Let a share-backed backup say what is in it

plugins/backups/api/routes.py

@@ -156,9 +156,30 @@ def asset_info(assetid):
     if revision is None:
         return success_response({'fields': [], 'sectioncount': 0})
 
+    try:
+        projection = revision.payload
+    except ValueError:
+        # payload deliberately raises on unreadable payloadjson so a corrupt row
+        # is named rather than mistaken for an opaque one. That is right for a
+        # download; for an at-a-glance card it would take the whole asset page
+        # down over one bad row, so the card degrades to empty instead.
+        current_app.logger.warning(
+            'backuprevision %s has unreadable payloadjson; info card empty',
+            revision.backuprevisionid)
+        return success_response({'fields': [], 'sectioncount': 0})
+
     data = kind.buildinfo(
-        revision.payload, assetid,
-        partmarkertypes=current_app.config.get('BACKUPS_PARTMARKER_TYPES'))
+        projection, assetid,
+        partmarkertypes=current_app.config.get('BACKUPS_PARTMARKER_TYPES'),
+        # A kind that diffs against its own previous revision needs to know
+        # which revision it is looking at. Passed generically rather than
+        # queried inside the kind, so the endpoint stays the single place that
+        # decides "latest".
+        revisionid=revision.backuprevisionid,
+        # The card names the PC a snapshot came from, and chains its diff on the
+        # same value. Passed from the column rather than read out of the stored
+        # manifest, which carries its own copy that can disagree.
+        sourcehostname=revision.sourcehostname)
     data['backuprevisionid'] = revision.backuprevisionid
     data['collectedat'] = _utciso(revision.collectedat)
     return success_response(data)