From b975fa2e9952f1f8ac2cddb15c287768bf9b0b4e Mon Sep 17 00:00:00 2001
From: Patrick Cloke <clokep@users.noreply.github.com>
Date: Fri, 24 Jul 2020 10:59:51 -0400
Subject: [PATCH] Convert state resolution to async/await (#7942)

---
 changelog.d/7942.misc                         |   1 +
 synapse/api/auth.py                           |  12 +-
 synapse/events/builder.py                     |   4 +-
 synapse/federation/sender/__init__.py         |   4 +-
 synapse/handlers/presence.py                  |   4 +-
 synapse/push/bulk_push_rule_evaluator.py      |   4 +-
 synapse/state/__init__.py                     |  95 +++++++---------
 synapse/state/v1.py                           |  15 +--
 synapse/state/v2.py                           | 107 ++++++++----------
 synapse/storage/data_stores/main/push_rule.py |   2 +-
 .../storage/data_stores/main/roommember.py    |   2 +-
 .../data_stores/main/user_directory.py        |   4 +-
 synapse/storage/persist_events.py             |   5 +-
 tests/federation/test_federation_sender.py    |  19 ++--
 tests/state/test_v2.py                        |  17 +--
 tests/storage/test_room.py                    |   8 +-
 tests/test_state.py                           |  72 +++++++-----
 tests/test_utils/__init__.py                  |   7 +-
 18 files changed, 198 insertions(+), 184 deletions(-)
 create mode 100644 changelog.d/7942.misc

diff --git a/changelog.d/7942.misc b/changelog.d/7942.misc
new file mode 100644
index 0000000000..b504cf4e6f
--- /dev/null
+++ b/changelog.d/7942.misc
@@ -0,0 +1 @@
+Convert state resolution to async/await.
diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 40dc62ef6c..b53e8451e5 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -127,8 +127,10 @@ class Auth(object):
         if current_state:
             member = current_state.get((EventTypes.Member, user_id), None)
         else:
-            member = yield self.state.get_current_state(
-                room_id=room_id, event_type=EventTypes.Member, state_key=user_id
+            member = yield defer.ensureDeferred(
+                self.state.get_current_state(
+                    room_id=room_id, event_type=EventTypes.Member, state_key=user_id
+                )
             )
         membership = member.membership if member else None
 
@@ -665,8 +667,10 @@ class Auth(object):
             )
             return member_event.membership, member_event.event_id
         except AuthError:
-            visibility = yield self.state.get_current_state(
-                room_id, EventTypes.RoomHistoryVisibility, ""
+            visibility = yield defer.ensureDeferred(
+                self.state.get_current_state(
+                    room_id, EventTypes.RoomHistoryVisibility, ""
+                )
             )
             if (
                 visibility
diff --git a/synapse/events/builder.py b/synapse/events/builder.py
index 92aadfe7ef..0bb216419a 100644
--- a/synapse/events/builder.py
+++ b/synapse/events/builder.py
@@ -106,8 +106,8 @@ class EventBuilder(object):
             Deferred[FrozenEvent]
         """
 
-        state_ids = yield self._state.get_current_state_ids(
-            self.room_id, prev_event_ids
+        state_ids = yield defer.ensureDeferred(
+            self._state.get_current_state_ids(self.room_id, prev_event_ids)
         )
         auth_ids = yield self._auth.compute_auth_events(self, state_ids)
 
diff --git a/synapse/federation/sender/__init__.py b/synapse/federation/sender/__init__.py
index 99ce73e081..ba4ddd2370 100644
--- a/synapse/federation/sender/__init__.py
+++ b/synapse/federation/sender/__init__.py
@@ -330,7 +330,9 @@ class FederationSender(object):
         room_id = receipt.room_id
 
         # Work out which remote servers should be poked and poke them.
-        domains = yield self.state.get_current_hosts_in_room(room_id)
+        domains = yield defer.ensureDeferred(
+            self.state.get_current_hosts_in_room(room_id)
+        )
         domains = [
             d
             for d in domains
diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 8e99c83d9d..b3a3bb8c3f 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -928,8 +928,8 @@ class PresenceHandler(BasePresenceHandler):
             # TODO: Check that this is actually a new server joining the
             # room.
 
-            user_ids = await self.state.get_current_users_in_room(room_id)
-            user_ids = list(filter(self.is_mine_id, user_ids))
+            users = await self.state.get_current_users_in_room(room_id)
+            user_ids = list(filter(self.is_mine_id, users))
 
             states_d = await self.current_state_for_users(user_ids)
 
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index 43ffe6faf0..472ddf9f7d 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -304,7 +304,9 @@ class RulesForRoom(object):
 
                 push_rules_delta_state_cache_metric.inc_hits()
             else:
-                current_state_ids = yield context.get_current_state_ids()
+                current_state_ids = yield defer.ensureDeferred(
+                    context.get_current_state_ids()
+                )
                 push_rules_delta_state_cache_metric.inc_misses()
 
             push_rules_state_size_counter.inc(len(current_state_ids))
diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py
index 495d9f04c8..25ccef5aa5 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -16,14 +16,12 @@
 
 import logging
 from collections import namedtuple
-from typing import Dict, Iterable, List, Optional, Set
+from typing import Awaitable, Dict, Iterable, List, Optional, Set
 
 import attr
 from frozendict import frozendict
 from prometheus_client import Histogram
 
-from twisted.internet import defer
-
 from synapse.api.constants import EventTypes
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, StateResolutionVersions
 from synapse.events import EventBase
@@ -31,6 +29,7 @@ from synapse.events.snapshot import EventContext
 from synapse.logging.utils import log_function
 from synapse.state import v1, v2
 from synapse.storage.data_stores.main.events_worker import EventRedactBehaviour
+from synapse.storage.roommember import ProfileInfo
 from synapse.types import StateMap
 from synapse.util import Clock
 from synapse.util.async_helpers import Linearizer
@@ -108,8 +107,7 @@ class StateHandler(object):
         self.hs = hs
         self._state_resolution_handler = hs.get_state_resolution_handler()
 
-    @defer.inlineCallbacks
-    def get_current_state(
+    async def get_current_state(
         self, room_id, event_type=None, state_key="", latest_event_ids=None
     ):
         """ Retrieves the current state for the room. This is done by
@@ -126,20 +124,20 @@ class StateHandler(object):
             map from (type, state_key) to event
         """
         if not latest_event_ids:
-            latest_event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
+            latest_event_ids = await self.store.get_latest_event_ids_in_room(room_id)
 
         logger.debug("calling resolve_state_groups from get_current_state")
-        ret = yield self.resolve_state_groups_for_events(room_id, latest_event_ids)
+        ret = await self.resolve_state_groups_for_events(room_id, latest_event_ids)
         state = ret.state
 
         if event_type:
             event_id = state.get((event_type, state_key))
             event = None
             if event_id:
-                event = yield self.store.get_event(event_id, allow_none=True)
+                event = await self.store.get_event(event_id, allow_none=True)
             return event
 
-        state_map = yield self.store.get_events(
+        state_map = await self.store.get_events(
             list(state.values()), get_prev_content=False
         )
         state = {
@@ -148,8 +146,7 @@ class StateHandler(object):
 
         return state
 
-    @defer.inlineCallbacks
-    def get_current_state_ids(self, room_id, latest_event_ids=None):
+    async def get_current_state_ids(self, room_id, latest_event_ids=None):
         """Get the current state, or the state at a set of events, for a room
 
         Args:
@@ -164,41 +161,38 @@ class StateHandler(object):
                 (event_type, state_key) -> event_id
         """
         if not latest_event_ids:
-            latest_event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
+            latest_event_ids = await self.store.get_latest_event_ids_in_room(room_id)
 
         logger.debug("calling resolve_state_groups from get_current_state_ids")
-        ret = yield self.resolve_state_groups_for_events(room_id, latest_event_ids)
+        ret = await self.resolve_state_groups_for_events(room_id, latest_event_ids)
         state = ret.state
 
         return state
 
-    @defer.inlineCallbacks
-    def get_current_users_in_room(self, room_id, latest_event_ids=None):
+    async def get_current_users_in_room(
+        self, room_id: str, latest_event_ids: Optional[List[str]] = None
+    ) -> Dict[str, ProfileInfo]:
         """
         Get the users who are currently in a room.
 
         Args:
-            room_id (str): The ID of the room.
-            latest_event_ids (List[str]|None): Precomputed list of latest
-                event IDs. Will be computed if None.
+            room_id: The ID of the room.
+            latest_event_ids: Precomputed list of latest event IDs. Will be computed if None.
         Returns:
-            Deferred[Dict[str,ProfileInfo]]: Dictionary of user IDs to their
-                profileinfo.
+            Dictionary of user IDs to their profileinfo.
         """
         if not latest_event_ids:
-            latest_event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
+            latest_event_ids = await self.store.get_latest_event_ids_in_room(room_id)
         logger.debug("calling resolve_state_groups from get_current_users_in_room")
-        entry = yield self.resolve_state_groups_for_events(room_id, latest_event_ids)
-        joined_users = yield self.store.get_joined_users_from_state(room_id, entry)
+        entry = await self.resolve_state_groups_for_events(room_id, latest_event_ids)
+        joined_users = await self.store.get_joined_users_from_state(room_id, entry)
         return joined_users
 
-    @defer.inlineCallbacks
-    def get_current_hosts_in_room(self, room_id):
-        event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
-        return (yield self.get_hosts_in_room_at_events(room_id, event_ids))
+    async def get_current_hosts_in_room(self, room_id):
+        event_ids = await self.store.get_latest_event_ids_in_room(room_id)
+        return await self.get_hosts_in_room_at_events(room_id, event_ids)
 
-    @defer.inlineCallbacks
-    def get_hosts_in_room_at_events(self, room_id, event_ids):
+    async def get_hosts_in_room_at_events(self, room_id, event_ids):
         """Get the hosts that were in a room at the given event ids
 
         Args:
@@ -208,12 +202,11 @@ class StateHandler(object):
         Returns:
             Deferred[list[str]]: the hosts in the room at the given events
         """
-        entry = yield self.resolve_state_groups_for_events(room_id, event_ids)
-        joined_hosts = yield self.store.get_joined_hosts(room_id, entry)
+        entry = await self.resolve_state_groups_for_events(room_id, event_ids)
+        joined_hosts = await self.store.get_joined_hosts(room_id, entry)
         return joined_hosts
 
-    @defer.inlineCallbacks
-    def compute_event_context(
+    async def compute_event_context(
         self, event: EventBase, old_state: Optional[Iterable[EventBase]] = None
     ):
         """Build an EventContext structure for the event.
@@ -278,7 +271,7 @@ class StateHandler(object):
             # otherwise, we'll need to resolve the state across the prev_events.
             logger.debug("calling resolve_state_groups from compute_event_context")
 
-            entry = yield self.resolve_state_groups_for_events(
+            entry = await self.resolve_state_groups_for_events(
                 event.room_id, event.prev_event_ids()
             )
 
@@ -295,7 +288,7 @@ class StateHandler(object):
         #
 
         if not state_group_before_event:
-            state_group_before_event = yield self.state_store.store_state_group(
+            state_group_before_event = await self.state_store.store_state_group(
                 event.event_id,
                 event.room_id,
                 prev_group=state_group_before_event_prev_group,
@@ -335,7 +328,7 @@ class StateHandler(object):
         state_ids_after_event[key] = event.event_id
         delta_ids = {key: event.event_id}
 
-        state_group_after_event = yield self.state_store.store_state_group(
+        state_group_after_event = await self.state_store.store_state_group(
             event.event_id,
             event.room_id,
             prev_group=state_group_before_event,
@@ -353,8 +346,7 @@ class StateHandler(object):
         )
 
     @measure_func()
-    @defer.inlineCallbacks
-    def resolve_state_groups_for_events(self, room_id, event_ids):
+    async def resolve_state_groups_for_events(self, room_id, event_ids):
         """ Given a list of event_ids this method fetches the state at each
         event, resolves conflicts between them and returns them.
 
@@ -373,7 +365,7 @@ class StateHandler(object):
         # map from state group id to the state in that state group (where
         # 'state' is a map from state key to event id)
         # dict[int, dict[(str, str), str]]
-        state_groups_ids = yield self.state_store.get_state_groups_ids(
+        state_groups_ids = await self.state_store.get_state_groups_ids(
             room_id, event_ids
         )
 
@@ -382,7 +374,7 @@ class StateHandler(object):
         elif len(state_groups_ids) == 1:
             name, state_list = list(state_groups_ids.items()).pop()
 
-            prev_group, delta_ids = yield self.state_store.get_state_group_delta(name)
+            prev_group, delta_ids = await self.state_store.get_state_group_delta(name)
 
             return _StateCacheEntry(
                 state=state_list,
@@ -391,9 +383,9 @@ class StateHandler(object):
                 delta_ids=delta_ids,
             )
 
-        room_version = yield self.store.get_room_version_id(room_id)
+        room_version = await self.store.get_room_version_id(room_id)
 
-        result = yield self._state_resolution_handler.resolve_state_groups(
+        result = await self._state_resolution_handler.resolve_state_groups(
             room_id,
             room_version,
             state_groups_ids,
@@ -402,8 +394,7 @@ class StateHandler(object):
         )
         return result
 
-    @defer.inlineCallbacks
-    def resolve_events(self, room_version, state_sets, event):
+    async def resolve_events(self, room_version, state_sets, event):
         logger.info(
             "Resolving state for %s with %d groups", event.room_id, len(state_sets)
         )
@@ -414,7 +405,7 @@ class StateHandler(object):
         state_map = {ev.event_id: ev for st in state_sets for ev in st}
 
         with Measure(self.clock, "state._resolve_events"):
-            new_state = yield resolve_events_with_store(
+            new_state = await resolve_events_with_store(
                 self.clock,
                 event.room_id,
                 room_version,
@@ -451,9 +442,8 @@ class StateResolutionHandler(object):
             reset_expiry_on_get=True,
         )
 
-    @defer.inlineCallbacks
     @log_function
-    def resolve_state_groups(
+    async def resolve_state_groups(
         self, room_id, room_version, state_groups_ids, event_map, state_res_store
     ):
         """Resolves conflicts between a set of state groups
@@ -479,13 +469,13 @@ class StateResolutionHandler(object):
             state_res_store (StateResolutionStore)
 
         Returns:
-            Deferred[_StateCacheEntry]: resolved state
+            _StateCacheEntry: resolved state
         """
         logger.debug("resolve_state_groups state_groups %s", state_groups_ids.keys())
 
         group_names = frozenset(state_groups_ids.keys())
 
-        with (yield self.resolve_linearizer.queue(group_names)):
+        with (await self.resolve_linearizer.queue(group_names)):
             if self._state_cache is not None:
                 cache = self._state_cache.get(group_names, None)
                 if cache:
@@ -517,7 +507,7 @@ class StateResolutionHandler(object):
             if conflicted_state:
                 logger.info("Resolving conflicted state for %r", room_id)
                 with Measure(self.clock, "state._resolve_events"):
-                    new_state = yield resolve_events_with_store(
+                    new_state = await resolve_events_with_store(
                         self.clock,
                         room_id,
                         room_version,
@@ -598,7 +588,7 @@ def resolve_events_with_store(
     state_sets: List[StateMap[str]],
     event_map: Optional[Dict[str, EventBase]],
     state_res_store: "StateResolutionStore",
-):
+) -> Awaitable[StateMap[str]]:
     """
     Args:
         room_id: the room we are working in
@@ -619,8 +609,7 @@ def resolve_events_with_store(
         state_res_store: a place to fetch events from
 
     Returns:
-        Deferred[dict[(str, str), str]]:
-            a map from (type, state_key) to event_id.
+        a map from (type, state_key) to event_id.
     """
     v = KNOWN_ROOM_VERSIONS[room_version]
     if v.state_res == StateResolutionVersions.V1:
diff --git a/synapse/state/v1.py b/synapse/state/v1.py
index 7b531a8337..ab5e24841d 100644
--- a/synapse/state/v1.py
+++ b/synapse/state/v1.py
@@ -15,9 +15,7 @@
 
 import hashlib
 import logging
-from typing import Callable, Dict, List, Optional
-
-from twisted.internet import defer
+from typing import Awaitable, Callable, Dict, List, Optional
 
 from synapse import event_auth
 from synapse.api.constants import EventTypes
@@ -32,12 +30,11 @@ logger = logging.getLogger(__name__)
 POWER_KEY = (EventTypes.PowerLevels, "")
 
 
-@defer.inlineCallbacks
-def resolve_events_with_store(
+async def resolve_events_with_store(
     room_id: str,
     state_sets: List[StateMap[str]],
     event_map: Optional[Dict[str, EventBase]],
-    state_map_factory: Callable,
+    state_map_factory: Callable[[List[str]], Awaitable],
 ):
     """
     Args:
@@ -56,7 +53,7 @@ def resolve_events_with_store(
 
         state_map_factory: will be called
             with a list of event_ids that are needed, and should return with
-            a Deferred of dict of event_id to event.
+            an Awaitable that resolves to a dict of event_id to event.
 
     Returns:
         Deferred[dict[(str, str), str]]:
@@ -80,7 +77,7 @@ def resolve_events_with_store(
 
     # dict[str, FrozenEvent]: a map from state event id to event. Only includes
     # the state events which are in conflict (and those in event_map)
-    state_map = yield state_map_factory(needed_events)
+    state_map = await state_map_factory(needed_events)
     if event_map is not None:
         state_map.update(event_map)
 
@@ -110,7 +107,7 @@ def resolve_events_with_store(
         "Asking for %d/%d auth events", len(new_needed_events), new_needed_event_count
     )
 
-    state_map_new = yield state_map_factory(new_needed_events)
+    state_map_new = await state_map_factory(new_needed_events)
     for event in state_map_new.values():
         if event.room_id != room_id:
             raise Exception(
diff --git a/synapse/state/v2.py b/synapse/state/v2.py
index bf6caa0946..6634955cdc 100644
--- a/synapse/state/v2.py
+++ b/synapse/state/v2.py
@@ -18,8 +18,6 @@ import itertools
 import logging
 from typing import Dict, List, Optional
 
-from twisted.internet import defer
-
 import synapse.state
 from synapse import event_auth
 from synapse.api.constants import EventTypes
@@ -32,14 +30,13 @@ from synapse.util import Clock
 logger = logging.getLogger(__name__)
 
 
-# We want to yield to the reactor occasionally during state res when dealing
+# We want to await to the reactor occasionally during state res when dealing
 # with large data sets, so that we don't exhaust the reactor. This is done by
-# yielding to reactor during loops every N iterations.
-_YIELD_AFTER_ITERATIONS = 100
+# awaiting to reactor during loops every N iterations.
+_AWAIT_AFTER_ITERATIONS = 100
 
 
-@defer.inlineCallbacks
-def resolve_events_with_store(
+async def resolve_events_with_store(
     clock: Clock,
     room_id: str,
     room_version: str,
@@ -87,7 +84,7 @@ def resolve_events_with_store(
 
     # Also fetch all auth events that appear in only some of the state sets'
     # auth chains.
-    auth_diff = yield _get_auth_chain_difference(state_sets, event_map, state_res_store)
+    auth_diff = await _get_auth_chain_difference(state_sets, event_map, state_res_store)
 
     full_conflicted_set = set(
         itertools.chain(
@@ -95,7 +92,7 @@ def resolve_events_with_store(
         )
     )
 
-    events = yield state_res_store.get_events(
+    events = await state_res_store.get_events(
         [eid for eid in full_conflicted_set if eid not in event_map],
         allow_rejected=True,
     )
@@ -118,14 +115,14 @@ def resolve_events_with_store(
         eid for eid in full_conflicted_set if _is_power_event(event_map[eid])
     )
 
-    sorted_power_events = yield _reverse_topological_power_sort(
+    sorted_power_events = await _reverse_topological_power_sort(
         clock, room_id, power_events, event_map, state_res_store, full_conflicted_set
     )
 
     logger.debug("sorted %d power events", len(sorted_power_events))
 
     # Now sequentially auth each one
-    resolved_state = yield _iterative_auth_checks(
+    resolved_state = await _iterative_auth_checks(
         clock,
         room_id,
         room_version,
@@ -148,13 +145,13 @@ def resolve_events_with_store(
     logger.debug("sorting %d remaining events", len(leftover_events))
 
     pl = resolved_state.get((EventTypes.PowerLevels, ""), None)
-    leftover_events = yield _mainline_sort(
+    leftover_events = await _mainline_sort(
         clock, room_id, leftover_events, pl, event_map, state_res_store
     )
 
     logger.debug("resolving remaining events")
 
-    resolved_state = yield _iterative_auth_checks(
+    resolved_state = await _iterative_auth_checks(
         clock,
         room_id,
         room_version,
@@ -174,8 +171,7 @@ def resolve_events_with_store(
     return resolved_state
 
 
-@defer.inlineCallbacks
-def _get_power_level_for_sender(room_id, event_id, event_map, state_res_store):
+async def _get_power_level_for_sender(room_id, event_id, event_map, state_res_store):
     """Return the power level of the sender of the given event according to
     their auth events.
 
@@ -188,11 +184,11 @@ def _get_power_level_for_sender(room_id, event_id, event_map, state_res_store):
     Returns:
         Deferred[int]
     """
-    event = yield _get_event(room_id, event_id, event_map, state_res_store)
+    event = await _get_event(room_id, event_id, event_map, state_res_store)
 
     pl = None
     for aid in event.auth_event_ids():
-        aev = yield _get_event(
+        aev = await _get_event(
             room_id, aid, event_map, state_res_store, allow_none=True
         )
         if aev and (aev.type, aev.state_key) == (EventTypes.PowerLevels, ""):
@@ -202,7 +198,7 @@ def _get_power_level_for_sender(room_id, event_id, event_map, state_res_store):
     if pl is None:
         # Couldn't find power level. Check if they're the creator of the room
         for aid in event.auth_event_ids():
-            aev = yield _get_event(
+            aev = await _get_event(
                 room_id, aid, event_map, state_res_store, allow_none=True
             )
             if aev and (aev.type, aev.state_key) == (EventTypes.Create, ""):
@@ -221,8 +217,7 @@ def _get_power_level_for_sender(room_id, event_id, event_map, state_res_store):
         return int(level)
 
 
-@defer.inlineCallbacks
-def _get_auth_chain_difference(state_sets, event_map, state_res_store):
+async def _get_auth_chain_difference(state_sets, event_map, state_res_store):
     """Compare the auth chains of each state set and return the set of events
     that only appear in some but not all of the auth chains.
 
@@ -235,7 +230,7 @@ def _get_auth_chain_difference(state_sets, event_map, state_res_store):
         Deferred[set[str]]: Set of event IDs
     """
 
-    difference = yield state_res_store.get_auth_chain_difference(
+    difference = await state_res_store.get_auth_chain_difference(
         [set(state_set.values()) for state_set in state_sets]
     )
 
@@ -292,8 +287,7 @@ def _is_power_event(event):
     return False
 
 
-@defer.inlineCallbacks
-def _add_event_and_auth_chain_to_graph(
+async def _add_event_and_auth_chain_to_graph(
     graph, room_id, event_id, event_map, state_res_store, auth_diff
 ):
     """Helper function for _reverse_topological_power_sort that add the event
@@ -314,7 +308,7 @@ def _add_event_and_auth_chain_to_graph(
         eid = state.pop()
         graph.setdefault(eid, set())
 
-        event = yield _get_event(room_id, eid, event_map, state_res_store)
+        event = await _get_event(room_id, eid, event_map, state_res_store)
         for aid in event.auth_event_ids():
             if aid in auth_diff:
                 if aid not in graph:
@@ -323,8 +317,7 @@ def _add_event_and_auth_chain_to_graph(
                 graph.setdefault(eid, set()).add(aid)
 
 
-@defer.inlineCallbacks
-def _reverse_topological_power_sort(
+async def _reverse_topological_power_sort(
     clock, room_id, event_ids, event_map, state_res_store, auth_diff
 ):
     """Returns a list of the event_ids sorted by reverse topological ordering,
@@ -344,26 +337,26 @@ def _reverse_topological_power_sort(
 
     graph = {}
     for idx, event_id in enumerate(event_ids, start=1):
-        yield _add_event_and_auth_chain_to_graph(
+        await _add_event_and_auth_chain_to_graph(
             graph, room_id, event_id, event_map, state_res_store, auth_diff
         )
 
-        # We yield occasionally when we're working with large data sets to
+        # We await occasionally when we're working with large data sets to
         # ensure that we don't block the reactor loop for too long.
-        if idx % _YIELD_AFTER_ITERATIONS == 0:
-            yield clock.sleep(0)
+        if idx % _AWAIT_AFTER_ITERATIONS == 0:
+            await clock.sleep(0)
 
     event_to_pl = {}
     for idx, event_id in enumerate(graph, start=1):
-        pl = yield _get_power_level_for_sender(
+        pl = await _get_power_level_for_sender(
             room_id, event_id, event_map, state_res_store
         )
         event_to_pl[event_id] = pl
 
-        # We yield occasionally when we're working with large data sets to
+        # We await occasionally when we're working with large data sets to
         # ensure that we don't block the reactor loop for too long.
-        if idx % _YIELD_AFTER_ITERATIONS == 0:
-            yield clock.sleep(0)
+        if idx % _AWAIT_AFTER_ITERATIONS == 0:
+            await clock.sleep(0)
 
     def _get_power_order(event_id):
         ev = event_map[event_id]
@@ -378,8 +371,7 @@ def _reverse_topological_power_sort(
     return sorted_events
 
 
-@defer.inlineCallbacks
-def _iterative_auth_checks(
+async def _iterative_auth_checks(
     clock, room_id, room_version, event_ids, base_state, event_map, state_res_store
 ):
     """Sequentially apply auth checks to each event in given list, updating the
@@ -405,7 +397,7 @@ def _iterative_auth_checks(
 
         auth_events = {}
         for aid in event.auth_event_ids():
-            ev = yield _get_event(
+            ev = await _get_event(
                 room_id, aid, event_map, state_res_store, allow_none=True
             )
 
@@ -420,7 +412,7 @@ def _iterative_auth_checks(
         for key in event_auth.auth_types_for_event(event):
             if key in resolved_state:
                 ev_id = resolved_state[key]
-                ev = yield _get_event(room_id, ev_id, event_map, state_res_store)
+                ev = await _get_event(room_id, ev_id, event_map, state_res_store)
 
                 if ev.rejected_reason is None:
                     auth_events[key] = event_map[ev_id]
@@ -438,16 +430,15 @@ def _iterative_auth_checks(
         except AuthError:
             pass
 
-        # We yield occasionally when we're working with large data sets to
+        # We await occasionally when we're working with large data sets to
         # ensure that we don't block the reactor loop for too long.
-        if idx % _YIELD_AFTER_ITERATIONS == 0:
-            yield clock.sleep(0)
+        if idx % _AWAIT_AFTER_ITERATIONS == 0:
+            await clock.sleep(0)
 
     return resolved_state
 
 
-@defer.inlineCallbacks
-def _mainline_sort(
+async def _mainline_sort(
     clock, room_id, event_ids, resolved_power_event_id, event_map, state_res_store
 ):
     """Returns a sorted list of event_ids sorted by mainline ordering based on
@@ -474,21 +465,21 @@ def _mainline_sort(
     idx = 0
     while pl:
         mainline.append(pl)
-        pl_ev = yield _get_event(room_id, pl, event_map, state_res_store)
+        pl_ev = await _get_event(room_id, pl, event_map, state_res_store)
         auth_events = pl_ev.auth_event_ids()
         pl = None
         for aid in auth_events:
-            ev = yield _get_event(
+            ev = await _get_event(
                 room_id, aid, event_map, state_res_store, allow_none=True
             )
             if ev and (ev.type, ev.state_key) == (EventTypes.PowerLevels, ""):
                 pl = aid
                 break
 
-        # We yield occasionally when we're working with large data sets to
+        # We await occasionally when we're working with large data sets to
         # ensure that we don't block the reactor loop for too long.
-        if idx != 0 and idx % _YIELD_AFTER_ITERATIONS == 0:
-            yield clock.sleep(0)
+        if idx != 0 and idx % _AWAIT_AFTER_ITERATIONS == 0:
+            await clock.sleep(0)
 
         idx += 1
 
@@ -498,23 +489,24 @@ def _mainline_sort(
 
     order_map = {}
     for idx, ev_id in enumerate(event_ids, start=1):
-        depth = yield _get_mainline_depth_for_event(
+        depth = await _get_mainline_depth_for_event(
             event_map[ev_id], mainline_map, event_map, state_res_store
         )
         order_map[ev_id] = (depth, event_map[ev_id].origin_server_ts, ev_id)
 
-        # We yield occasionally when we're working with large data sets to
+        # We await occasionally when we're working with large data sets to
         # ensure that we don't block the reactor loop for too long.
-        if idx % _YIELD_AFTER_ITERATIONS == 0:
-            yield clock.sleep(0)
+        if idx % _AWAIT_AFTER_ITERATIONS == 0:
+            await clock.sleep(0)
 
     event_ids.sort(key=lambda ev_id: order_map[ev_id])
 
     return event_ids
 
 
-@defer.inlineCallbacks
-def _get_mainline_depth_for_event(event, mainline_map, event_map, state_res_store):
+async def _get_mainline_depth_for_event(
+    event, mainline_map, event_map, state_res_store
+):
     """Get the mainline depths for the given event based on the mainline map
 
