Fix sqite locking bugs present on sytest

Comments do the explaining.

roomserver/storage/shared/storage.go

@@ -492,15 +492,32 @@ func (d *Database) StoreEvent(
 		if roomInfo == nil && len(prevEvents) > 0 {
 			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("expected room %q to exist", event.RoomID())
 		}
+		// Create an updater - NB: on sqlite this WILL create a txn as we are directly calling the shared DB form of
+		// GetLatestEventsForUpdate - not via the SQLiteDatabase form which has `nil` txns. This
+		// function only does SELECTs though so the created txn (at this point) is just a read txn like
+		// any other so this is fine. If we ever update GetLatestEventsForUpdate or NewLatestEventsUpdater
+		// to do writes however then this will need to go inside `Writer.Do`.
 		updater, err = d.GetLatestEventsForUpdate(ctx, *roomInfo)
 		if err != nil {
 			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("NewLatestEventsUpdater: %w", err)
 		}
-		if err = updater.StorePreviousEvents(eventNID, prevEvents); err != nil {
+		// Ensure that we atomically store prev events AND commit them. If we don't wrap StorePreviousEvents
-			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("updater.StorePreviousEvents: %w", err)
+		// and EndTransaction in a writer then it's possible for a new write txn to be made between the two
+		// function calls which will then fail with 'database is locked'. This new write txn would HAVE to be
+		// something like SetRoomAlias/RemoveRoomAlias as normal input events are already done sequentially due to
+		// SupportsConcurrentRoomInputs() == false on sqlite, though this does not apply to setting room aliases
+		// as they don't go via InputRoomEvents
+		err = d.Writer.Do(d.DB, updater.txn, func(txn *sql.Tx) error {
+			if err = updater.StorePreviousEvents(eventNID, prevEvents); err != nil {
+				return fmt.Errorf("updater.StorePreviousEvents: %w", err)
+			}
+			succeeded := true
+			err = sqlutil.EndTransaction(updater, &succeeded)
+			return err
+		})
+		if err != nil {
+			return 0, types.StateAtEvent{}, nil, "", err
 		}
-		succeeded := true
-		err = sqlutil.EndTransaction(updater, &succeeded)
 	}
 
 	return roomNID, types.StateAtEvent{