Test case for an edgy race

changelog.d/11701.misc

@@ -0,0 +1 @@
+Add a test for [an edge case](https://github.com/matrix-org/synapse/pull/11532#discussion_r769104461) in the `/sync` logic.

tests/handlers/test_sync.py

@@ -11,22 +11,22 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
 from typing import Optional
-from unittest.mock import Mock
+from unittest.mock import MagicMock, Mock, patch
 
 from synapse.api.constants import EventTypes, JoinRules
 from synapse.api.errors import Codes, ResourceLimitError
 from synapse.api.filtering import Filtering
 from synapse.api.room_versions import RoomVersions
-from synapse.handlers.sync import SyncConfig
+from synapse.handlers.sync import SyncConfig, SyncResult
 from synapse.rest import admin
 from synapse.rest.client import knock, login, room
 from synapse.server import HomeServer
 from synapse.types import UserID, create_requester
 
 import tests.unittest
 import tests.utils
+from tests.test_utils import make_awaitable
 
 
 class SyncTestCase(tests.unittest.HomeserverTestCase):
@@ -186,6 +186,97 @@ def test_unknown_room_version(self):
  self.assertNotIn(invite_room, [r.room_id for r in result.invited])
  self.assertNotIn(knock_room, [r.room_id for r in result.knocked])
 
+ def test_ban_wins_race_with_join(self):
+ """Rooms shouldn't appear under "joined" if a join loses a race to a ban.
+
+ A complicated edge case. Imagine the following scenario:
+
+ * you attempt to join a room
+ * racing with that is a ban which comes in over federation, which ends up with
+ an earlier stream_ordering than the join.
+ * you get a sync response with a sync token which is _after_ the ban, but before
+ the join
+ * now your join lands; it is a valid event because its `prev_event`s predate the
+ ban, but will not make it into current_state_events (because bans win over
+ joins in state res, essentially).
+ * When we do a sync from the incremental sync, the only event in the timeline
+ is your join ... and yet you aren't joined.
+
+ The ban coming in over federation isn't crucial for this behaviour; the key
+ requirements are:
+ 1. the homeserver generates a join event with prev_events that precede the ban
+ (so that it passes the "are you banned" test)
+ 2. the join event has a stream_ordering after that of the ban.
+
+ We use monkeypatching to artificially trigger condition (1).
+ """
+ # A local user Alice creates a room.
+ owner = self.register_user("alice", "password")
+ owner_tok = self.login(owner, "password")
+ room_id = self.helper.create_room_as(owner, is_public=True, tok=owner_tok)
+
+ # Do a sync as Alice to get the latest event in the room.
+ alice_sync_result: SyncResult = self.get_success(
+ self.sync_handler.wait_for_sync_for_user(
+ create_requester(owner), generate_sync_config(owner)
+ )
+ )
+ self.assertEqual(len(alice_sync_result.joined), 1)
+ self.assertEqual(alice_sync_result.joined[0].room_id, room_id)
+ last_room_creation_event_id = (
+ alice_sync_result.joined[0].timeline.events[-1].event_id
+ )
+
+ # Eve, a ne'er-do-well, registers.
+ eve = self.register_user("eve", "password")
+ eve_token = self.login(eve, "password")
+
+ # Alice preemptively bans Eve.
+ self.helper.ban(room_id, owner, eve, tok=owner_tok)
+
+ # Eve syncs.
+ eve_requester = create_requester(eve)
+ eve_sync_config = generate_sync_config(eve)
+ eve_sync_after_ban: SyncResult = self.get_success(
+ self.sync_handler.wait_for_sync_for_user(eve_requester, eve_sync_config)
+ )
+
+ # Sanity check this sync result. We shouldn't be joined to the room.
+ self.assertEqual(eve_sync_after_ban.joined, [])
+
+ # Eve tries to join the room. We monkey patch the internal logic which selects
+ # the prev_events used when creating the join event, such that the ban does not
+ # precede the join.
+ mocked_get_prev_events = patch.object(
+ self.hs.get_datastore(),
+ "get_prev_events_for_room",
+ new_callable=MagicMock,
+ return_value=make_awaitable([last_room_creation_event_id]),
+ )
+ with mocked_get_prev_events:
+ self.helper.join(room_id, eve, tok=eve_token)
+
+ # Eve makes a second, incremental sync.
+ eve_incremental_sync_after_join: SyncResult = self.get_success(
+ self.sync_handler.wait_for_sync_for_user(
+ eve_requester,
+ eve_sync_config,
+ since_token=eve_sync_after_ban.next_batch,
+ )
+ )
+ # Eve should not see herself as joined to the room.
+ self.assertEqual(eve_incremental_sync_after_join.joined, [])
+
+ # If we did a third initial sync, we should _still_ see eve is not joined to the room.
+ eve_initial_sync_after_join: SyncResult = self.get_success(
+ self.sync_handler.wait_for_sync_for_user(
+ eve_requester,
+ eve_sync_config,
+ since_token=None,
+ )
+ )
+ self.assertEqual(eve_initial_sync_after_join.joined, [])
+
 
 _request_key = 0
 

tests/rest/client/utils.py

@@ -196,6 +196,16 @@ def leave(self, room=None, user=None, expect_code=200, tok=None):
  expect_code=expect_code,
  )
 
+ def ban(self, room: str, src: str, targ: str, **kwargs: object):
+ """A convenience helper: `change_membership` with `membership` preset to "ban"."""
+ self.change_membership(
+ room=room,
+ src=src,
+ targ=targ,
+ membership=Membership.BAN,
+ **kwargs,
+ )
+
  def change_membership(
  self,
  room: str,