DataSource: move update_events from a class to an instance attribute

Currently, DataSourceAzure updates self.update_events in __init__. As
update_events is a class attribute on DataSource, this updates it for
all instances of classes derived from DataSource including those for
other clouds. This means that if DataSourceAzure is even instantiated,
its behaviour is applied to whichever data source ends up being used for
boot.

To address this, update_events is moved from a class attribute to an
instance attribute (that is therefore populated at instantiation time).
This retains the defaults for all DataSource sub-class instances, but
avoids them being able to mutate the state in instances of other
DataSource sub-classes.

update_events is only ever referenced on an instance of DataSource (or a
sub-class); no code relies on it being a class attribute. (In fact,
it's only used within methods on DataSource or its sub-classes, so it
doesn't even _need_ to remain public, though I think it's appropriate
for it to be public.)

DataSourceScaleway is also updated to move update_events from a
class attribute to an instance attribute, as the class attribute would
now be masked by the DataSource instance attribute.

LP: #1819913

3 files changed, 20 insertions, 4 deletions

diff --git a/cloudinit/sources/DataSourceScaleway.py b/cloudinit/sources/DataSourceScaleway.py
index b573b382..54bfc1fe 100644
--- a/cloudinit/sources/DataSourceScaleway.py
+++ b/cloudinit/sources/DataSourceScaleway.py
@@ -171,10 +171,11 @@ def query_data_api(api_type, api_address, retries, timeout):
 
 class DataSourceScaleway(sources.DataSource):
     dsname = "Scaleway"
-    update_events = {'network': [EventType.BOOT_NEW_INSTANCE, EventType.BOOT]}
 
     def __init__(self, sys_cfg, distro, paths):
         super(DataSourceScaleway, self).__init__(sys_cfg, distro, paths)
+        self.update_events = {
+            'network': {EventType.BOOT_NEW_INSTANCE, EventType.BOOT}}
 
         self.ds_cfg = util.mergemanydict([
             util.get_cfg_by_path(sys_cfg, ["datasource", "Scaleway"], {}),
diff --git a/cloudinit/sources/__init__.py b/cloudinit/sources/__init__.py
index e6966b31..1604932d 100644
--- a/cloudinit/sources/__init__.py
+++ b/cloudinit/sources/__init__.py
@@ -164,9 +164,6 @@ class DataSource(object):
     # A datasource which supports writing network config on each system boot
     # would call update_events['network'].add(EventType.BOOT).
 
-    # Default: generate network config on new instance id (first boot).
-    update_events = {'network': set([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
     # values are reset via clear_cached_attrs during any update_metadata call.
@@ -191,6 +188,9 @@ class DataSource(object):
         self.vendordata = None
         self.vendordata_raw = None
 
+        # Default: generate network config on new instance id (first boot).
+        self.update_events = {'network': {EventType.BOOT_NEW_INSTANCE}}
+
         self.ds_cfg = util.get_cfg_by_path(
             self.sys_cfg, ("datasource", self.dsname), {})
         if not self.ds_cfg:
diff --git a/cloudinit/sources/tests/test_init.py b/cloudinit/sources/tests/test_init.py
index 6378e98b..cb1912be 100644
--- a/cloudinit/sources/tests/test_init.py
+++ b/cloudinit/sources/tests/test_init.py
@@ -575,6 +575,21 @@ class TestDataSource(CiTestCase):
             " events: New instance first boot",
             self.logs.getvalue())
 
+    def test_data_sources_cant_mutate_update_events_for_others(self):
+        """update_events shouldn't be changed for other DSes (LP: #1819913)"""
+
+        class ModifyingDS(DataSource):
+
+            def __init__(self, sys_cfg, distro, paths):
+                # This mirrors what DataSourceAzure does which causes LP:
+                # #1819913
+                DataSource.__init__(self, sys_cfg, distro, paths)
+                self.update_events['network'].add(EventType.BOOT)
+
+        before_update_events = copy.deepcopy(self.datasource.update_events)
+        ModifyingDS(self.sys_cfg, self.distro, self.paths)
+        self.assertEqual(before_update_events, self.datasource.update_events)
+
 
 class TestRedactSensitiveData(CiTestCase):