diff --git a/tests/rest/client/test_third_party_rules.py b/tests/rest/client/test_third_party_rules.py
index 0ae4029640..38ac9be113 100644
--- a/tests/rest/client/test_third_party_rules.py
+++ b/tests/rest/client/test_third_party_rules.py
@@ -15,6 +15,7 @@ import threading
from typing import Dict
from unittest.mock import Mock
+from synapse.api.constants import EventTypes
from synapse.events import EventBase
from synapse.events.third_party_rules import load_legacy_third_party_event_rules
from synapse.module_api import ModuleApi
@@ -327,3 +328,86 @@ class ThirdPartyRulesTestCase(unittest.HomeserverTestCase):
correctly.
"""
self.helper.create_room_as(self.user_id, tok=self.tok, expect_code=403)
+
+ def test_sent_event_end_up_in_room_state(self):
+ """Tests that a state event sent by a module while processing another state event
+ doesn't get dropped from the state of the room. This is to guard against a bug
+ where Synapse has been observed doing so, see https://github.com/matrix-org/synapse/issues/10830
+ """
+ event_type = "org.matrix.test_state"
+
+ # This content will be updated later on, and since we actually use a reference on
+ # the dict it does the right thing. It's a bit hacky but a handy way of making
+ # sure the state actually gets updated.
+ event_content = {"i": -1}
+
+ api = self.hs.get_module_api()
+
+ # Define a callback that sends a custom event on power levels update.
+ async def test_fn(event: EventBase, state_events):
+ if event.is_state and event.type == EventTypes.PowerLevels:
+ await api.create_and_send_event_into_room(
+ {
+ "room_id": event.room_id,
+ "sender": event.sender,
+ "type": event_type,
+ "content": event_content,
+ "state_key": "",
+ }
+ )
+ return True, None
+
+ self.hs.get_third_party_event_rules()._check_event_allowed_callbacks = [test_fn]
+
+ # Sometimes the bug might not happen the first time the event type is added
+ # to the state but might happen when an event updates the state of the room for
+ # that type, so we test updating the state several times.
+ for i in range(5):
+ # Update the content of the custom state event to be sent by the callback.
+ event_content["i"] = i
+
+ # Update the room's power levels with a different value each time so Synapse
+ # doesn't consider an update redundant.
+ self._update_power_levels(event_default=i)
+
+ # Check that the new event made it to the room's state.
+ channel = self.make_request(
+ method="GET",
+ path="/rooms/" + self.room_id + "/state/" + event_type,
+ access_token=self.tok,
+ )
+
+ self.assertEqual(channel.code, 200)
+ self.assertEqual(channel.json_body["i"], i)
+
+ def _update_power_levels(self, event_default: int = 0):
+ """Updates the room's power levels.
+
+ Args:
+ event_default: Value to use for 'events_default'.
+ """
+ self.helper.send_state(
+ room_id=self.room_id,
+ event_type=EventTypes.PowerLevels,
+ body={
+ "ban": 50,
+ "events": {
+ "m.room.avatar": 50,
+ "m.room.canonical_alias": 50,
+ "m.room.encryption": 100,
+ "m.room.history_visibility": 100,
+ "m.room.name": 50,
+ "m.room.power_levels": 100,
+ "m.room.server_acl": 100,
+ "m.room.tombstone": 100,
+ },
+ "events_default": event_default,
+ "invite": 0,
+ "kick": 50,
+ "redact": 50,
+ "state_default": 50,
+ "users": {self.user_id: 100},
+ "users_default": 0,
+ },
+ tok=self.tok,
+ )
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