGUSR2 is delivered; the handler and the hotplug re-scan behavior are unchanged.
• --kill-whom=main requires systemd >= 252 (Debian bookworm floor); the device already relies on modern systemctl (ceralive-addon-reconciler.service).
• Complementary hardening (Restart=on-failure on avahi-daemon.service) is handled separately in image-building-pipeline as defense-in-depth.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on this repository. Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: ed6bafb3-9d32-4f88-973b-c0507de48d83

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/udev-sigusr2-scoped-kill

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}