Skip attempt to get state at backwards-extremities (#12173)

We don't *have* the state at a backwards-extremity, so this is never going to
do anything useful.

changelog.d/12173.misc

+Avoid trying to calculate the state at outlier events.

synapse/handlers/federation.py

@@ -23,8 +23,6 @@ from signedjson.key import decode_verify_key_bytes
 from signedjson.sign import verify_signed_json
 from unpaddedbase64 import decode_base64
 
-from twisted.internet import defer
-
 from synapse import event_auth
 from synapse.api.constants import EventContentFields, EventTypes, Membership
 from synapse.api.errors import (
@@ -45,11 +43,7 @@ from synapse.events.snapshot import EventContext
 from synapse.events.validator import EventValidator
 from synapse.federation.federation_client import InvalidResponseError
 from synapse.http.servlet import assert_params_in_dict
-from synapse.logging.context import (
-    make_deferred_yieldable,
-    nested_logging_context,
-    preserve_fn,
-)
+from synapse.logging.context import nested_logging_context
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.replication.http.federation import (
     ReplicationCleanRoomRestServlet,
@@ -355,56 +349,8 @@ class FederationHandler:
         if success:
             return True
 
-        # Huh, well *those* domains didn't work out. Lets try some domains
-        # from the time.
-
-        tried_domains = set(likely_domains)
-        tried_domains.add(self.server_name)
-
-        event_ids = list(extremities.keys())
-
-        logger.debug("calling resolve_state_groups in _maybe_backfill")
-        resolve = preserve_fn(self.state_handler.resolve_state_groups_for_events)
-        states_list = await make_deferred_yieldable(
-            defer.gatherResults(
-                [resolve(room_id, [e]) for e in event_ids], consumeErrors=True
-            )
-        )
-
-        # A map from event_id to state map of event_ids.
-        state_ids: Dict[str, StateMap[str]] = dict(
-            zip(event_ids, [s.state for s in states_list])
-        )
-
-        state_map = await self.store.get_events(
-            [e_id for ids in state_ids.values() for e_id in ids.values()],
-            get_prev_content=False,
-        )
-
-        # A map from event_id to state map of events.
-        state_events: Dict[str, StateMap[EventBase]] = {
-            key: {
-                k: state_map[e_id]
-                for k, e_id in state_dict.items()
-                if e_id in state_map
-            }
-            for key, state_dict in state_ids.items()
-        }
-
-        for e_id in event_ids:
-            likely_extremeties_domains = get_domains_from_state(state_events[e_id])
-
-            success = await try_backfill(
-                [
-                    dom
-                    for dom, _ in likely_extremeties_domains
-                    if dom not in tried_domains
-                ]
-            )
-            if success:
-                return True
-
-            tried_domains.update(dom for dom, _ in likely_extremeties_domains)
+        # TODO: we could also try servers which were previously in the room, but
+        #   are no longer.
 
         return False