Return an immutable value from get_latest_event_ids_in_room. (#16326)

5 files changed, 14 insertions, 15 deletions

diff --git a/synapse/events/builder.py b/synapse/events/builder.py
index 1165c017ba..43469b170f 100644
--- a/synapse/events/builder.py
+++ b/synapse/events/builder.py
@@ -103,7 +103,7 @@ class EventBuilder:
 
     async def build(
         self,
-        prev_event_ids: StrCollection,
+        prev_event_ids: List[str],
         auth_event_ids: Optional[List[str]],
         depth: Optional[int] = None,
     ) -> EventBase:
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index d32d224d56..eedde97ab0 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -723,12 +723,11 @@ class FederationEventHandler:
         if not prevs - seen:
             return
 
-        latest_list = await self._store.get_latest_event_ids_in_room(room_id)
+        latest_frozen = await self._store.get_latest_event_ids_in_room(room_id)
 
         # We add the prev events that we have seen to the latest
         # list to ensure the remote server doesn't give them to us
-        latest = set(latest_list)
-        latest |= seen
+        latest = seen | latest_frozen
 
         logger.info(
             "Requesting missing events between %s and %s",
@@ -1976,8 +1975,7 @@ class FederationEventHandler:
             # partial and full state and may not be accurate.
             return
 
-        extrem_ids_list = await self._store.get_latest_event_ids_in_room(event.room_id)
-        extrem_ids = set(extrem_ids_list)
+        extrem_ids = await self._store.get_latest_event_ids_in_room(event.room_id)
         prev_event_ids = set(event.prev_event_ids())
 
         if extrem_ids == prev_event_ids:
diff --git a/synapse/storage/controllers/persist_events.py b/synapse/storage/controllers/persist_events.py
index 6864f93090..f39ae2d635 100644
--- a/synapse/storage/controllers/persist_events.py
+++ b/synapse/storage/controllers/persist_events.py
@@ -19,6 +19,7 @@ import logging
 from collections import deque
 from typing import (
     TYPE_CHECKING,
+    AbstractSet,
     Any,
     Awaitable,
     Callable,
@@ -618,7 +619,7 @@ class EventsPersistenceStorageController:
                         )
 
                     for room_id, ev_ctx_rm in events_by_room.items():
-                        latest_event_ids = set(
+                        latest_event_ids = (
                             await self.main_store.get_latest_event_ids_in_room(room_id)
                         )
                         new_latest_event_ids = await self._calculate_new_extremities(
@@ -740,7 +741,7 @@ class EventsPersistenceStorageController:
         self,
         room_id: str,
         event_contexts: List[Tuple[EventBase, EventContext]],
-        latest_event_ids: Collection[str],
+        latest_event_ids: AbstractSet[str],
     ) -> Set[str]:
         """Calculates the new forward extremities for a room given events to
         persist.
@@ -758,8 +759,6 @@ class EventsPersistenceStorageController:
             and not event.internal_metadata.is_soft_failed()
         ]
 
-        latest_event_ids = set(latest_event_ids)
-
         # start with the existing forward extremities
         result = set(latest_event_ids)
 
@@ -798,7 +797,7 @@ class EventsPersistenceStorageController:
         self,
         room_id: str,
         events_context: List[Tuple[EventBase, EventContext]],
-        old_latest_event_ids: Set[str],
+        old_latest_event_ids: AbstractSet[str],
         new_latest_event_ids: Set[str],
     ) -> Tuple[Optional[StateMap[str]], Optional[StateMap[str]], Set[str]]:
         """Calculate the current state dict after adding some new events to
diff --git a/synapse/storage/databases/main/event_federation.py b/synapse/storage/databases/main/event_federation.py
index 09de8f55e2..afffa54985 100644
--- a/synapse/storage/databases/main/event_federation.py
+++ b/synapse/storage/databases/main/event_federation.py
@@ -19,6 +19,7 @@ from typing import (
     TYPE_CHECKING,
     Collection,
     Dict,
+    FrozenSet,
     Iterable,
     List,
     Optional,
@@ -47,7 +48,7 @@ from synapse.storage.database import (
 from synapse.storage.databases.main.events_worker import EventsWorkerStore
 from synapse.storage.databases.main.signatures import SignatureWorkerStore
 from synapse.storage.engines import PostgresEngine, Sqlite3Engine
-from synapse.types import JsonDict, StrCollection, StrSequence
+from synapse.types import JsonDict, StrCollection
 from synapse.util import json_encoder
 from synapse.util.caches.descriptors import cached
 from synapse.util.caches.lrucache import LruCache
@@ -1179,13 +1180,14 @@ class EventFederationWorkerStore(SignatureWorkerStore, EventsWorkerStore, SQLBas
         )
 
     @cached(max_entries=5000, iterable=True)
-    async def get_latest_event_ids_in_room(self, room_id: str) -> StrSequence:
-        return await self.db_pool.simple_select_onecol(
+    async def get_latest_event_ids_in_room(self, room_id: str) -> FrozenSet[str]:
+        event_ids = await self.db_pool.simple_select_onecol(
             table="event_forward_extremities",
             keyvalues={"room_id": room_id},
             retcol="event_id",
             desc="get_latest_event_ids_in_room",
         )
+        return frozenset(event_ids)
 
     async def get_min_depth(self, room_id: str) -> Optional[int]:
         """For the given room, get the minimum depth we have seen for it."""
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 0c1ed75240..bc8474a589 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -222,7 +222,7 @@ class PersistEventsStore:
 
             for room_id, latest_event_ids in new_forward_extremities.items():
                 self.store.get_latest_event_ids_in_room.prefill(
-                    (room_id,), list(latest_event_ids)
+                    (room_id,), frozenset(latest_event_ids)
                 )
 
     async def _get_events_which_are_prevs(self, event_ids: Iterable[str]) -> List[str]: