[PATCH v3 3/3] 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>
---
v3: pass $@ to mount-flatpak rather than each individual argument.
v2: https://spectrum-os.org/lists/archives/spectrum-devel/20251124195921.24441-1-hi@alyssa.is/

 host/rootfs/default.nix               | 10 +++---
 host/rootfs/file-list.mk              |  1 +
 host/rootfs/image/usr/bin/run-flatpak | 51 +++++++++++++++++++++++++++
 3 files changed, 58 insertions(+), 4 deletions(-)
 create mode 100755 host/rootfs/image/usr/bin/run-flatpak

diff --git a/host/rootfs/default.nix b/host/rootfs/default.nix
index 941c04e..c93a06b 100644
--- a/host/rootfs/default.nix
+++ b/host/rootfs/default.nix
@@ -11,8 +11,9 @@ pkgsStatic.callPackage (
 { spectrum-host-tools
 , lib, stdenvNoCC, nixos, runCommand, writeClosure, erofs-utils, s6-rc
 , busybox, cloud-hypervisor, cryptsetup, dbus, execline, inkscape
-, iproute2, inotify-tools, jq, mdevd, s6, s6-linux-init, socat
-, util-linuxMinimal, virtiofsd, xorg, xdg-desktop-portal-spectrum-host
+, iproute2, inotify-tools, jq, mdevd, mount-flatpak, s6, s6-linux-init
+, socat, util-linuxMinimal, virtiofsd, xorg
+, xdg-desktop-portal-spectrum-host
 }:
 
 let
@@ -34,8 +35,9 @@ let
 
   packages = [
     cloud-hypervisor cryptsetup dbus execline inotify-tools iproute2
-    jq mdevd s6 s6-linux-init s6-rc socat spectrum-host-tools
-    util-linuxMinimal virtiofsd xdg-desktop-portal-spectrum-host
+    jq mdevd mount-flatpak s6 s6-linux-init s6-rc socat
+    spectrum-host-tools util-linuxMinimal virtiofsd
+    xdg-desktop-portal-spectrum-host
 
     (busybox.override {
       # Use a separate file as it is a bit too big.
diff --git a/host/rootfs/file-list.mk b/host/rootfs/file-list.mk
index 7625c54..67529c0 100644
--- a/host/rootfs/file-list.mk
+++ b/host/rootfs/file-list.mk
@@ -43,6 +43,7 @@ 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/vm-console \
 	image/usr/bin/vm-import \
diff --git a/host/rootfs/image/usr/bin/run-flatpak b/host/rootfs/image/usr/bin/run-flatpak
new file mode 100755
index 0000000..f7b9bc4
--- /dev/null
+++ b/host/rootfs/image/usr/bin/run-flatpak
@@ -0,0 +1,51 @@
+#!/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 { redirfd -w 1 /run/configs/${id}/fs/type echo flatpak }
+  if { cd /run/configs/${id}/fs mount-flatpak $@ }
+  if {
+    ln -s /usr/lib/spectrum/img/appvm/blk /usr/lib/spectrum/img/appvm/vmlinux
+      /run/configs/${id}
+  }
+
+  if { ln -s /run/configs/${id} ${dir}/config }
+
+  if { create-vm-dependencies $id }
+
+  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 $id }
+
+if {
+  elglob -0 flatpak_dir_mounts /run/configs/${id}/fs/flatpak/*/*/*/*/*
+  forx -E mount {
+    ${dir}/fs/doc
+    /run/configs/${id}/fs/flatpak/repo/config
+    $flatpak_dir_mounts
+    /run/configs/${id}/fs/flatpak
+    ${dir}/fs/config
+  }
+  umount $mount
+}
+
+rm -r $dir /run/configs/${id}
-- 
2.51.0