Don't invalidate the entire event cache when we purge history (#16905)

We do this by adding support to the LRU cache for "extra indices" based
on the cached value. This allows us to efficiently map from room ID to
the cached events and only invalidate those.

2 files changed, 9 insertions, 7 deletions

diff --git a/synapse/storage/databases/main/cache.py b/synapse/storage/databases/main/cache.py
index 7314d87404..bfd492d95d 100644
--- a/synapse/storage/databases/main/cache.py
+++ b/synapse/storage/databases/main/cache.py
@@ -373,7 +373,7 @@ class CacheInvalidationWorkerStore(SQLBaseStore):
         deleted.
         """
 
-        self._invalidate_local_get_event_cache_all()  # type: ignore[attr-defined]
+        self._invalidate_local_get_event_cache_room_id(room_id)  # type: ignore[attr-defined]
 
         self._attempt_to_invalidate_cache("have_seen_event", (room_id,))
         self._attempt_to_invalidate_cache("get_latest_event_ids_in_room", (room_id,))
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index 1fd458b510..9c3775bb7c 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -268,6 +268,8 @@ class EventsWorkerStore(SQLBaseStore):
         ] = AsyncLruCache(
             cache_name="*getEvent*",
             max_size=hs.config.caches.event_cache_size,
+            # `extra_index_cb` Returns a tuple as that is the key type
+            extra_index_cb=lambda _, v: (v.event.room_id,),
         )
 
         # Map from event ID to a deferred that will result in a map from event
@@ -782,9 +784,9 @@ class EventsWorkerStore(SQLBaseStore):
 
         if missing_events_ids:
 
-            async def get_missing_events_from_cache_or_db() -> Dict[
-                str, EventCacheEntry
-            ]:
+            async def get_missing_events_from_cache_or_db() -> (
+                Dict[str, EventCacheEntry]
+            ):
                 """Fetches the events in `missing_event_ids` from the database.
 
                 Also creates entries in `self._current_event_fetches` to allow
@@ -910,12 +912,12 @@ class EventsWorkerStore(SQLBaseStore):
         self._event_ref.pop(event_id, None)
         self._current_event_fetches.pop(event_id, None)
 
-    def _invalidate_local_get_event_cache_all(self) -> None:
-        """Clears the in-memory get event caches.
+    def _invalidate_local_get_event_cache_room_id(self, room_id: str) -> None:
+        """Clears the in-memory get event caches for a room.
 
         Used when we purge room history.
         """
-        self._get_event_cache.clear()
+        self._get_event_cache.invalidate_on_extra_index_local((room_id,))
         self._event_ref.clear()
         self._current_event_fetches.clear()