test: add a mock ModemManager for basic coverage of sd-networkd's integration

Just the minimal setup and test case required to cover
https://github.com/systemd/systemd/issues/41389 for now, can be
expanded in the future

Boring boilerplate is bot-made, don't @ me

Co-developed-by: Claude Opus 4.6 noreply@anthropic.com

test/test-network/conf/25-wwan-ipv4v6.network

@@ -0,0 +1,12 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Match]
+Name=dummy98
+
+[Network]
+LLDP=no
+LinkLocalAddressing=no
+IPv6AcceptRA=no
+
+[MobileNetwork]
+APN=internet.test
+IPFamily=both

test/test-network/conf/mock-modem-manager.conf

@@ -0,0 +1,13 @@
+<!DOCTYPE busconfig PUBLIC
+ "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
+ "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
+<busconfig>
+  <!-- Allow root to own and interact with the mock ModemManager service -->
+  <policy user="root">
+    <allow own="org.freedesktop.ModemManager1"/>
+    <allow send_destination="org.freedesktop.ModemManager1"/>
+  </policy>
+  <policy context="default">
+    <allow send_destination="org.freedesktop.ModemManager1"/>
+  </policy>
+</busconfig>

test/test-network/systemd-networkd-tests.py

@@ -73,6 +73,7 @@ resolvectl_bin = shutil.which('resolvectl', path=which_paths)
 timedatectl_bin = shutil.which('timedatectl', path=which_paths)
 udevadm_bin = shutil.which('udevadm', path=which_paths)
 test_ndisc_send = None
+test_modem_manager_mock = None
 build_dir = None
 source_dir = None
 
@@ -973,6 +974,28 @@ def start_radvd(*additional_options, config_file):
 def stop_radvd():
     stop_by_pid_file(radvd_pid_file)
 
+def start_modem_manager_mock(*additional_options):
+    dbus_policy_src = os.path.join(networkd_ci_temp_dir, 'mock-modem-manager.conf')
+    cp(dbus_policy_src, '/etc/dbus-1/system.d/mock-modem-manager.conf')
+    check_output('systemctl reload dbus.service')
+
+    command = ' '.join([test_modem_manager_mock] + list(additional_options))
+    with open('/run/systemd/system/test-modem-manager-mock.service', mode='w', encoding='utf-8') as f:
+        f.write('[Unit]\n'
+                'Description=Mock ModemManager for networkd testing\n'
+                '[Service]\n'
+                'Type=notify\n'
+                'BusName=org.freedesktop.ModemManager1\n'
+                f'ExecStart={command}\n')
+    check_output('systemctl daemon-reload')
+    check_output('systemctl start test-modem-manager-mock.service')
+
+def stop_modem_manager_mock():
+    call('systemctl stop test-modem-manager-mock.service')
+    rm_f('/run/systemd/system/test-modem-manager-mock.service')
+    call('systemctl daemon-reload')
+    rm_f('/etc/dbus-1/system.d/mock-modem-manager.conf')
+
 def radvd_check_config(config_file):
     if not shutil.which('radvd'):
         print('radvd is not installed, assuming the config check failed')
@@ -1099,6 +1122,7 @@ def tear_down_common():
     stop_dnsmasq()
     stop_isc_dhcpd()
     stop_radvd()
+    stop_modem_manager_mock()
 
     # 2. remove modules
     call_quiet('rmmod netdevsim')
@@ -9567,6 +9591,54 @@ class NetworkdSysctlTest(unittest.TestCase, Utilities):
         self.assertNotIn("changed sysctl '/proc/sys/net/ipv6/conf/dummy98/max_addresses'", log)
         self.assertNotIn("Sysctl monitor BPF returned error", log)
 
+class NetworkdWWANTests(unittest.TestCase, Utilities):
+
+    def setUp(self):
+        setup_common()
+
+    def tearDown(self):
+        tear_down_common()
+
+    def test_wwan_ipv4v6_static(self):
+        """Test WWAN bearer with both IPv4 and IPv6 static configuration.
+
+        Regression test for https://github.com/systemd/systemd/issues/41389
+        """
+        if not os.path.exists(test_modem_manager_mock):
+            self.skipTest(f'{test_modem_manager_mock} does not exist.')
+
+        copy_network_unit('12-dummy.netdev', '25-wwan-ipv4v6.network')
+        try:
+            start_modem_manager_mock(
+                '--ifname', 'dummy98',
+                '--ipv4-address', '100.120.244.160',
+                '--ipv4-gateway', '100.120.244.161',
+                '--ipv4-prefix', '26',
+                '--ipv6-address', '2001:db8::1',
+                '--ipv6-gateway', '2001:db8::2',
+                '--ipv6-prefix', '64',
+            )
+        except (subprocess.CalledProcessError, PermissionError, OSError) as e:
+            self.skipTest(f'Failed to start mock ModemManager: {e}')
+        start_networkd()
+        self.wait_online('dummy98:routable')
+
+        output = check_output('ip -4 address show dev dummy98')
+        print(output)
+        self.assertIn('100.120.244.160/26', output)
+
+        output = check_output('ip -6 address show dev dummy98')
+        print(output)
+        self.assertIn('2001:db8::1/64', output)
+
+        output = check_output('ip -4 route show dev dummy98')
+        print(output)
+        self.assertIn('default via 100.120.244.161', output)
+
+        output = check_output('ip -6 route show dev dummy98')
+        print(output)
+        self.assertIn('default via 2001:db8::2', output)
+
 if __name__ == '__main__':
     parser = argparse.ArgumentParser()
     parser.add_argument('--build-dir', help='Path to build dir', dest='build_dir')
@@ -9626,6 +9698,11 @@ if __name__ == '__main__':
     else:
         test_ndisc_send = '/usr/lib/tests/test-ndisc-send'
 
+    if build_dir:
+        test_modem_manager_mock = os.path.normpath(os.path.join(build_dir, 'test-modem-manager-mock'))
+    else:
+        test_modem_manager_mock = '/usr/lib/systemd/tests/unit-tests/manual/test-modem-manager-mock'
+
     if asan_options:
         env.update({'ASAN_OPTIONS': asan_options})
     if lsan_options: