Repository: matrix-org/synapse

Issue:
Message retention purge jobs can incorrectly think there's no event to purge from a room
An interesting bug I found while investigating #6683:

When figuring out which topological token to start a purge job at, we need to do the following:

1. Figure out a timestamp before which events will be purged
2. Select the first stream ordering after that timestamp
3. Select info about the first event after that stream ordering
4. Build a topological token from that info

In some situations (e.g. quiet rooms with a short max_lifetime), there not be an event after the stream ordering at step 3, therefore we abort the purge with the error `No event found`.


Discussion:

Resolving patch:
diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -156,7 +156,7 @@ def purge_history_for_rooms_in_range(self, min_ms, max_ms):
 
             stream_ordering = yield self.store.find_first_stream_ordering_after_ts(ts)
 
-            r = yield self.store.get_room_event_after_stream_ordering(
+            r = yield self.store.get_room_event_before_stream_ordering(
                 room_id, stream_ordering,
             )
             if not r:
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -107,7 +107,7 @@ async def on_POST(self, request, room_id, event_id):
 
             stream_ordering = await self.store.find_first_stream_ordering_after_ts(ts)
 
-            r = await self.store.get_room_event_after_stream_ordering(
+            r = await self.store.get_room_event_before_stream_ordering(
                 room_id, stream_ordering
             )
             if not r:
diff --git a/synapse/storage/data_stores/main/stream.py b/synapse/storage/data_stores/main/stream.py
--- a/synapse/storage/data_stores/main/stream.py
+++ b/synapse/storage/data_stores/main/stream.py
@@ -525,8 +525,8 @@ def get_recent_event_ids_for_room(self, room_id, limit, end_token):
 
         return rows, token
 
-    def get_room_event_after_stream_ordering(self, room_id, stream_ordering):
-        """Gets details of the first event in a room at or after a stream ordering
+    def get_room_event_before_stream_ordering(self, room_id, stream_ordering):
+        """Gets details of the first event in a room at or before a stream ordering
 
         Args:
             room_id (str):
@@ -541,15 +541,15 @@ def _f(txn):
             sql = (
                 "SELECT stream_ordering, topological_ordering, event_id"
                 " FROM events"
-                " WHERE room_id = ? AND stream_ordering >= ?"
+                " WHERE room_id = ? AND stream_ordering <= ?"
                 " AND NOT outlier"
-                " ORDER BY stream_ordering"
+                " ORDER BY stream_ordering DESC"
                 " LIMIT 1"
             )
             txn.execute(sql, (room_id, stream_ordering))
             return txn.fetchone()
 
-        return self.db.runInteraction("get_room_event_after_stream_ordering", _f)
+        return self.db.runInteraction("get_room_event_before_stream_ordering", _f)
 
     @defer.inlineCallbacks
     def get_room_events_max_id(self, room_id=None):


Test patch: