Accept & store thread IDs for receipts (implement MSC3771). (#13782)

Updates the `/receipts` endpoint and receipt EDU handler to parse a
`thread_id` from the body and insert it in the database.

synapse/rest/client/receipts.py

@@ -49,6 +49,7 @@ class ReceiptRestServlet(RestServlet):
             ReceiptTypes.READ_PRIVATE,
             ReceiptTypes.FULLY_READ,
         }
+        self._msc3771_enabled = hs.config.experimental.msc3771_enabled
 
     async def on_POST(
         self, request: SynapseRequest, room_id: str, receipt_type: str, event_id: str
@@ -61,7 +62,17 @@ class ReceiptRestServlet(RestServlet):
                 f"Receipt type must be {', '.join(self._known_receipt_types)}",
             )
 
-        parse_json_object_from_request(request, allow_empty_body=False)
+        body = parse_json_object_from_request(request)
+
+        # Pull the thread ID, if one exists.
+        thread_id = None
+        if self._msc3771_enabled:
+            if "thread_id" in body:
+                thread_id = body.get("thread_id")
+                if not thread_id or not isinstance(thread_id, str):
+                    raise SynapseError(
+                        400, "thread_id field must be a non-empty string"
+                    )
 
         await self.presence_handler.bump_presence_active_time(requester.user)
 
@@ -77,6 +88,7 @@ class ReceiptRestServlet(RestServlet):
                 receipt_type,
                 user_id=requester.user.to_string(),
                 event_id=event_id,
+                thread_id=thread_id,
             )
 
         return 200, {}