Fix sync waiting for an invalid token from the "future" (#17386)

Fixes https://github.com/element-hq/synapse/issues/17274, hopefully.

Basically, old versions of Synapse could advance streams without
persisting anything in the DB (fixed in #17229). On restart those
updates would get lost, and so the position of the stream would revert
to an older position. If this happened across an upgrade to a later
Synapse version which included #17215, then sync could get blocked
indefinitely (until the stream advanced to the position in the token).

We fix this by bounding the stream positions we'll wait for to the
maximum position of the underlying stream ID generator.

1 files changed, 5 insertions, 6 deletions

diff --git a/synapse/storage/databases/main/room.py b/synapse/storage/databases/main/room.py
index d5627b1d6e..80a4bf95f2 100644
--- a/synapse/storage/databases/main/room.py
+++ b/synapse/storage/databases/main/room.py
@@ -59,11 +59,7 @@ from synapse.storage.database import (
 )
 from synapse.storage.databases.main.cache import CacheInvalidationWorkerStore
 from synapse.storage.types import Cursor
-from synapse.storage.util.id_generators import (
-    AbstractStreamIdGenerator,
-    IdGenerator,
-    MultiWriterIdGenerator,
-)
+from synapse.storage.util.id_generators import IdGenerator, MultiWriterIdGenerator
 from synapse.types import JsonDict, RetentionPolicy, StrCollection, ThirdPartyInstanceID
 from synapse.util import json_encoder
 from synapse.util.caches.descriptors import cached, cachedList
@@ -151,7 +147,7 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
 
         self.config: HomeServerConfig = hs.config
 
-        self._un_partial_stated_rooms_stream_id_gen: AbstractStreamIdGenerator
+        self._un_partial_stated_rooms_stream_id_gen: MultiWriterIdGenerator
 
         self._un_partial_stated_rooms_stream_id_gen = MultiWriterIdGenerator(
             db_conn=db_conn,
@@ -1409,6 +1405,9 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
             instance_name
         )
 
+    def get_un_partial_stated_rooms_id_generator(self) -> MultiWriterIdGenerator:
+        return self._un_partial_stated_rooms_stream_id_gen
+
     async def get_un_partial_stated_rooms_between(
         self, last_id: int, current_id: int, room_ids: Collection[str]
     ) -> Set[str]: