Prune inbound federation queues if they get too long

changelog.d/10390.misc

@@ -0,0 +1 @@
+Prune inbound federation inbound queues for a room if they get too large.

synapse/federation/federation_server.py

@@ -998,6 +998,23 @@ async def _process_incoming_pdus_in_room_inner(
 
             origin, event = next
 
+            # Prune the event queue if its getting large.
+            #
+            # We do this *after* handling the first event as the common case is
+            # that the queue is empty (/has the single event in), and so there's
+            # no need to do this check.
+            pruned = await self.store.prune_staged_events_in_room(room_id, room_version)
+            if pruned:
+                # If we have pruned the queue check we need to refetch the next
+                # event to handle.
+                next = await self.store.get_next_staged_event_for_room(
+                    room_id, room_version
+                )
+                if not next:
+                    break
+
+                origin, event = next
+
             lock = await self.store.try_acquire_lock(
                 _INBOUND_EVENT_HANDLING_LOCK_NAME, room_id
             )

synapse/storage/databases/main/event_federation.py

@@ -16,11 +16,11 @@
 from queue import Empty, PriorityQueue
 from typing import Collection, Dict, Iterable, List, Optional, Set, Tuple
 
-from prometheus_client import Gauge
+from prometheus_client import Counter, Gauge
 
 from synapse.api.constants import MAX_DEPTH
 from synapse.api.errors import StoreError
-from synapse.api.room_versions import RoomVersion
+from synapse.api.room_versions import EventFormatVersions, RoomVersion
 from synapse.events import EventBase, make_event_from_dict
 from synapse.metrics.background_process_metrics import wrap_as_background_process
 from synapse.storage._base import SQLBaseStore, db_to_json, make_in_list_sql_clause
@@ -44,6 +44,12 @@
     "The total number of events in the inbound federation staging",
 )
 
+pdus_pruned_from_federation_queue = Counter(
+    "synapse_federation_server_number_inbound_pdu_pruned",
+    "The number of events in the inbound federation staging that have been "
+    "pruned due to the queue getting too long",
+)
+
 logger = logging.getLogger(__name__)
 
 
@@ -1207,6 +1213,78 @@ def _get_next_staged_event_for_room_txn(txn):
 
         return origin, event
 
+    async def prune_staged_events_in_room(
+        self,
+        room_id: str,
+        room_version: RoomVersion,
+    ) -> bool:
+        """Checks if there are lots of staged events for the room, and if so
+        prune them down.
+
+        Returns:
+            Whether any events were pruned
+        """
+
+        # First check the size of the queue.
+        count = await self.db_pool.simple_select_one_onecol(
+            table="federation_inbound_events_staging",
+            keyvalues={"room_id": room_id},
+            retcol="COALESCE(COUNT(*), 0)",
+            desc="prune_staged_events_in_room_count",
+        )
+
+        if count < 100:
+            return False
+
+        # If the queue is too large, then we want clear the entire queue,
+        # keeping only the forward extremities (i.e. the events not referenced
+        # by other events in the queue). We do this as that we can always
+        # backpaginate in all the events we have dropped.
+        rows = await self.db_pool.simple_select_list(
+            table="federation_inbound_events_staging",
+            keyvalues={"room_id": room_id},
+            retcols=("event_id", "event_json"),
+            desc="prune_staged_events_in_room_fetch",
+        )
+
+        # Find the set of events referenced by those in the queue, as well as
+        # collecting all the event IDs in the queue.
+        referenced_events: Set[str] = set()
+        seen_events: Set[str] = set()
+        for row in rows:
+            seen_events.add(row["event_id"])
+            event_d = db_to_json(row["event_json"])
+
+            # We don't bother parsing the dicts into full blown event objects,
+            # as that is needlessly expensive.
+            if room_version.event_format == EventFormatVersions.V1:
+                referenced_events.update(
+                    event_id for event_id, _ in event_d.get("prev_events", [])
+                )
+            else:
+                referenced_events.update(event_d.get("prev_events", []))
+
+        to_delete = referenced_events & seen_events
+        if not to_delete:
+            return False
+
+        pdus_pruned_from_federation_queue.inc(len(to_delete))
+        logger.info(
+            "Pruning %d events in room %s from federation queue",
+            len(to_delete),
+            room_id,
+        )
+
+        await self.db_pool.simple_delete_many(
+            table="federation_inbound_events_staging",
+            keyvalues={"room_id": room_id},
+            iterable=to_delete,
+            column="event_id",
+            desc="prune_staged_events_in_room_delete",
+        )
+
+        return True
+
     async def get_all_rooms_with_staged_incoming_events(self) -> List[str]:
         """Get the room IDs of all events currently staged."""
         return await self.db_pool.simple_select_onecol(