CI: Add mkosi boot tests

Using the new mkosi Github Action, we can add some simple boot tests
for the systemd mkosi configs. This makes sure these keep working
as expected.

.github/workflows/mkosi.yml

@@ -0,0 +1,42 @@
+name: mkosi
+
+# Simple boot tests that build and boot the mkosi images generated by the mkosi config files in .mkosi.
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  ci:
+    runs-on: ubuntu-20.04
+    strategy:
+      fail-fast: false
+      matrix:
+        distro:
+          - arch
+          - debian
+          - ubuntu
+          - fedora
+
+    steps:
+    - uses: actions/checkout@v2
+    - uses: systemd/mkosi@v8
+
+    - name: Install
+      run: sudo apt-get update && sudo apt-get install --no-install-recommends
+        ovmf
+        python3-pexpect
+        qemu-system-x86-64
+
+    - name: Build ${{ matrix.distro }}
+      run: sudo python3 -m mkosi --default .mkosi/mkosi.${{ matrix.distro }} --password= --qemu-headless build
+
+    - name: Boot ${{ matrix.distro }} systemd-nspawn
+      run: sudo ./.github/workflows/test_mkosi_boot.py python3 -m mkosi --default .mkosi/mkosi.${{ matrix.distro }} --password= --qemu-headless boot
+
+    - name: Boot ${{ matrix.distro }} QEMU
+      run: sudo ./.github/workflows/test_mkosi_boot.py python3 -m mkosi --default .mkosi/mkosi.${{ matrix.distro }} --password= --qemu-headless qemu

.github/workflows/test_mkosi_boot.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+import pexpect
+import sys
+
+
+def run() -> None:
+    p = pexpect.spawnu(" ".join(sys.argv[1:]), logfile=sys.stdout, timeout=300)
+
+    p.expect("login:")
+    p.sendline("root")
+
+    p.expect("#")
+    p.sendline("systemctl poweroff")
+
+    p.expect(pexpect.EOF)
+
+
+try:
+    run()
+except pexpect.EOF:
+    print("UNEXPECTED EOF")
+    sys.exit(1)
+except pexpect.TIMEOUT:
+    print("TIMED OUT")
+    sys.exit(1)