Let a share-backed backup say what is in it

plugins/backups/models/backup.py

@@ -67,7 +67,10 @@ class BackupRevision(db.Model):
     # (assetid, backupkind).
     contenthash = db.Column(db.String(64), nullable=False, index=True)
 
-    # Populated for storagebackend='shopdb' only. MEDIUMTEXT holding serialized
+    # Populated for storagebackend='shopdb', and optionally for 'share' kinds
+    # that send a manifest describing what their snapshot contains (the bytes
+    # still live on the share; this is the index, not the content).
+    # MEDIUMTEXT holding serialized
     # JSON rather than a native JSON column, matching geenforce.manifestjson.
     # Text also means the bytes come back exactly as written, so the canonical
     # key ordering the codec produces survives the round trip - a native JSON
@@ -105,7 +108,18 @@ class BackupRevision(db.Model):
 
     @property
     def payload(self):
-        """Decoded payloadjson, or None for opaque (share-backed) kinds."""
+        """Decoded payloadjson, or None when the row stores nothing.
+
+        A 'shopdb' kind always has one: the parsed projection ShopDB is
+        authoritative for. A 'share' kind has one only when its client sends a
+        manifest describing the snapshot's contents, and None otherwise - which
+        includes every share revision written before manifests existed.
+
+        Note the manifest is NOT covered by contenthash: the hash attests the
+        bytes on the share, so two posts with identical content but different
+        manifests dedup to one row. apply_collector_payload backfills a missing
+        manifest on a no-op for that reason.
+        """
         if not self.payloadjson:
             return None
         try: