Fix MIME policy failure on python version upgrade (SC-201)

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