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fix(systemd): give the receiver service its own VT instead of tty1

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The receiver service never started on the Pi: systemd blocks before
exec in acquire_terminal(), waiting for a controlling terminal that
the console session or getty on tty1 already owns. Symptoms: the
status shows the main PID as "(screencast)" with Tasks:1 and ~40ms
CPU, a silent journal, and no mDNS announcement — the binary never
ran. Reproduced locally with a transient unit (the desktop's Wayland
session owns tty1) and diagnosed by gdb-attaching the stuck process.

The service now owns a dedicated free VT (tty7) so acquisition is
immediate, with a tolerant ExecStartPre chvt to make the screencast
the on-screen console at boot. tty1 keeps the console login;
Ctrl+Alt+F1/Ctrl+Alt+F7 switch between them. The previous advice to
enable Console Autologin actively caused the hang and is removed
from the unit, RUNBOOK, and install script notes.

Validated locally: the same properties via systemd-run give a
running receiver with all ~15 threads alive that announces itself
and is found by --discover; verified unit passes systemd-analyze
verify; graceful stop withdraws mDNS.
This commit is contained in:
Florian Egger
2026-09-07 13:45:47 +02:00
parent 414c2c39ee
commit 343c6b45e6
4 changed files with 33 additions and 12 deletions
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docs/RUNBOOK.md
+6 -2
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@@ -96,9 +96,13 @@ The receiver does not need X11, Wayland, or any window manager: SDL3's
KMSDRM backend renders straight to the kernel display pipeline. This is
the recommended setup for small boards (Raspberry Pi OS **Lite**):
1. `sudo raspi-config` → System Options → Boot → **Console Autologin**.
2. No desktop may be enabled — a running compositor holds the DRM master
1. No desktop may be enabled — a running compositor holds the DRM master
and the receiver cannot take it (the two modes are mutually exclusive).
No console autologin is needed either.
2. The service renders on its **own VT (tty7)** and switches to it at
boot: `Ctrl+Alt+F1` returns to the console login, `Ctrl+Alt+F7` back
to the screencast. (A service must not target tty1: acquiring a tty
that the console session or getty owns blocks the start forever.)
3. Disable console blanking: append `consoleblank=0` to
`/boot/firmware/cmdline.txt` and reboot.