Accept threaded receipts for events related to the root event. (#14174)

The root node of a thread (and events related to it) are considered
"part of a thread" when validating receipts. This allows clients which
show the root node in both the main timeline and the threaded timeline
to easily send receipts in either.

Note that threaded notifications are not created for these events, these
events created notifications on the main timeline.

synapse/rest/client/receipts.py

@@ -15,7 +15,7 @@
 import logging
 from typing import TYPE_CHECKING, Tuple
 
-from synapse.api.constants import ReceiptTypes
+from synapse.api.constants import MAIN_TIMELINE, ReceiptTypes
 from synapse.api.errors import Codes, SynapseError
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
@@ -83,7 +83,7 @@ class ReceiptRestServlet(RestServlet):
                 )
 
             # Ensure the event ID roughly correlates to the thread ID.
-            if thread_id != await self._main_store.get_thread_id(event_id):
+            if not await self._is_event_in_thread(event_id, thread_id):
                 raise SynapseError(
                     400,
                     f"event_id {event_id} is not related to thread {thread_id}",
@@ -109,6 +109,46 @@ class ReceiptRestServlet(RestServlet):
 
         return 200, {}
 
+    async def _is_event_in_thread(self, event_id: str, thread_id: str) -> bool:
+        """
+        The event must be related to the thread ID (in a vague sense) to ensure
+        clients aren't sending bogus receipts.
+
+        A thread ID is considered valid for a given event E if:
+
+        1. E has a thread relation which matches the thread ID;
+        2. E has another event which has a thread relation to E matching the
+           thread ID; or
+        3. E is recursively related (via any rel_type) to an event which
+           satisfies 1 or 2.
+
+        Given the following DAG:
+
+            A <---[m.thread]-- B <--[m.annotation]-- C
+            ^
+            |--[m.reference]-- D <--[m.annotation]-- E
+
+        It is valid to send a receipt for thread A on A, B, C, D, or E.
+
+        It is valid to send a receipt for the main timeline on A, D, and E.
+
+        Args:
+            event_id: The event ID to check.
+            thread_id: The thread ID the event is potentially part of.
+
+        Returns:
+            True if the event belongs to the given thread, otherwise False.
+        """
+
+        # If the receipt is on the main timeline, it is enough to check whether
+        # the event is directly related to a thread.
+        if thread_id == MAIN_TIMELINE:
+            return MAIN_TIMELINE == await self._main_store.get_thread_id(event_id)
+
+        # Otherwise, check if the event is directly part of a thread, or is the
+        # root message (or related to the root message) of a thread.
+        return thread_id == await self._main_store.get_thread_id_for_receipts(event_id)
+
 
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ReceiptRestServlet(hs).register(http_server)