Remove `_get_events_cache` check optimisation from `_have_seen_events_dict` (#14161)

changelog.d/14161.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in 1.30.0 where purging and rejoining a room without restarting in-between would result in a broken room.

synapse/storage/databases/main/events_worker.py

@@ -1502,21 +1502,15 @@ class EventsWorkerStore(SQLBaseStore):
         Returns:
              a dict {event_id -> bool}
         """
-        # if the event cache contains the event, obviously we've seen it.
-
-        cache_results = {
-            event_id
-            for event_id in event_ids
-            if await self._get_event_cache.contains((event_id,))
-        }
-        results = dict.fromkeys(cache_results, True)
-        remaining = [
-            event_id for event_id in event_ids if event_id not in cache_results
-        ]
-        if not remaining:
-            return results
+        # TODO: We used to query the _get_event_cache here as a fast-path before
+        #  hitting the database. For if an event were in the cache, we've presumably
+        #  seen it before.
+        #
+        #  But this is currently an invalid assumption due to the _get_event_cache
+        #  not being invalidated when purging events from a room. The optimisation can
+        #  be re-added after https://github.com/matrix-org/synapse/issues/13476
 
-        def have_seen_events_txn(txn: LoggingTransaction) -> None:
+        def have_seen_events_txn(txn: LoggingTransaction) -> Dict[str, bool]:
             # we deliberately do *not* query the database for room_id, to make the
             # query an index-only lookup on `events_event_id_key`.
             #
@@ -1524,16 +1518,17 @@ class EventsWorkerStore(SQLBaseStore):
 
             sql = "SELECT event_id FROM events AS e WHERE "
             clause, args = make_in_list_sql_clause(
-                txn.database_engine, "e.event_id", remaining
+                txn.database_engine, "e.event_id", event_ids
             )
             txn.execute(sql + clause, args)
             found_events = {eid for eid, in txn}
 
             # ... and then we can update the results for each key
-            results.update({eid: (eid in found_events) for eid in remaining})
+            return {eid: (eid in found_events) for eid in event_ids}
 
-        await self.db_pool.runInteraction("have_seen_events", have_seen_events_txn)
-        return results
+        return await self.db_pool.runInteraction(
+            "have_seen_events", have_seen_events_txn
+        )
 
     @cached(max_entries=100000, tree=True)
     async def have_seen_event(self, room_id: str, event_id: str) -> bool:

tests/storage/databases/main/test_events_worker.py

@@ -90,18 +90,6 @@ class HaveSeenEventsTestCase(unittest.HomeserverTestCase):
             self.assertEqual(res, {self.event_ids[0]})
             self.assertEqual(ctx.get_resource_usage().db_txn_count, 0)
 
-    def test_query_via_event_cache(self):
-        # fetch an event into the event cache
-        self.get_success(self.store.get_event(self.event_ids[0]))
-
-        # looking it up should now cause no db hits
-        with LoggingContext(name="test") as ctx:
-            res = self.get_success(
-                self.store.have_seen_events(self.room_id, [self.event_ids[0]])
-            )
-            self.assertEqual(res, {self.event_ids[0]})
-            self.assertEqual(ctx.get_resource_usage().db_txn_count, 0)
-
     def test_persisting_event_invalidates_cache(self):
         """
         Test to make sure that the `have_seen_event` cache