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

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.

.agents/MEMORY.md

@@ -34,9 +34,18 @@ loopback; current phase is Phase 7.
 - **systemd autostart**: `systemd/screencast-receiver.service` is a
   template (`__SC_RECEIVER_BIN__`/`__SC_RECEIVER_USER__`); the install
   script substitutes and enables it (opt out: `SC_RECEIVER_SERVICE=0`),
-  adding the run user to video/render/input for headless KMSDRM. The unit
-  gets a controlling tty (StandardInput=tty, tty1) because SDL's KMSDRM
-  backend needs one for VT handling.
+  adding the run user to video/render/input for headless KMSDRM.
+- **systemd tty trap (hit on the real Pi, reproduced locally)**: with
+  StandardInput=tty + TTYPath=tty1, the service NEVER started — systemd
+  blocks PRE-EXEC in acquire_terminal() waiting for a tty that the console
+  session/getty already owns. Symptoms: status shows the main PID as
+  "(screencast)" with Tasks:1 and ~40ms CPU, silent journal, no mDNS.
+  Diagnosed by gdb-attaching the stuck process (acquire_terminal backtrace)
+  after reproducing with a local transient unit. Fix: the service owns a
+  dedicated free VT (tty7) + tolerant `ExecStartPre=-/usr/bin/chvt 7`;
+  tty1 keeps the console. Also: `systemctl status` Tasks counts THREADS
+  (a healthy receiver shows ~15), and "Console Autologin" advice was
+  WRONG (it made the hang deterministic) — removed from all docs.
 - **Fullscreen headless rendering**: under the KMSDRM video driver (no
   window manager) the renderer goes fullscreen automatically; --fullscreen
   forces it on desktops. Aspect is preserved via

docs/RUNBOOK.md

@@ -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.
 

scripts/install-receiver.sh

@@ -184,8 +184,10 @@ The receiver is installed and starts automatically at boot.
 Notes for small boards (e.g. Pi Zero 2 W):
   - No window manager is required: the receiver renders directly to the
     kernel display pipeline (SDL KMSDRM). Use Raspberry Pi OS Lite with
-    Console Autologin (raspi-config) and no desktop enabled — a running
-    compositor would own the display and the receiver could not start.
+    NO desktop enabled — a running compositor would own the display and
+    the receiver could not start. No console autologin is needed either.
+  - The service owns tty7 and switches to it at boot (Ctrl+Alt+F1 returns
+    to the console login, Ctrl+Alt+F7 to the screencast).
   - Disable console blanking so the picture never goes dark: add
     consoleblank=0 to /boot/firmware/cmdline.txt and reboot.
   - Decoding is software H.264; expect smooth playback for small streams

systemd/screencast-receiver.service

@@ -26,12 +26,16 @@ StartLimitBurst=10
 [Service]
 Type=simple
 User=__SC_RECEIVER_USER__
+# SDL's KMSDRM backend wants a tty. The service gets its OWN virtual
+# terminal: systemd must acquire the controlling terminal before exec, and
+# acquiring one that another session owns (tty1: getty, console autologin,
+# or a desktop) blocks the start FOREVER in a pre-exec placeholder process
+# (systemctl status shows the main PID as "(screencast)"). tty7 is free on
+# stock systems; tty1 keeps the console login.
+ExecStartPre=-/usr/bin/chvt 7
 ExecStart=__SC_RECEIVER_BIN__ --receive
-# SDL's KMSDRM backend (headless console) expects a controlling terminal
-# for its VT handling; system services have none by default. Harmless in
-# desktop mode, where the process never touches the tty.
 StandardInput=tty
-TTYPath=/dev/tty1
+TTYPath=/dev/tty7
 # SIGTERM (systemctl stop) shuts the receiver down gracefully, which
 # withdraws its mDNS announcement. Never SIGKILL it; the systemd default
 # KillMode escalates only after the graceful timeout.
@@ -54,6 +58,8 @@ WantedBy=multi-user.target
 # to the kernel via KMSDRM):
 #   * Raspberry Pi OS Lite, no desktop enabled (a running compositor would
 #     hold the DRM master and the receiver could not take it)
-#   * raspi-config → System Options → Boot → Console Autologin
+#   * no console autologin needed: the service owns tty7 and switches to it
+#     at boot; Ctrl+Alt+F1 returns to the console login, Ctrl+Alt+F7 to the
+#     screencast
 #   * disable console blanking: add consoleblank=0 to
 #     /boot/firmware/cmdline.txt