[PATCH v5 6/6] host/rootfs: add run-flatpak script

[Alyssa Ross <hi@alyssa.is>]

This is the entrypoint for running Flatpak applications.

It would be good to only add mounts for the VM in virtiofsd's mount
namespace, so we don't need to do lots of manual unmounts, but that's
a wider change affecting more than just Flatpak.

I've tested this by copying my host's Flatpak repository into a disk
image, and attaching that as a drive to the VM.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
---
v5: no change
v4: https://spectrum-os.org/lists/archives/spectrum-devel/20251201044534.977524-9-hi@alyssa.is/

 host/rootfs/default.nix               | 12 +++----
 host/rootfs/file-list.mk              |  1 +
 host/rootfs/image/usr/bin/run-flatpak | 47 +++++++++++++++++++++++++++
 3 files changed, 54 insertions(+), 6 deletions(-)
 create mode 100755 host/rootfs/image/usr/bin/run-flatpak

diff --git a/host/rootfs/default.nix b/host/rootfs/default.nix
index 57dd7a9..ca2084f 100644
--- a/host/rootfs/default.nix
+++ b/host/rootfs/default.nix
@@ -12,9 +12,9 @@ pkgsMusl.callPackage (
 , lib, stdenvNoCC, nixos, runCommand, writeClosure, erofs-utils, s6-rc
 , btrfs-progs, busybox, cloud-hypervisor, cosmic-files, crosvm
 , cryptsetup, dejavu_fonts, dbus, execline, foot, fuse3, iproute2
-, inotify-tools, jq, kmod, mdevd, mesa, s6, s6-linux-init, socat
-, systemd, util-linuxMinimal, virtiofsd, westonLite
-, xdg-desktop-portal, xdg-desktop-portal-gtk
+, inotify-tools, jq, kmod, mdevd, mesa, mount-flatpak, s6
+, s6-linux-init, socat, systemd, util-linuxMinimal, virtiofsd
+, westonLite, xdg-desktop-portal, xdg-desktop-portal-gtk
 , xdg-desktop-portal-spectrum-host
 }:
 
@@ -26,9 +26,9 @@ let
 
   packages = [
     btrfs-progs cloud-hypervisor cosmic-files crosvm cryptsetup dbus
-    execline fuse3 inotify-tools iproute2 jq kmod mdevd s6 s6-linux-init
-    s6-rc socat spectrum-host-tools spectrum-router util-linuxMinimal virtiofsd
-    xdg-desktop-portal-spectrum-host
+    execline fuse3 inotify-tools iproute2 jq kmod mdevd mount-flatpak s6
+    s6-linux-init s6-rc socat spectrum-host-tools spectrum-router
+    util-linuxMinimal virtiofsd xdg-desktop-portal-spectrum-host
 
     (foot.override { allowPgo = false; })
 
diff --git a/host/rootfs/file-list.mk b/host/rootfs/file-list.mk
index bfe3940..df22bce 100644
--- a/host/rootfs/file-list.mk
+++ b/host/rootfs/file-list.mk
@@ -55,6 +55,7 @@ FILES = \
 	image/usr/bin/assign-devices \
 	image/usr/bin/create-vm-dependencies \
 	image/usr/bin/run-appimage \
+	image/usr/bin/run-flatpak \
 	image/usr/bin/run-vmm \
 	image/usr/bin/spectrum-update \
 	image/usr/bin/vm-console \
diff --git a/host/rootfs/image/usr/bin/run-flatpak b/host/rootfs/image/usr/bin/run-flatpak
new file mode 100755
index 0000000..d7914a7
--- /dev/null
+++ b/host/rootfs/image/usr/bin/run-flatpak
@@ -0,0 +1,47 @@
+#!/bin/execlineb -W
+# SPDX-License-Identifier: EUPL-1.2+
+# SPDX-FileCopyrightText: 2024-2025 Alyssa Ross <hi@alyssa.is>
+
+backtick -E dir { mktemp -d /run/vm/by-id/XXXXXX }
+backtick -E id { basename -- $dir }
+
+if {
+  elgetpositionals
+
+  if { mkdir -p /run/configs/${id}/fs }
+
+  if {
+    ln -s /usr/lib/spectrum/img/appvm/blk /usr/lib/spectrum/img/appvm/vmlinux
+      /run/configs/${id}
+  }
+
+  if { ln -s /run/configs/${id} ${dir}/config }
+
+  if { create-vm-dependencies $id }
+
+  if {
+    nsenter --mount=${dir}/mount
+    cd ${dir}/fs/config
+    if { redirfd -w 1 type echo flatpak }
+    mount-flatpak $@
+  }
+
+  piperw 4 3
+  background {
+    fdclose 3
+    fdmove 0 4
+
+    # Wait for the VMM to be up, then start the VM.
+    if { redirfd -w 1 /dev/null head -1 }
+    vm-start $id
+  }
+  fdclose 4
+
+  foreground { run-vmm $id }
+}
+
+if { s6-instance-delete -- /run/service/vm-services $id }
+
+if { umount ${dir}/mount } # mount namespace
+if { umount ${dir}/mount } # private bind mount
+rm -r $dir /run/configs/${id}
-- 
2.51.0