Apply join rate limiter outside the lineariser (#16441)

changelog.d/16441.misc

@@ -0,0 +1 @@
+Improve rate limiting logic.

synapse/handlers/room_member.py

@@ -382,8 +382,10 @@ async def _local_membership_update(
         and persist a new event for the new membership change.
 
         Args:
-            requester:
-            target:
+            requester: User requesting the membership change, i.e. the sender of the
+                desired membership event.
+            target: Use whose membership should change, i.e. the state_key of the
+                desired membership event.
             room_id:
             membership:
 
@@ -415,7 +417,6 @@ async def _local_membership_update(
         Returns:
             Tuple of event ID and stream ordering position
         """
-
         user_id = target.to_string()
 
         if content is None:
@@ -475,21 +476,6 @@ async def _local_membership_update(
                     (EventTypes.Member, user_id), None
                 )
 
-                if event.membership == Membership.JOIN:
-                    newly_joined = True
-                    if prev_member_event_id:
-                        prev_member_event = await self.store.get_event(
-                            prev_member_event_id
-                        )
-                        newly_joined = prev_member_event.membership != Membership.JOIN
-
-                    # Only rate-limit if the user actually joined the room, otherwise we'll end
-                    # up blocking profile updates.
-                    if newly_joined and ratelimit:
-                        await self._join_rate_limiter_local.ratelimit(requester)
-                        await self._join_rate_per_room_limiter.ratelimit(
-                            requester, key=room_id, update=False
-                        )
                 with opentracing.start_active_span("handle_new_client_event"):
                     result_event = (
                         await self.event_creation_handler.handle_new_client_event(
@@ -618,6 +604,25 @@ async def update_membership(
         Raises:
             ShadowBanError if a shadow-banned requester attempts to send an invite.
         """
+        if ratelimit:
+            if action == Membership.JOIN:
+                # Only rate-limit if the user isn't already joined to the room, otherwise
+                # we'll end up blocking profile updates.
+                (
+                    current_membership,
+                    _,
+                ) = await self.store.get_local_current_membership_for_user_in_room(
+                    requester.user.to_string(),
+                    room_id,
+                )
+                if current_membership != Membership.JOIN:
+                    await self._join_rate_limiter_local.ratelimit(requester)
+                    await self._join_rate_per_room_limiter.ratelimit(
+                        requester, key=room_id, update=False
+                    )
+            elif action == Membership.INVITE:
+                await self.ratelimit_invite(requester, room_id, target.to_string())
+
         if action == Membership.INVITE and requester.shadow_banned:
             # We randomly sleep a bit just to annoy the requester.
             await self.clock.sleep(random.randint(1, 10))
@@ -794,8 +799,6 @@ async def update_membership_locked(
 
         if effective_membership_state == Membership.INVITE:
             target_id = target.to_string()
-            if ratelimit:
-                await self.ratelimit_invite(requester, room_id, target_id)
 
             # block any attempts to invite the server notices mxid
             if target_id == self._server_notices_mxid:

tests/rest/client/test_rooms.py

@@ -1444,6 +1444,30 @@ def test_join_local_ratelimit(self) -> None:
             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}}}
+    )
+    def test_join_attempts_local_ratelimit(self) -> None:
+        """Tests that unsuccessful joins that end up being denied are rate-limited."""
+        # Create 4 rooms
+        room_ids = [
+            self.helper.create_room_as(self.user_id, is_public=True) for _ in range(4)
+        ]
+        # Pre-emptively ban the user who will attempt to join.
+        joiner_user_id = self.register_user("joiner", "secret")
+        for room_id in room_ids:
+            self.helper.ban(room_id, self.user_id, joiner_user_id)
+
+        # Now make a new user try to join some of them.
+        # The user can make 3 requests, each of which should be denied.
+        for room_id in room_ids[0:3]:
+            self.helper.join(room_id, joiner_user_id, expect_code=HTTPStatus.FORBIDDEN)
+
+        # The fourth attempt should be rate limited.
+        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}}}
     )