mkosi: install dlopen optional dependencies for debian/ubuntu builds

mkosi.images/system/mkosi.conf.d/10-debian-ubuntu/mkosi.conf.d/network.conf

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[Match]
+Environment=NO_BUILD=1
+
+[Content]
+WithNetwork=yes

mkosi.images/system/mkosi.conf.d/10-debian-ubuntu/mkosi.postinst

@@ -0,0 +1,29 @@
+#!/bin/bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -e
+
+# By default Suggests are not installed (and often Recommends are disabled too), which means we will miss
+# the dlopen optional dependencies, but the tests need them, so parse them from the package metadata and
+# install them. This is not an issue when building locally, as the build and runtime images are the same,
+# so they would get installed as build dependencies anyway.
+
+if [ "$1" = "build" ] || ! ((NO_BUILD)); then
+    exit 0
+fi
+
+# Query the Recommends and Suggests of all systemd packages, by matching on the version
+systemd_version="$(dpkg-query --showformat '${Version}' --show systemd)"
+mapfile -t systemd_packages < <( dpkg --list | grep '^ii' | grep "$systemd_version" | awk '{print $2}' | tr '\n' ' ' )
+extra_packages=()
+# shellcheck disable=SC2068
+for package in ${systemd_packages[@]}; do
+    # We are looking for dlopens, so filter for libraries
+    mapfile -t -O "${#extra_packages[@]}" extra_packages < <(dpkg-query --showformat '${Suggests}' --show "$package" | sed -e "s/, /\n/g" -e "s/|.*//" | grep "lib")
+    mapfile -t -O "${#extra_packages[@]}" extra_packages < <(dpkg-query --showformat '${Recommends}' --show "$package" | sed -e "s/, /\n/g" -e "s/|.*//" | grep "lib")
+done
+
+if [ "${#extra_packages[@]}" -eq 0 ]; then
+    exit 0
+fi
+
+apt install "${extra_packages[@]}"