[PATCH 1/8] host/rootfs: poll active consoles for serial getty

[Alyssa Ross <hi@alyssa.is>]

Previously, the active consoles were only checked once, so if a
console appeared later, getty wouldn't be run for it.  This meant that
if the console was set to a serial device whose driver was not
built-in, it might have no shell if the module was not loaded until
after rc.init had already run.  Also, if a console device was removed,
its instance of the serial-getty would hang around.

To fix this, replace the getty instantiation in rc.init with a service
that will create and destroy serial-getty instances to match the
currently configured consoles, which will rerun whenever the
configured consoles change.

Fixes: b096279 ("host/rootfs: run getty for active serial consoles")
Signed-off-by: Alyssa Ross <hi@alyssa.is>
---
 host/rootfs/Makefile                          |  1 +
 host/rootfs/default.nix                       |  8 ++--
 .../service/serial-getty-generator/run        | 43 +++++++++++++++++++
 host/rootfs/etc/s6-linux-init/scripts/rc.init |  9 ----
 4 files changed, 48 insertions(+), 13 deletions(-)
 create mode 100755 host/rootfs/etc/s6-linux-init/run-image/service/serial-getty-generator/run

diff --git a/host/rootfs/Makefile b/host/rootfs/Makefile
index d54538b..d5b9b55 100644
--- a/host/rootfs/Makefile
+++ b/host/rootfs/Makefile
@@ -31,6 +31,7 @@ FILES = \
 	etc/s6-linux-init/run-image/service/getty-tty4/run \
 	etc/s6-linux-init/run-image/service/s6-svscan-log/notification-fd \
 	etc/s6-linux-init/run-image/service/s6-svscan-log/run \
+	etc/s6-linux-init/run-image/service/serial-getty-generator/run \
 	etc/s6-linux-init/run-image/service/serial-getty/notification-fd \
 	etc/s6-linux-init/run-image/service/serial-getty/run \
 	etc/s6-linux-init/run-image/service/serial-getty/template/run \
diff --git a/host/rootfs/default.nix b/host/rootfs/default.nix
index b50d090..bfc1ba8 100644
--- a/host/rootfs/default.nix
+++ b/host/rootfs/default.nix
@@ -9,8 +9,8 @@ pkgsStatic.callPackage (
 { start-vmm
 , lib, stdenvNoCC, nixos, runCommand, writeClosure, erofs-utils, s6-rc
 , bcachefs-tools, busybox, cloud-hypervisor, cryptsetup, dbus, execline
-, e2fsprogs, inkscape, jq, kmod, mdevd, s6, s6-linux-init, socat
-, util-linuxMinimal, virtiofsd, xorg
+, e2fsprogs, inkscape, inotify-tools, jq, kmod, mdevd, s6
+, s6-linux-init, socat, util-linuxMinimal, virtiofsd, xorg
 , xdg-desktop-portal-spectrum-host
 }:
 
@@ -137,8 +137,8 @@ let
   foot = pkgsGui.foot.override { allowPgo = false; };
 
   packages = [
-    bcachefs-tools cloud-hypervisor dbus execline jq kmod mdevd s6
-    s6-linux-init s6-rc socat start-vmm virtiofsd
+    bcachefs-tools cloud-hypervisor dbus execline inotify-tools jq
+    kmod mdevd s6 s6-linux-init s6-rc socat start-vmm virtiofsd
     xdg-desktop-portal-spectrum-host
 
     (cryptsetup.override {
diff --git a/host/rootfs/etc/s6-linux-init/run-image/service/serial-getty-generator/run b/host/rootfs/etc/s6-linux-init/run-image/service/serial-getty-generator/run
new file mode 100755
index 0000000..445604f
--- /dev/null
+++ b/host/rootfs/etc/s6-linux-init/run-image/service/serial-getty-generator/run
@@ -0,0 +1,43 @@
+#!/bin/execlineb -P
+# SPDX-License-Identifier: EUPL-1.2+
+# SPDX-FileCopyrightText: 2024-2025 Alyssa Ross <hi@alyssa.is>
+
+piperw 3 4
+background {
+  fdclose 3
+  fdmove 2 4
+  inotifywait -e MODIFY /sys/class/tty/console/active
+}
+fdclose 4
+importas -i inotifywait_pid !
+
+foreground {
+  if { fdmove 0 3 grep -qx "Watches established." }
+  background { fdmove 0 3 cat }
+  fdclose 3
+
+  # Wait until inotifywait is ready before updating serial gettys,
+  # so that changes won't be missed in between updating and starting
+  # inotifywait.
+  pipeline { s6-instance-list /run/service/serial-getty }
+  pipeline { sort }
+  fdmove -c 3 0
+  
+  redirfd -r 0 /sys/class/tty/console/active
+  pipeline { tr " " "\n" }
+  pipeline { sort }
+  
+  pipeline { comm -3 - /proc/self/fd/3 }
+  forstdin -Ep line
+  case -N $line {
+    "	?tty[0-9]*" { }
+    "	(.*)" {
+      importas -i tty 1
+      s6-instance-delete /run/service/serial-getty $tty
+    }
+  }
+  s6-instance-create /run/service/serial-getty $line
+}
+
+# Block until the active consoles change, then let s6 restart us.
+wait -- $inotifywait_pid
diff --git a/host/rootfs/etc/s6-linux-init/scripts/rc.init b/host/rootfs/etc/s6-linux-init/scripts/rc.init
index c778714..674fd38 100755
--- a/host/rootfs/etc/s6-linux-init/scripts/rc.init
+++ b/host/rootfs/etc/s6-linux-init/scripts/rc.init
@@ -2,15 +2,6 @@
 # SPDX-License-Identifier: EUPL-1.2+
 # SPDX-FileCopyrightText: 2020-2022, 2024 Alyssa Ross <hi@alyssa.is>
 
-background {
-  redirfd -r 0 /sys/class/tty/console/active
-  withstdinas active
-  importas -isu active active
-  forx -po0 -E tty { $active }
-  case $tty { tty[0-9]* { } }
-  s6-instance-create /run/service/serial-getty $tty
-}
-
 if { s6-rc-init -c /etc/s6-rc /run/service }
 
 if { mount --make-shared /run }
-- 
2.49.0