Fix room creation being rate limited too aggressively since Synapse v…

…1.69.0. (#14314)

* Introduce a test for the old behaviour which we want to restore

* Reintroduce the old behaviour in a simpler way

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <[email protected]>

* Use 1 credit instead of 2 for creating a room: be more lenient than before

Notably, the UI in Element Web was still broken after restoring to prior behaviour.

After discussion, we agreed that it would be sensible to increase the limit.

Signed-off-by: Olivier Wilkinson (reivilibre) <[email protected]>

changelog.d/14314.bugfix

@@ -0,0 +1 @@
+Fix room creation being rate limited too aggressively since Synapse v1.69.0.

synapse/api/ratelimiting.py

@@ -343,6 +343,7 @@ async def ratelimit(
         requester: Requester,
         update: bool = True,
         is_admin_redaction: bool = False,
+        n_actions: int = 1,
     ) -> None:
         """Ratelimits requests.
 
@@ -355,6 +356,8 @@ async def ratelimit(
             is_admin_redaction: Whether this is a room admin/moderator
                 redacting an event. If so then we may apply different
                 ratelimits depending on config.
+            n_actions: Multiplier for the number of actions to apply to the
+                rate limiter at once.
 
         Raises:
             LimitExceededError if the request should be ratelimited
@@ -383,12 +386,15 @@ async def ratelimit(
         if is_admin_redaction and self.admin_redaction_ratelimiter:
             # If we have separate config for admin redactions, use a separate
             # ratelimiter as to not have user_ids clash
-            await self.admin_redaction_ratelimiter.ratelimit(requester, update=update)
+            await self.admin_redaction_ratelimiter.ratelimit(
+                requester, update=update, n_actions=n_actions
+            )
         else:
             # Override rate and burst count per-user
             await self.request_ratelimiter.ratelimit(
                 requester,
                 rate_hz=messages_per_second,
                 burst_count=burst_count,
                 update=update,
+                n_actions=n_actions,
             )

synapse/handlers/room.py

@@ -559,7 +559,6 @@ async def clone_existing_room(
             invite_list=[],
             initial_state=initial_state,
             creation_content=creation_content,
-            ratelimit=False,
         )
 
         # Transfer membership events
@@ -753,6 +752,10 @@ async def create_room(
                 )
 
         if ratelimit:
+            # Rate limit once in advance, but don't rate limit the individual
+            # events in the room — room creation isn't atomic and it's very
+            # janky if half the events in the initial state don't make it because
+            # of rate limiting.
             await self.request_ratelimiter.ratelimit(requester)
 
         room_version_id = config.get(
@@ -913,7 +916,6 @@ async def create_room(
             room_alias=room_alias,
             power_level_content_override=power_level_content_override,
             creator_join_profile=creator_join_profile,
-            ratelimit=ratelimit,
         )
 
         if "name" in config:
@@ -1037,7 +1039,6 @@ async def _send_events_for_new_room(
         room_alias: Optional[RoomAlias] = None,
         power_level_content_override: Optional[JsonDict] = None,
         creator_join_profile: Optional[JsonDict] = None,
-        ratelimit: bool = True,
     ) -> Tuple[int, str, int]:
         """Sends the initial events into a new room. Sends the room creation, membership,
         and power level events into the room sequentially, then creates and batches up the
@@ -1046,6 +1047,8 @@ async def _send_events_for_new_room(
         `power_level_content_override` doesn't apply when initial state has
         power level state event content.
 
+        Rate limiting should already have been applied by this point.
+
         Returns:
             A tuple containing the stream ID, event ID and depth of the last
             event sent to the room.
@@ -1144,7 +1147,7 @@ async def send(
             creator.user,
             room_id,
             "join",
-            ratelimit=ratelimit,
+            ratelimit=False,
             content=creator_join_profile,
             new_room=True,
             prev_event_ids=[last_sent_event_id],
@@ -1269,7 +1272,10 @@ async def send(
             events_to_send.append((encryption_event, encryption_context))
 
         last_event = await self.event_creation_handler.handle_new_client_event(
-            creator, events_to_send, ignore_shadow_ban=True
+            creator,
+            events_to_send,
+            ignore_shadow_ban=True,
+            ratelimit=False,
         )
         assert last_event.internal_metadata.stream_ordering is not None
         return last_event.internal_metadata.stream_ordering, last_event.event_id, depth

tests/rest/client/test_rooms.py

@@ -54,6 +54,7 @@
 from tests.storage.test_stream import PaginationTestCase
 from tests.test_utils import make_awaitable
 from tests.test_utils.event_injection import create_event
+from tests.unittest import override_config
 
 PATH_PREFIX = b"/_matrix/client/api/v1"
 
@@ -871,6 +872,41 @@ async def user_may_join_room_tuple(
         self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
         self.assertEqual(join_mock.call_count, 0)
 
+    def _create_basic_room(self) -> Tuple[int, object]:
+        """
+        Tries to create a basic room and returns the response code.
+        """
+        channel = self.make_request(
+            "POST",
+            "/createRoom",
+            {},
+        )
+        return channel.code, channel.json_body
+
+    @override_config(
+        {
+            "rc_message": {"per_second": 0.2, "burst_count": 10},
+        }
+    )
+    def test_room_creation_ratelimiting(self) -> None:
+        """
+        Regression test for #14312, where ratelimiting was made too strict.
+        Clients should be able to create 10 rooms in a row
+        without hitting rate limits, using default rate limit config.
+        (We override rate limiting config back to its default value.)
+
+        To ensure we don't make ratelimiting too generous accidentally,
+        also check that we can't create an 11th room.
+        """
+
+        for _ in range(10):
+            code, json_body = self._create_basic_room()
+            self.assertEqual(code, HTTPStatus.OK, json_body)
+
+        # The 6th room hits the rate limit.
+        code, json_body = self._create_basic_room()
+        self.assertEqual(code, HTTPStatus.TOO_MANY_REQUESTS, json_body)
+
 
 class RoomTopicTestCase(RoomBase):
     """Tests /rooms/$room_id/topic REST events."""
@@ -1390,10 +1426,22 @@ def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
     )
     def test_join_local_ratelimit(self) -> None:
         """Tests that local joins are actually rate-limited."""
-        for _ in range(3):
-            self.helper.create_room_as(self.user_id)
+        # Create 4 rooms
+        room_ids = [
+            self.helper.create_room_as(self.user_id, is_public=True) for _ in range(4)
+        ]
+
+        joiner_user_id = self.register_user("joiner", "secret")
+        # Now make a new user try to join some of them.
 
-        self.helper.create_room_as(self.user_id, expect_code=429)
+        # The user can join 3 rooms
+        for room_id in room_ids[0:3]:
+            self.helper.join(room_id, joiner_user_id)
+
+        # But the user cannot join a 4th room
+        self.helper.join(
+            room_ids[3], joiner_user_id, expect_code=HTTPStatus.TOO_MANY_REQUESTS
+        )
 
     @unittest.override_config(
         {"rc_joins": {"local": {"per_second": 0.5, "burst_count": 3}}}