try to overlay flatpak in live session

this is a bit complicated unfortunately

mkosi.extra/usr/bin/_kde-linux-volatile-flatpak

@@ -0,0 +1,29 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2024 Harald Sitter <sitter@kde.org>
+
+set -eux
+
+if ! grep "kdeos.live=1" /proc/cmdline; then
+    echo "kdeos.live=1 not in cmdline"
+    exit 1
+fi
+
+if [ "$(readlink --canonicalize /dev/disk/by-label/KDEOSLive)" != "$(readlink --canonicalize /dev/gpt-auto-root)" ]; then
+    echo "gpt-auto-root is not KDEOSLive"
+    exit 1
+fi
+
+# Replicate what systemd-volatile-root does. Unfortunately we can't just use the volatile root because you can't
+# overlay with an overlay. This means we potentialy claim a lot of RAM (25% for root + 15% for flatpaks).
+# Not ideal but there's no way I can see to get access to the volatile-sysroot (it gets unmounted and removed
+# before systemd-volatile-root exits).
+
+mkdir --mode 0700 /run/kde-linux/
+mkdir --mode 0700 /run/kde-linux/overlay-var-lib-flatpak
+mount -t tmpfs tmpfs /run/kde-linux/overlay-var-lib-flatpak -o mode=0755,size=15%,nr_inodes=1m
+mkdir --mode 0755 /run/kde-linux/overlay-var-lib-flatpak/upper
+mkdir --mode 0755 /run/kde-linux/overlay-var-lib-flatpak/work
+
+mount -t overlay overlay /var/lib/flatpak \
+    -o lowerdir=/var/lib/flatpak,upperdir=/run/kde-linux/overlay-var-lib-flatpak/upper,workdir=/run/kde-linux/overlay-var-lib-flatpak/work

mkosi.extra/usr/lib/systemd/system-generators/kde-linux-live-generator

@@ -44,7 +44,7 @@ cat <<- EOF > "$normal_dir/var-lib-flatpak.mount"
 # Generated by $(basename "$0")
 [Unit]
 Description=Mount unit for /var/lib/flatpak
-Before=systemd-volatile-root.service
+Before=kde-linux-volatile-var-lib-flatpak.service
 
 [Mount]
 What=/dev/disk/by-label/KDEOSLive

mkosi.extra/usr/lib/systemd/system-preset/50-systemdos.preset

@@ -13,6 +13,7 @@ enable cups.service
 
 enable kde-linux-btrfs.service
 enable kde-linux-live-setup.service
+enable kde-linux-volatile-var-lib-flatpak.service
 enable plasma-setup-live-system.service
 enable kdeos-overlay.service
 enable var-lib-snapd-snap.mount

mkosi.extra/usr/lib/systemd/system/kde-linux-volatile-var-lib-flatpak.service

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2024 Harald Sitter <sitter@kde.org>
+
+[Unit]
+Description=Volatile /var/lib/flatpak File Systems
+Before=display-manager.service
+After=var.mount
+ConditionKernelCommandLine=kdeos.live=1
+
+[Service]
+ExecStart=/usr/bin/_kde-linux-volatile-flatpak
+
+[Install]
+WantedBy=multi-user.target