Allow disabling of network activation

In canonical#919 (81299de), we refactored some of the code used to bring up
networks across distros. Previously, the call to bring up network
interfaces during 'init' stage unintentionally resulted in a no-op
such that network interfaces were NEVER brought up by cloud-init, even
if new network interfaces were found after crawling the metadata.

The code was altered to bring up these discovered network interfaces.
On ubuntu, this results in a 'netplan apply' call during 'init' stage
for any ubuntu-based distro on a datasource that has a NETWORK
dependency. On GCE, this additional 'netplan apply' conflicts with the
google-guest-agent service, resulting in an instance that can no
be connected to.

This commit adds a 'disable_network_activation' option that can be
enabled in /etc/cloud.cfg to disable the activation of network
interfaces in 'init' stage.

cloudinit/cmd/main.py

@@ -348,6 +348,11 @@ def main_init(name, args):
         util.del_file(os.path.join(path_helper.get_cpath("data"), "no-net"))
 
     # Stage 5
+    if util.get_cfg_option_bool(init.cfg, 'disable_network_activation'):
+        bring_up_interfaces = False
+    else:
+        bring_up_interfaces = not args.local
+        LOG.debug(bring_up_interfaces)
     try:
         init.fetch(existing=existing)
         # if in network mode, and the datasource is local
@@ -367,7 +372,7 @@ def main_init(name, args):
             util.logexc(LOG, ("No instance datasource found!"
                               " Likely bad things to come!"))
         if not args.force:
-            init.apply_network_config(bring_up=not args.local)
+            init.apply_network_config(bring_up=bring_up_interfaces)
             LOG.debug("[%s] Exiting without datasource", mode)
             if mode == sources.DSMODE_LOCAL:
                 return (None, [])
@@ -388,7 +393,7 @@ def main_init(name, args):
         # dhcp clients to advertize this hostname to any DDNS services
         # LP: #1746455.
         _maybe_set_hostname(init, stage='local', retry_stage='network')
-    init.apply_network_config(bring_up=bool(mode != sources.DSMODE_LOCAL))
+    init.apply_network_config(bring_up=bring_up_interfaces)
 
     if mode == sources.DSMODE_LOCAL:
         if init.datasource.dsmode != mode:

cloudinit/distros/__init__.py

@@ -227,8 +227,11 @@ def apply_network_config(self, netconfig, bring_up=False) -> bool:
 
         # Now try to bring them up
         if bring_up:
+            LOG.debug('Bringing up newly configured network interfaces')
             network_activator = activators.select_activator()
             network_activator.bring_up_all_interfaces(network_state)
+        else:
+            LOG.debug("Not bringing up newly configured network interfaces")
         return False
 
     def apply_network_config_names(self, netconfig):

tests/integration_tests/datasources/test_network_dependency.py

@@ -0,0 +1,40 @@
+import pytest
+
+from tests.integration_tests.clouds import IntegrationCloud
+from tests.integration_tests.conftest import get_validated_source
+
+
+def _setup_custom_image(session_cloud: IntegrationCloud):
+    """Like `setup_image` in conftest.py, but with customized content."""
+    source = get_validated_source(session_cloud)
+    if not source.installs_new_version():
+        return
+    client = session_cloud.launch()
+
+    # Insert our "disable_network_activation" file here
+    client.write_to_file(
+        '/etc/cloud/cloud.cfg.d/99-disable-network-activation.cfg',
+        'disable_network_activation: true\n',
+    )
+
+    client.install_new_cloud_init(source)
+    # Even if we're keeping instances, we don't want to keep this
+    # one around as it was just for image creation
+    client.destroy()
+
+
+# This test should be able to work on any cloud whose datasource specifies
+# a NETWORK dependency
+@pytest.mark.gce
+@pytest.mark.ubuntu  # Because netplan
+def test_network_activation_disabled(session_cloud: IntegrationCloud):
+    """Test that the network is not activated during init mode."""
+    _setup_custom_image(session_cloud)
+    with session_cloud.launch() as client:
+        assert client.execute('systemctl is-active google-guest-agent').ok
+        log = client.read_from_file('/var/log/cloud-init.log')
+
+    assert "Running command ['netplan', 'apply']" not in log
+
+    assert 'Not bringing up newly configured network interfaces' in log
+    assert 'Bringing up newly configured network interfaces' not in log