introduce an upgrade framework and related testing (#659)

This commit does the following:

* introduces the `cloudinit.persistence` module, containing
  `CloudInitPickleMixin` which provides lightweight versioning of
  objects' pickled representations (and associated testing)
* introduces a basic upgrade testing framework (in
  `cloudinit.tests.test_upgrade`) which unpickles pickles from previous
  versions of cloud-init (stored in `tests/data/old_pickles`) and tests
  invariants that the current cloud-init codebase expects
* uses the versioning framework to address an upgrade issue where
  `Distro.networking` could get into an unexpected state, and uses the
  upgrade testing framework to confirm that the issue is addressed

1 files changed, 45 insertions, 0 deletions

diff --git a/cloudinit/tests/test_upgrade.py b/cloudinit/tests/test_upgrade.py
new file mode 100644
index 00000000..f79a2536
--- /dev/null
+++ b/cloudinit/tests/test_upgrade.py
@@ -0,0 +1,45 @@
+# Copyright (C) 2020 Canonical Ltd.
+#
+# Author: Daniel Watkins <oddbloke@ubuntu.com>
+#
+# This file is part of cloud-init. See LICENSE file for license information.
+
+"""Upgrade testing for cloud-init.
+
+This module tests cloud-init's behaviour across upgrades.  Specifically, it
+specifies a set of invariants that the current codebase expects to be true (as
+tests in ``TestUpgrade``) and then checks that these hold true after unpickling
+``obj.pkl``s from previous versions of cloud-init; those pickles are stored in
+``tests/data/old_pickles/``.
+"""
+
+import operator
+import pathlib
+
+import pytest
+
+from cloudinit.stages import _pkl_load
+from cloudinit.tests.helpers import resourceLocation
+
+
+class TestUpgrade:
+    @pytest.fixture(
+        params=pathlib.Path(resourceLocation("old_pickles")).glob("*.pkl"),
+        scope="class",
+        ids=operator.attrgetter("name"),
+    )
+    def previous_obj_pkl(self, request):
+        """Load each pickle to memory once, then run all tests against it.
+
+        Test implementations _must not_ modify the ``previous_obj_pkl`` which
+        they are passed, as that will affect tests that run after them.
+        """
+        return _pkl_load(str(request.param))
+
+    def test_networking_set_on_distro(self, previous_obj_pkl):
+        """We always expect to have ``.networking`` on ``Distro`` objects."""
+        assert previous_obj_pkl.distro.networking is not None
+
+    def test_blacklist_drivers_set_on_networking(self, previous_obj_pkl):
+        """We always expect Networking.blacklist_drivers to be initialised."""
+        assert previous_obj_pkl.distro.networking.blacklist_drivers is None