     Args:
@@ -541,7 +533,7 @@ def _get_mainline_depth_for_event(event, mainline_map, event_map, state_res_stor
         event = None
 
         for aid in auth_events:
-            aev = yield _get_event(
+            aev = await _get_event(
                 room_id, aid, event_map, state_res_store, allow_none=True
             )
             if aev and (aev.type, aev.state_key) == (EventTypes.PowerLevels, ""):
@@ -552,8 +544,7 @@ def _get_mainline_depth_for_event(event, mainline_map, event_map, state_res_stor
     return 0
 
 
-@defer.inlineCallbacks
-def _get_event(room_id, event_id, event_map, state_res_store, allow_none=False):
+async def _get_event(room_id, event_id, event_map, state_res_store, allow_none=False):
     """Helper function to look up event in event_map, falling back to looking
     it up in the store
 
@@ -569,7 +560,7 @@ def _get_event(room_id, event_id, event_map, state_res_store, allow_none=False):
         Deferred[Optional[FrozenEvent]]
     """
     if event_id not in event_map:
-        events = yield state_res_store.get_events([event_id], allow_rejected=True)
+        events = await state_res_store.get_events([event_id], allow_rejected=True)
         event_map.update(events)
     event = event_map.get(event_id)
 
diff --git a/synapse/storage/data_stores/main/push_rule.py b/synapse/storage/data_stores/main/push_rule.py
index d181488db7..c229248101 100644
--- a/synapse/storage/data_stores/main/push_rule.py
+++ b/synapse/storage/data_stores/main/push_rule.py
@@ -259,7 +259,7 @@ class PushRulesWorkerStore(
             # To do this we set the state_group to a new object as object() != object()
             state_group = object()
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
         result = yield self._bulk_get_push_rules_for_room(
             event.room_id, state_group, current_state_ids, event=event
         )
diff --git a/synapse/storage/data_stores/main/roommember.py b/synapse/storage/data_stores/main/roommember.py
index 29765890ee..a92e401e88 100644
--- a/synapse/storage/data_stores/main/roommember.py
+++ b/synapse/storage/data_stores/main/roommember.py
@@ -497,7 +497,7 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             # To do this we set the state_group to a new object as object() != object()
             state_group = object()
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
         result = yield self._get_joined_users_from_context(
             event.room_id, state_group, current_state_ids, event=event, context=context
         )
diff --git a/synapse/storage/data_stores/main/user_directory.py b/synapse/storage/data_stores/main/user_directory.py
index 6b8130bf0f..942e51fd3a 100644
--- a/synapse/storage/data_stores/main/user_directory.py
+++ b/synapse/storage/data_stores/main/user_directory.py
@@ -198,7 +198,9 @@ class UserDirectoryBackgroundUpdateStore(StateDeltasStore):
                     room_id
                 )
 
-                users_with_profile = yield state.get_current_users_in_room(room_id)
+                users_with_profile = yield defer.ensureDeferred(
+                    state.get_current_users_in_room(room_id)
+                )
                 user_ids = set(users_with_profile)
 
                 # Update each user in the user directory.
diff --git a/synapse/storage/persist_events.py b/synapse/storage/persist_events.py
index fa46041676..78fbdcdee8 100644
--- a/synapse/storage/persist_events.py
+++ b/synapse/storage/persist_events.py
@@ -29,7 +29,6 @@ from synapse.events import FrozenEvent
 from synapse.events.snapshot import EventContext
 from synapse.logging.context import PreserveLoggingContext, make_deferred_yieldable
 from synapse.metrics.background_process_metrics import run_as_background_process
-from synapse.state import StateResolutionStore
 from synapse.storage.data_stores import DataStores
 from synapse.storage.data_stores.main.events import DeltaState
 from synapse.types import StateMap
@@ -648,6 +647,10 @@ class EventsPersistenceStorage(object):
             room_version = await self.main_store.get_room_version_id(room_id)
 
         logger.debug("calling resolve_state_groups from preserve_events")
+
+        # Avoid a circular import.
+        from synapse.state import StateResolutionStore
+
         res = await self._state_resolution_handler.resolve_state_groups(
             room_id,
             room_version,
diff --git a/tests/federation/test_federation_sender.py b/tests/federation/test_federation_sender.py
index 1a9bd5f37d..d1bd18da39 100644
--- a/tests/federation/test_federation_sender.py
+++ b/tests/federation/test_federation_sender.py
@@ -26,21 +26,24 @@ from synapse.rest import admin
 from synapse.rest.client.v1 import login
 from synapse.types import JsonDict, ReadReceipt
 
