Fix MIME policy failure on python version upgrade (#934)

Python 3.6 added a new `policy` attribute to `MIMEMultipart`.
MIMEMultipart may be part of the cached object pickle of a datasource.
Upgrading from an old version of python to 3.6+ will cause the
datasource to be invalid after pickle load.

This commit uses the upgrade framework to attempt to access the mime
message and fail early (thus discarding the cache) if we cannot.
Commit 78e89b0 should fix this issue more generally.

cloudinit/sources/__init__.py

@@ -75,6 +75,10 @@
                                  _NETCFG_SOURCE_NAMES)(*_NETCFG_SOURCE_NAMES)
 
 
+class DatasourceUnpickleUserDataError(Exception):
+    """Raised when userdata is unable to be unpickled due to python upgrades"""
+
+
 class DataSourceNotFoundException(Exception):
     pass
 
@@ -239,6 +243,20 @@ def _unpickle(self, ci_pkl_version: int) -> None:
             self.vendordata2 = None
         if not hasattr(self, 'vendordata2_raw'):
             self.vendordata2_raw = None
+        if hasattr(self, 'userdata') and self.userdata is not None:
+            # If userdata stores MIME data, on < python3.6 it will be
+            # missing the 'policy' attribute that exists on >=python3.6.
+            # Calling str() on the userdata will attempt to access this
+            # policy attribute. This will raise an exception, causing
+            # the pickle load to fail, so cloud-init will discard the cache
+            try:
+                str(self.userdata)
+            except AttributeError as e:
+                LOG.debug(
+                    "Unable to unpickle datasource: %s."
+                    " Ignoring current cache.", e
+                )
+                raise DatasourceUnpickleUserDataError() from e
 
     def __str__(self):
         return type_utils.obj_name(self)

cloudinit/stages.py

@@ -1070,6 +1070,8 @@ def _pkl_load(fname):
         return None
     try:
         return pickle.loads(pickle_contents)
+    except sources.DatasourceUnpickleUserDataError:
+        return None
     except Exception:
         util.logexc(LOG, "Failed loading pickled blob from %s", fname)
         return None