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, 16 insertions, 1 deletions

diff --git a/cloudinit/distros/__init__.py b/cloudinit/distros/__init__.py
index fac8cf67..1e118472 100755
--- a/cloudinit/distros/__init__.py
+++ b/cloudinit/distros/__init__.py
@@ -23,6 +23,7 @@ from cloudinit import net
 from cloudinit.net import eni
 from cloudinit.net import network_state
 from cloudinit.net import renderers
+from cloudinit import persistence
 from cloudinit import ssh_util
 from cloudinit import type_utils
 from cloudinit import subp
@@ -62,7 +63,7 @@ PREFERRED_NTP_CLIENTS = ['chrony', 'systemd-timesyncd', 'ntp', 'ntpdate']
 LDH_ASCII_CHARS = string.ascii_letters + string.digits + "-"
 
 
-class Distro(metaclass=abc.ABCMeta):
+class Distro(persistence.CloudInitPickleMixin, metaclass=abc.ABCMeta):
 
     usr_lib_exec = "/usr/lib"
     hosts_fn = "/etc/hosts"
@@ -77,12 +78,26 @@ class Distro(metaclass=abc.ABCMeta):
     # subclasses
     shutdown_options_map = {'halt': '-H', 'poweroff': '-P', 'reboot': '-r'}
 
+    _ci_pkl_version = 1
+
     def __init__(self, name, cfg, paths):
         self._paths = paths
         self._cfg = cfg
         self.name = name
         self.networking = self.networking_cls()
 
+    def _unpickle(self, ci_pkl_version: int) -> None:
+        """Perform deserialization fixes for Distro."""
+        if "networking" not in self.__dict__ or not self.networking.__dict__:
+            # This is either a Distro pickle with no networking attribute OR
+            # this is a Distro pickle with a networking attribute but from
+            # before ``Networking`` had any state (meaning that
+            # Networking.__setstate__ will not be called).  In either case, we
+            # want to ensure that `self.networking` is freshly-instantiated:
+            # either because it isn't present at all, or because it will be
+            # missing expected instance state otherwise.
+            self.networking = self.networking_cls()
+
     @abc.abstractmethod
     def install_packages(self, pkglist):
         raise NotImplementedError()