Fix a rare bug where initial /syncs would fail

changelog.d/15383.bugfix

@@ -0,0 +1 @@
+Fix a rare bug introduced in Synapse 1.66.0 where initial syncs would fail when the user had been kicked from a faster joined room that had not finished syncing.

synapse/handlers/sync.py

@@ -943,6 +943,8 @@ async def compute_state_delta(
 
                 timeline_state = {}
 
+                # Membership events to fetch that can be found in the room state, or in
+                # the case of partial state rooms, the auth events of timeline events.
                 members_to_fetch = set()
                 first_event_by_sender_map = {}
                 for event in batch.events:
@@ -964,9 +966,19 @@ async def compute_state_delta(
                     # (if we are) to fix https://github.com/vector-im/riot-web/issues/7209
                     # We only need apply this on full state syncs given we disabled
                     # LL for incr syncs in #3840.
-                    members_to_fetch.add(sync_config.user.to_string())
-
-                state_filter = StateFilter.from_lazy_load_member_list(members_to_fetch)
+                    # We don't insert ourselves into `members_to_fetch`, because in some
+                    # rare cases (an empty event batch with a now_token after the user's
+                    # leave in a partial state room which another local user has
+                    # joined), the room state will be missing our membership and there
+                    # is no guarantee that our membership will be in the auth events of
+                    # timeline events when the room is partial stated.
+                    state_filter = StateFilter.from_lazy_load_member_list(
+                        members_to_fetch.union((sync_config.user.to_string(),))
+                    )
+                else:
+                    state_filter = StateFilter.from_lazy_load_member_list(
+                        members_to_fetch
+                    )
 
                 # We are happy to use partial state to compute the `/sync` response.
                 # Since partial state may not include the lazy-loaded memberships we
@@ -988,7 +1000,9 @@ async def compute_state_delta(
             # sync's timeline and the start of the current sync's timeline.
             # See the docstring above for details.
             state_ids: StateMap[str]
-
+            # We need to know whether the state we fetch may be partial, so check
+            # whether the room is partial stated *before* fetching it.
+            is_partial_state_room = await self.store.is_partial_state_room(room_id)
             if full_state:
                 if batch:
                     state_at_timeline_end = (
@@ -1119,7 +1133,7 @@ async def compute_state_delta(
             # If we only have partial state for the room, `state_ids` may be missing the
             # memberships we wanted. We attempt to find some by digging through the auth
             # events of timeline events.
-            if lazy_load_members and await self.store.is_partial_state_room(room_id):
+            if lazy_load_members and is_partial_state_room:
                 assert members_to_fetch is not None
                 assert first_event_by_sender_map is not None