Recursively fetch the thread for receipts & notifications. (#13824)

Consider an event to be part of a thread if you can follow a
chain of relations up to a thread root.

Part of MSC3773 & MSC3771.

1 files changed, 20 insertions, 2 deletions

diff --git a/synapse/rest/client/receipts.py b/synapse/rest/client/receipts.py
index f3ff156abe..287dfdd69e 100644
--- a/synapse/rest/client/receipts.py
+++ b/synapse/rest/client/receipts.py
@@ -16,7 +16,7 @@ import logging
 from typing import TYPE_CHECKING, Tuple
 
 from synapse.api.constants import ReceiptTypes
-from synapse.api.errors import SynapseError
+from synapse.api.errors import Codes, SynapseError
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
 from synapse.http.site import SynapseRequest
@@ -43,6 +43,7 @@ class ReceiptRestServlet(RestServlet):
         self.receipts_handler = hs.get_receipts_handler()
         self.read_marker_handler = hs.get_read_marker_handler()
         self.presence_handler = hs.get_presence_handler()
+        self._main_store = hs.get_datastores().main
 
         self._known_receipt_types = {
             ReceiptTypes.READ,
@@ -71,7 +72,24 @@ class ReceiptRestServlet(RestServlet):
                 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"
+                        400,
+                        "thread_id field must be a non-empty string",
+                        Codes.INVALID_PARAM,
+                    )
+
+                if receipt_type == ReceiptTypes.FULLY_READ:
+                    raise SynapseError(
+                        400,
+                        f"thread_id is not compatible with {ReceiptTypes.FULLY_READ} receipts.",
+                        Codes.INVALID_PARAM,
+                    )
+
+                # Ensure the event ID roughly correlates to the thread ID.
+                if thread_id != await self._main_store.get_thread_id(event_id):
+                    raise SynapseError(
+                        400,
+                        f"event_id {event_id} is not related to thread {thread_id}",
+                        Codes.INVALID_PARAM,
                     )
 
         await self.presence_handler.bump_presence_active_time(requester.user)