Initial hotplug support (#936)

Adds a udev script which will invoke a hotplug hook script on all net
add events. The script will write some udev arguments to a systemd FIFO
socket (to ensure we have only instance of cloud-init running at a
time), which is then read by a new service that calls a new 'cloud-init
devel hotplug-hook' command to handle the new event.

This hotplug-hook command will:
- Fetch the pickled datsource
- Verify that the hotplug event is supported/enabled
- Update the metadata for the datasource
- Ensure the hotplugged device exists within the datasource
- Apply the config change on the datasource metadata
- Bring up the new interface (or apply global network configuration)
- Save the updated metadata back to the pickle cache

Also scattered in some unrelated typing where helpful

1 files changed, 9 insertions, 2 deletions

diff --git a/cloudinit/distros/__init__.py b/cloudinit/distros/__init__.py
index 2caa8bc2..7bdf2197 100755
--- a/cloudinit/distros/__init__.py
+++ b/cloudinit/distros/__init__.py
@@ -206,8 +206,15 @@ class Distro(persistence.CloudInitPickleMixin, metaclass=abc.ABCMeta):
     def generate_fallback_config(self):
         return net.generate_fallback_config()
 
-    def apply_network_config(self, netconfig, bring_up=False):
-        # apply network config netconfig
+    def apply_network_config(self, netconfig, bring_up=False) -> bool:
+        """Apply the network config.
+
+        If bring_up is True, attempt to bring up the passed in devices. If
+        devices is None, attempt to bring up devices returned by
+        _write_network_config.
+
+        Returns True if any devices failed to come up, otherwise False.
+        """
         # This method is preferred to apply_network which only takes
         # a much less complete network config format (interfaces(5)).
         network_state = parse_net_config_data(netconfig)