Return thread receipts down sync.

synapse/storage/databases/main/receipts.py

@@ -412,6 +412,14 @@ def f(txn: LoggingTransaction) -> List[Dict[str, Any]]:
             "_get_linearized_receipts_for_rooms", f
         )
 
+        # Map of room ID to a dictionary in the form that sync wants it.
+        #
+        # room ID -> dict of:
+        #   type: str
+        #   room_id: str
+        #   content: event ID -> dict of:
+        #       receipt type -> dict of:
+        #           user ID -> data
         results: JsonDict = {}
         for row in txn_results:
             # We want a single event per room, since we want to batch the
@@ -427,6 +435,8 @@ def f(txn: LoggingTransaction) -> List[Dict[str, Any]]:
             receipt_type = event_entry.setdefault(row["receipt_type"], {})
 
             receipt_type[row["user_id"]] = db_to_json(row["data"])
+            if row["thread_id"]:
+                receipt_type[row["user_id"]]["thread_id"] = row["thread_id"]
 
         results = {
             room_id: [results[room_id]] if room_id in results else []