Reduce auth chains fetched during v2 state res. (#6952)

The state res v2 algorithm only cares about the difference between auth
chains, so we can pass in the known common state to the `get_auth_chain`
storage function so that it can ignore those events.

synapse/storage/data_stores/main/event_federation.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 import itertools
 import logging
+from typing import List, Optional, Set
 
 from six.moves import range
 from six.moves.queue import Empty, PriorityQueue
@@ -46,21 +47,37 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
             event_ids, include_given=include_given
         ).addCallback(self.get_events_as_list)
 
-    def get_auth_chain_ids(self, event_ids, include_given=False):
+    def get_auth_chain_ids(
+        self,
+        event_ids: List[str],
+        include_given: bool = False,
+        ignore_events: Optional[Set[str]] = None,
+    ):
         """Get auth events for given event_ids. The events *must* be state events.
 
         Args:
-            event_ids (list): state events
-            include_given (bool): include the given events in result
+            event_ids: state events
+            include_given: include the given events in result
+            ignore_events: Set of events to exclude from the returned auth
+                chain. This is useful if the caller will just discard the
+                given events anyway, and saves us from figuring out their auth
+                chains if not required.
 
         Returns:
             list of event_ids
         """
         return self.db.runInteraction(
-            "get_auth_chain_ids", self._get_auth_chain_ids_txn, event_ids, include_given
+            "get_auth_chain_ids",
+            self._get_auth_chain_ids_txn,
+            event_ids,
+            include_given,
+            ignore_events,
         )
 
-    def _get_auth_chain_ids_txn(self, txn, event_ids, include_given):
+    def _get_auth_chain_ids_txn(self, txn, event_ids, include_given, ignore_events):
+        if ignore_events is None:
+            ignore_events = set()
+
         if include_given:
             results = set(event_ids)
         else:
@@ -80,6 +97,7 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
                 txn.execute(base_sql + clause, list(args))
                 new_front.update([r[0] for r in txn])
 
+            new_front -= ignore_events
             new_front -= results
 
             front = new_front