Add resiliant repair for raw manifest data on sync

CHANGES/+sync_repair_manifest_data.bugfix

@@ -0,0 +1 @@
+Add transparent repair code during Manifest sync to redownload the raw data in case it was lost during an upgrade.

pulp_container/app/tasks/sync_stages.py

@@ -82,7 +82,15 @@ async def _check_for_existing_manifest(self, download_tag):
             digest=digest, pulp_domain=get_domain()
         ).afirst():
             raw_text_data = manifest.data
-            content_data = json.loads(raw_text_data)
+            if raw_text_data is None:
+                # This situation allegedly happens on some upgrade paths.
+                # A migration should mark the field not Null eventually. At that point this
+                # workaround can be removed.
+                content_data, raw_text_data, _ = await self._download_manifest_data(response.url)
+                manifest.data = raw_text_data
+                await manifest.asave()
+            else:
+                content_data = json.loads(raw_text_data)
         else:
             content_data, raw_text_data, response = await self._download_manifest_data(response.url)