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, 17 insertions, 0 deletions

diff --git a/cloudinit/event.py b/cloudinit/event.py
new file mode 100644
index 00000000..f7b311fb
--- /dev/null
+++ b/cloudinit/event.py
@@ -0,0 +1,17 @@
+# This file is part of cloud-init. See LICENSE file for license information.
+
+"""Classes and functions related to event handling."""
+
+
+# Event types which can generate maintenance requests for cloud-init.
+class EventType(object):
+    BOOT = "System boot"
+    BOOT_NEW_INSTANCE = "New instance first boot"
+
+    # TODO: Cloud-init will grow support for the follow event types:
+    # UDEV
+    # METADATA_CHANGE
+    # USER_REQUEST
+
+
+# vi: ts=4 expandtab