Allow user control over update events (#834)

Control is currently limited to boot events, though this should
allow us to more easily incorporate HOTPLUG support. Disabling
'instance-first-boot' is not supported as we apply networking config
too early in boot to have processed userdata (along with the fact
that this would be a pretty big foot-gun).

The concept of update events on datasource has been split into
supported update events and default update events. Defaults will be
used if there is no user-defined update events, but user-defined
events won't be supplied if they aren't supported.
When applying the networking config, we now check to see if the event
is supported by the datasource as well as if it is enabled.

Configuration looks like:
updates:
  network:
    when: ['boot']

1 files changed, 29 insertions, 12 deletions

diff --git a/cloudinit/sources/__init__.py b/cloudinit/sources/__init__.py
index 7d74f8d9..a07c4b4f 100644
--- a/cloudinit/sources/__init__.py
+++ b/cloudinit/sources/__init__.py
@@ -13,6 +13,7 @@ import copy
 import json
 import os
 from collections import namedtuple
+from typing import Dict, List
 
 from cloudinit import dmi
 from cloudinit import importer
@@ -22,7 +23,7 @@ from cloudinit import type_utils
 from cloudinit import user_data as ud
 from cloudinit import util
 from cloudinit.atomic_helper import write_json
-from cloudinit.event import EventType
+from cloudinit.event import EventScope, EventType
 from cloudinit.filters import launch_index
 from cloudinit.persistence import CloudInitPickleMixin
 from cloudinit.reporting import events
@@ -175,12 +176,23 @@ class DataSource(CloudInitPickleMixin, metaclass=abc.ABCMeta):
 
     # The datasource defines a set of supported EventTypes during which
     # the datasource can react to changes in metadata and regenerate
-    # network configuration on metadata changes.
-    # A datasource which supports writing network config on each system boot
-    # would call update_events['network'].add(EventType.BOOT).
+    # network configuration on metadata changes. These are defined in
+    # `supported_network_events`.
+    # The datasource also defines a set of default EventTypes that the
+    # datasource can react to. These are the event types that will be used
+    # if not overridden by the user.
+    # A datasource requiring to write network config on each system boot
+    # would call default_update_events['network'].add(EventType.BOOT).
 
     # Default: generate network config on new instance id (first boot).
-    update_events = {'network': set([EventType.BOOT_NEW_INSTANCE])}
+    supported_update_events = {EventScope.NETWORK: {
+        EventType.BOOT_NEW_INSTANCE,
+        EventType.BOOT,
+        EventType.BOOT_LEGACY,
+    }}
+    default_update_events = {EventScope.NETWORK: {
+        EventType.BOOT_NEW_INSTANCE,
+    }}
 
     # N-tuple listing default values for any metadata-related class
     # attributes cached on an instance by a process_data runs. These attribute
@@ -648,10 +660,12 @@ class DataSource(CloudInitPickleMixin, metaclass=abc.ABCMeta):
     def get_package_mirror_info(self):
         return self.distro.get_package_mirror_info(data_source=self)
 
-    def update_metadata(self, source_event_types):
+    def update_metadata_if_supported(
+        self, source_event_types: List[EventType]
+    ) -> bool:
         """Refresh cached metadata if the datasource supports this event.
 
-        The datasource has a list of update_events which
+        The datasource has a list of supported_update_events which
         trigger refreshing all cached metadata as well as refreshing the
         network configuration.
 
@@ -661,9 +675,9 @@ class DataSource(CloudInitPickleMixin, metaclass=abc.ABCMeta):
         @return True if the datasource did successfully update cached metadata
             due to source_event_type.
         """
-        supported_events = {}
+        supported_events = {}  # type: Dict[EventScope, set]
         for event in source_event_types:
-            for update_scope, update_events in self.update_events.items():
+            for update_scope, update_events in self.supported_update_events.items():  # noqa: E501
                 if event in update_events:
                     if not supported_events.get(update_scope):
                         supported_events[update_scope] = set()
@@ -671,7 +685,8 @@ class DataSource(CloudInitPickleMixin, metaclass=abc.ABCMeta):
         for scope, matched_events in supported_events.items():
             LOG.debug(
                 "Update datasource metadata and %s config due to events: %s",
-                scope, ', '.join(matched_events))
+                scope.value,
+                ', '.join([event.value for event in matched_events]))
             # Each datasource has a cached config property which needs clearing
             # Once cleared that config property will be regenerated from
             # current metadata.
@@ -682,7 +697,7 @@ class DataSource(CloudInitPickleMixin, metaclass=abc.ABCMeta):
             if result:
                 return True
         LOG.debug("Datasource %s not updated for events: %s", self,
-                  ', '.join(source_event_types))
+                  ', '.join([event.value for event in source_event_types]))
         return False
 
     def check_instance_id(self, sys_cfg):
@@ -789,7 +804,9 @@ def find_source(sys_cfg, distro, paths, ds_deps, cfg_list, pkg_list, reporter):
             with myrep:
                 LOG.debug("Seeing if we can get any data from %s", cls)
                 s = cls(sys_cfg, distro, paths)
-                if s.update_metadata([EventType.BOOT_NEW_INSTANCE]):
+                if s.update_metadata_if_supported(
+                    [EventType.BOOT_NEW_INSTANCE]
+                ):
                     myrep.message = "found %s data from %s" % (mode, name)
                     return (s, type_utils.obj_name(cls))
         except Exception: