diff options
Diffstat (limited to 'synapse/handlers/federation.py')
| -rw-r--r-- | synapse/handlers/federation.py | 65 |
1 files changed, 57 insertions, 8 deletions
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py index 43f2986f89..014dab2940 100644 --- a/synapse/handlers/federation.py +++ b/synapse/handlers/federation.py @@ -943,15 +943,26 @@ class FederationHandler(BaseHandler): return events - async def maybe_backfill(self, room_id, current_depth): + async def maybe_backfill( + self, room_id: str, current_depth: int, limit: int + ) -> bool: """Checks the database to see if we should backfill before paginating, and if so do. + + Args: + room_id + current_depth: The depth from which we're paginating from. This is + used to decide if we should backfill and what extremities to + use. + limit: The number of events that the pagination request will + return. This is used as part of the heuristic to decide if we + should back paginate. """ extremities = await self.store.get_oldest_events_with_depth_in_room(room_id) if not extremities: logger.debug("Not backfilling as no extremeties found.") - return + return False # We only want to paginate if we can actually see the events we'll get, # as otherwise we'll just spend a lot of resources to get redacted @@ -1004,16 +1015,54 @@ class FederationHandler(BaseHandler): sorted_extremeties_tuple = sorted(extremities.items(), key=lambda e: -int(e[1])) max_depth = sorted_extremeties_tuple[0][1] + # If we're approaching an extremity we trigger a backfill, otherwise we + # no-op. + # + # We chose twice the limit here as then clients paginating backwards + # will send pagination requests that trigger backfill at least twice + # using the most recent extremity before it gets removed (see below). We + # chose more than one times the limit in case of failure, but choosing a + # much larger factor will result in triggering a backfill request much + # earlier than necessary. + if current_depth - 2 * limit > max_depth: + logger.debug( + "Not backfilling as we don't need to. %d < %d - 2 * %d", + max_depth, + current_depth, + limit, + ) + return False + + logger.debug( + "room_id: %s, backfill: current_depth: %s, max_depth: %s, extrems: %s", + room_id, + current_depth, + max_depth, + sorted_extremeties_tuple, + ) + + # We ignore extremities that have a greater depth than our current depth + # as: + # 1. we don't really care about getting events that have happened + # before our current position; and + # 2. we have likely previously tried and failed to backfill from that + # extremity, so to avoid getting "stuck" requesting the same + # backfill repeatedly we drop those extremities. + filtered_sorted_extremeties_tuple = [ + t for t in sorted_extremeties_tuple if int(t[1]) <= current_depth + ] + + # However, we need to check that the filtered extremities are non-empty. + # If they are empty then either we can a) bail or b) still attempt to + # backill. We opt to try backfilling anyway just in case we do get + # relevant events. + if filtered_sorted_extremeties_tuple: + sorted_extremeties_tuple = filtered_sorted_extremeties_tuple + # We don't want to specify too many extremities as it causes the backfill # request URI to be too long. extremities = dict(sorted_extremeties_tuple[:5]) - if current_depth > max_depth: - logger.debug( - "Not backfilling as we don't need to. %d < %d", max_depth, current_depth - ) - return - # Now we need to decide which hosts to hit first. # First we try hosts that are already in the room |