Order in-flight state group queries in biggest-first order (#11610)

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

synapse/storage/databases/state/store.py

@@ -25,6 +25,7 @@ from typing import (
 )
 
 import attr
+from sortedcontainers import SortedDict
 
 from twisted.internet import defer
 
@@ -72,6 +73,24 @@ class _GetStateGroupDelta:
         return len(self.delta_ids) if self.delta_ids else 0
 
 
+def state_filter_rough_priority_comparator(
+    state_filter: StateFilter,
+) -> Tuple[int, int]:
+    """
+    Returns a comparable value that roughly indicates the relative size of this
+    state filter compared to others.
+    'Larger' state filters should sort first when using ascending order, so
+    this is essentially the opposite of 'size'.
+    It should be treated as a rough guide only and should not be interpreted to
+    have any particular meaning. The representation may also change
+
+    The current implementation returns a tuple of the form:
+        * -1 for include_others, 0 otherwise
+        * -(number of entries in state_filter.types)
+    """
+    return -int(state_filter.include_others), -len(state_filter.types)
+
+
 class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
     """A data store for fetching/storing state groups."""
 
@@ -127,7 +146,7 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
         # Current ongoing get_state_for_groups in-flight requests
         # {group ID -> {StateFilter -> ObservableDeferred}}
         self._state_group_inflight_requests: Dict[
-            int, Dict[StateFilter, AbstractObservableDeferred[StateMap[str]]]
+            int, SortedDict[StateFilter, AbstractObservableDeferred[StateMap[str]]]
         ] = {}
 
         def get_max_state_group_txn(txn: Cursor) -> int:
@@ -279,7 +298,10 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
 
         # The list of ongoing requests which will help narrow the current request.
         reusable_requests = []
-        for (request_state_filter, request_deferred) in inflight_requests.items():
+
+        # Iterate over existing requests in roughly biggest-first order.
+        for request_state_filter in inflight_requests:
+            request_deferred = inflight_requests[request_state_filter]
             new_state_filter_left_over = state_filter_left_over.approx_difference(
                 request_state_filter
             )
@@ -358,7 +380,9 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
         observable_deferred = ObservableDeferred(request_deferred, consumeErrors=True)
 
         # Insert the ObservableDeferred into the cache
-        group_request_dict = self._state_group_inflight_requests.setdefault(group, {})
+        group_request_dict = self._state_group_inflight_requests.setdefault(
+            group, SortedDict(state_filter_rough_priority_comparator)
+        )
         group_request_dict[db_state_filter] = observable_deferred
 
         return await make_deferred_yieldable(observable_deferred.observe())