Fix races in room stats (and other) updates. (#6187)

Hopefully this will fix the occasional failures we were seeing in the room directory.

The problem was that events are not necessarily persisted (and `current_state_delta_stream` updated) in the same order as their stream_id. So for instance current_state_delta 9 might be persisted *before* current_state_delta 8. Then, when the room stats saw stream_id 9, it assumed it had done everything up to 9, and never came back to do stream_id 8.

We can solve this easily by only processing up to the stream_id where we know all events have been persisted.

synapse/storage/state_deltas.py

@@ -21,7 +21,7 @@ logger = logging.getLogger(__name__)
 
 
 class StateDeltasStore(SQLBaseStore):
-    def get_current_state_deltas(self, prev_stream_id):
+    def get_current_state_deltas(self, prev_stream_id: int, max_stream_id: int):
         """Fetch a list of room state changes since the given stream id
 
         Each entry in the result contains the following fields:
@@ -36,15 +36,27 @@ class StateDeltasStore(SQLBaseStore):
 
         Args:
             prev_stream_id (int): point to get changes since (exclusive)
+            max_stream_id (int): the point that we know has been correctly persisted
+               - ie, an upper limit to return changes from.
 
         Returns:
-            Deferred[list[dict]]: results
+            Deferred[tuple[int, list[dict]]: A tuple consisting of:
+               - the stream id which these results go up to
+               - list of current_state_delta_stream rows. If it is empty, we are
+                 up to date.
         """
         prev_stream_id = int(prev_stream_id)
+
+        # check we're not going backwards
+        assert prev_stream_id <= max_stream_id
+
         if not self._curr_state_delta_stream_cache.has_any_entity_changed(
             prev_stream_id
         ):
-            return []
+            # if the CSDs haven't changed between prev_stream_id and now, we
+            # know for certain that they haven't changed between prev_stream_id and
+            # max_stream_id.
+            return max_stream_id, []
 
         def get_current_state_deltas_txn(txn):
             # First we calculate the max stream id that will give us less than
@@ -54,21 +66,29 @@ class StateDeltasStore(SQLBaseStore):
             sql = """
                 SELECT stream_id, count(*)
                 FROM current_state_delta_stream
-                WHERE stream_id > ?
+                WHERE stream_id > ? AND stream_id <= ?
                 GROUP BY stream_id
                 ORDER BY stream_id ASC
                 LIMIT 100
             """
-            txn.execute(sql, (prev_stream_id,))
+            txn.execute(sql, (prev_stream_id, max_stream_id))
 
             total = 0
-            max_stream_id = prev_stream_id
-            for max_stream_id, count in txn:
+
+            for stream_id, count in txn:
                 total += count
                 if total > 100:
                     # We arbitarily limit to 100 entries to ensure we don't
                     # select toooo many.
+                    logger.debug(
+                        "Clipping current_state_delta_stream rows to stream_id %i",
+                        stream_id,
+                    )
+                    clipped_stream_id = stream_id
                     break
+            else:
+                # if there's no problem, we may as well go right up to the max_stream_id
+                clipped_stream_id = max_stream_id
 
             # Now actually get the deltas
             sql = """
@@ -77,8 +97,8 @@ class StateDeltasStore(SQLBaseStore):
                 WHERE ? < stream_id AND stream_id <= ?
                 ORDER BY stream_id ASC
             """
-            txn.execute(sql, (prev_stream_id, max_stream_id))
-            return self.cursor_to_dict(txn)
+            txn.execute(sql, (prev_stream_id, clipped_stream_id))
+            return clipped_stream_id, self.cursor_to_dict(txn)
 
         return self.runInteraction(
             "get_current_state_deltas", get_current_state_deltas_txn