update_metadata: a datasource can support network re-config every boot

Very basic type definitions are now defined to distinguish 'boot'
events from 'new instance (first boot)'. Event types will now be handed
to a datasource.update_metadata method which can determine whether
to refresh its metadata and re-render configuration based on that
source event.

A datasource can 'subscribe' to an event by setting up the update_events
attribute on the datasource class which describe what config scope is
updated by a list of matching events. By default datasources will have
the following update_events: {'network': [EventType.BOOT_NEW_INSTANCE]}
This setting says the datasource will re-write network configuration only
on first boot of a new instance or when the instance id changes.

New methods are now present on the datasource:
  - clear_cached_attrs: Resets cached datasource attributes to values
    listed in datasource.cached_attr_defaults. This is performed prior to
    processing a fresh metadata process to avoid keeping old/invalid
    cached data around.
  - update_metadata: accepts source_event_types to determine if the
    metadata should be crawled again and processed

1 files changed, 10 insertions, 4 deletions

diff --git a/cloudinit/stages.py b/cloudinit/stages.py
index 286607bf..c132b57d 100644
--- a/cloudinit/stages.py
+++ b/cloudinit/stages.py
@@ -22,6 +22,8 @@ from cloudinit.handlers import cloud_config as cc_part
 from cloudinit.handlers import shell_script as ss_part
 from cloudinit.handlers import upstart_job as up_part
 
+from cloudinit.event import EventType
+
 from cloudinit import cloud
 from cloudinit import config
 from cloudinit import distros
@@ -648,10 +650,14 @@ class Init(object):
         except Exception as e:
             LOG.warning("Failed to rename devices: %s", e)
 
-        if (self.datasource is not NULL_DATA_SOURCE and
-                not self.is_new_instance()):
-            LOG.debug("not a new instance. network config is not applied.")
-            return
+        if self.datasource is not NULL_DATA_SOURCE:
+            if not self.is_new_instance():
+                if not self.datasource.update_metadata([EventType.BOOT]):
+                    LOG.debug(
+                        "No network config applied. Neither a new instance"
+                        " nor datasource network update on '%s' event",
+                        EventType.BOOT)
+                    return
 
         LOG.info("Applying network configuration from %s bringup=%s: %s",
                  src, bring_up, netcfg)