test: Fix networkd test for an already running service

Mount tmpfses over the networkd and resolved config and state
directories, and stop the services beforehand. This ensures that the
test does not mess with an existing networkd/resolved setup. At least
for ethernet setups, this does not sever existing links, so is good
enough for the CI cases we are interested in (QEMU and LXC).

Relax the skip check to only skip the test when trying to run this on
real iron, but start running it in virtual machines now.

This allows us to run the test on Ubuntu 18.04 LTS in CI, which uses
both services by default.

test/networkd-test.py

@@ -37,16 +37,36 @@ NETWORKD_WAIT_ONLINE = shutil.which('systemd-networkd-wait-online',
 
 RESOLV_CONF = '/run/systemd/resolve/resolv.conf'
 
+tmpmounts = []
+running_units = []
+stopped_units = []
+
 
 def setUpModule():
+    global tmpmounts
+
     """Initialize the environment, and perform sanity checks on it."""
     if NETWORKD_WAIT_ONLINE is None:
         raise OSError(errno.ENOENT, 'systemd-networkd-wait-online not found')
 
-    # Do not run any tests if the system is using networkd already.
-    if subprocess.call(['systemctl', 'is-active', '--quiet',
-                        'systemd-networkd.service']) == 0:
-        raise unittest.SkipTest('networkd is already active')
+    # Do not run any tests if the system is using networkd already and it's not virtualized
+    if (subprocess.call(['systemctl', 'is-active', '--quiet', 'systemd-networkd.service']) == 0 and
+            subprocess.call(['systemd-detect-virt', '--quiet']) != 0):
+        raise unittest.SkipTest('not virtualized and networkd is already active')
+    # Ensure we don't mess with an existing networkd config
+    for u in ['systemd-networkd.socket', 'systemd-networkd', 'systemd-resolved']:
+        if subprocess.call(['systemctl', 'is-active', '--quiet', u]) == 0:
+            subprocess.call(['systemctl', 'stop', u])
+            running_units.append(u)
+        else:
+            stopped_units.append(u)
+    for d in ['/etc/systemd/network', '/run/systemd/network',
+              '/run/systemd/netif', '/run/systemd/resolve']:
+        if os.path.isdir(d):
+            subprocess.check_call(["mount", "-t", "tmpfs", "none", d])
+            tmpmounts.append(d)
+    if os.path.isdir('/run/systemd/resolve'):
+        os.chmod('/run/systemd/resolve', 0o755)
 
     # Avoid "Failed to open /dev/tty" errors in containers.
     os.environ['SYSTEMD_LOG_TARGET'] = 'journal'
@@ -60,6 +80,16 @@ def setUpModule():
     subprocess.check_call(['adduser', '--system', '--no-create-home', 'systemd-network'])
 
 
+def tearDownModule():
+    global tmpmounts
+    for d in tmpmounts:
+        subprocess.check_call(["umount", d])
+    for u in stopped_units:
+        subprocess.call(["systemctl", "stop", u])
+    for u in running_units:
+        subprocess.call(["systemctl", "restart", u])
+
+
 class NetworkdTestingUtilities:
     """Provide a set of utility functions to facilitate networkd tests.