Fix room creation being rate limited too aggressively since Synapse v1.69.0. (#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) <oliverw@matrix.org>

* 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) <oliverw@matrix.org>

4 files changed, 70 insertions, 9 deletions

diff --git a/changelog.d/14314.bugfix b/changelog.d/14314.bugfix
new file mode 100644
index 0000000000..8be47ee083
--- /dev/null
+++ b/changelog.d/14314.bugfix
@@ -0,0 +1 @@
+Fix room creation being rate limited too aggressively since Synapse v1.69.0.
\ No newline at end of file
diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py
index 044c7d4926..511790c7c5 100644
--- a/synapse/api/ratelimiting.py
+++ b/synapse/api/ratelimiting.py
@@ -343,6 +343,7 @@ class RequestRatelimiter:
         requester: Requester,
         update: bool = True,
         is_admin_redaction: bool = False,
+        n_actions: int = 1,
     ) -> None:
         """Ratelimits requests.
 
@@ -355,6 +356,8 @@ class RequestRatelimiter:
             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
@@ -383,7 +386,9 @@ class RequestRatelimiter:
         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(
@@ -391,4 +396,5 @@ class RequestRatelimiter:
                 rate_hz=messages_per_second,
                 burst_count=burst_count,
                 update=update,
+                n_actions=n_actions,
             )
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 638f54051a..d74b675adc 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -559,7 +559,6 @@ class RoomCreationHandler:
             invite_list=[],
             initial_state=initial_state,
             creation_content=creation_content,
-            ratelimit=False,
         )
 
         # Transfer membership events
@@ -753,6 +752,10 @@ class RoomCreationHandler:
                 )
 
         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 @@ class RoomCreationHandler:
             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 @@ class RoomCreationHandler:
         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 @@ class RoomCreationHandler:
         `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 @@ class RoomCreationHandler:
             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 @@ class RoomCreationHandler:
             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
diff --git a/tests/rest/client/test_rooms.py b/tests/rest/client/test_rooms.py
index 716366eb90..1084d4ad9d 100644
--- a/tests/rest/client/test_rooms.py
+++ b/tests/rest/client/test_rooms.py
@@ -54,6 +54,7 @@ from tests.http.server._base import make_request_with_cancellation_test
 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 @@ class RoomsCreateTestCase(RoomBase):
         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 @@ class RoomJoinRatelimitTestCase(RoomBase):
     )
     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}}}