+from tests.test_utils import make_awaitable
 from tests.unittest import HomeserverTestCase, override_config
 
 
 class FederationSenderReceiptsTestCases(HomeserverTestCase):
     def make_homeserver(self, reactor, clock):
+        mock_state_handler = Mock(spec=["get_current_hosts_in_room"])
+        # Ensure a new Awaitable is created for each call.
+        mock_state_handler.get_current_hosts_in_room.side_effect = lambda room_Id: make_awaitable(
+            ["test", "host2"]
+        )
         return self.setup_test_homeserver(
-            state_handler=Mock(spec=["get_current_hosts_in_room"]),
+            state_handler=mock_state_handler,
             federation_transport_client=Mock(spec=["send_transaction"]),
         )
 
     @override_config({"send_federation": True})
     def test_send_receipts(self):
-        mock_state_handler = self.hs.get_state_handler()
-        mock_state_handler.get_current_hosts_in_room.return_value = ["test", "host2"]
-
         mock_send_transaction = (
             self.hs.get_federation_transport_client().send_transaction
         )
@@ -81,9 +84,6 @@ class FederationSenderReceiptsTestCases(HomeserverTestCase):
     def test_send_receipts_with_backoff(self):
         """Send two receipts in quick succession; the second should be flushed, but
         only after 20ms"""
-        mock_state_handler = self.hs.get_state_handler()
-        mock_state_handler.get_current_hosts_in_room.return_value = ["test", "host2"]
-
         mock_send_transaction = (
             self.hs.get_federation_transport_client().send_transaction
         )
@@ -164,7 +164,6 @@ class FederationSenderDevicesTestCases(HomeserverTestCase):
 
     def make_homeserver(self, reactor, clock):
         return self.setup_test_homeserver(
-            state_handler=Mock(spec=["get_current_hosts_in_room"]),
             federation_transport_client=Mock(spec=["send_transaction"]),
         )
 
@@ -174,10 +173,6 @@ class FederationSenderDevicesTestCases(HomeserverTestCase):
         return c
 
     def prepare(self, reactor, clock, hs):
-        # stub out get_current_hosts_in_room
-        mock_state_handler = hs.get_state_handler()
-        mock_state_handler.get_current_hosts_in_room.return_value = ["test", "host2"]
-
         # stub out get_users_who_share_room_with_user so that it claims that
         # `@user2:host2` is in the room
         def get_users_who_share_room_with_user(user_id):
diff --git a/tests/state/test_v2.py b/tests/state/test_v2.py
index 38f9b423ef..f2955a9c69 100644
--- a/tests/state/test_v2.py
+++ b/tests/state/test_v2.py
@@ -14,6 +14,7 @@
 # limitations under the License.
 
 import itertools
+from typing import List
 
 import attr
 
@@ -432,7 +433,7 @@ class StateTestCase(unittest.TestCase):
                     state_res_store=TestStateResolutionStore(event_map),
                 )
 
-                state_before = self.successResultOf(state_d)
+                state_before = self.successResultOf(defer.ensureDeferred(state_d))
 
             state_after = dict(state_before)
             if fake_event.state_key is not None:
@@ -581,7 +582,7 @@ class SimpleParamStateTestCase(unittest.TestCase):
             state_res_store=TestStateResolutionStore(self.event_map),
         )
 
-        state = self.successResultOf(state_d)
+        state = self.successResultOf(defer.ensureDeferred(state_d))
 
         self.assert_dict(self.expected_combined_state, state)
 
@@ -608,9 +609,11 @@ class TestStateResolutionStore(object):
             Deferred[dict[str, FrozenEvent]]: Dict from event_id to event.
         """
 
-        return {eid: self.event_map[eid] for eid in event_ids if eid in self.event_map}
+        return defer.succeed(
+            {eid: self.event_map[eid] for eid in event_ids if eid in self.event_map}
+        )
 
-    def _get_auth_chain(self, event_ids):
+    def _get_auth_chain(self, event_ids: List[str]) -> List[str]:
         """Gets the full auth chain for a set of events (including rejected
         events).
 
@@ -622,10 +625,10 @@ class TestStateResolutionStore(object):
                presence of rejected events
 
         Args:
-            event_ids (list): The event IDs of the events to fetch the auth
+            event_ids: The event IDs of the events to fetch the auth
                 chain for. Must be state events.
         Returns:
-            Deferred[list[str]]: List of event IDs of the auth chain.
+            List of event IDs of the auth chain.
         """
 
         # Simple DFS for auth chain
@@ -648,4 +651,4 @@ class TestStateResolutionStore(object):
         chains = [frozenset(self._get_auth_chain(a)) for a in auth_sets]
 
         common = set(chains[0]).intersection(*chains[1:])
-        return set(chains[0]).union(*chains[1:]) - common
+        return defer.succeed(set(chains[0]).union(*chains[1:]) - common)
diff --git a/tests/storage/test_room.py b/tests/storage/test_room.py
index b1dceb2918..1d77b4a2d6 100644
--- a/tests/storage/test_room.py
+++ b/tests/storage/test_room.py
@@ -109,7 +109,9 @@ class RoomEventsStoreTestCase(unittest.TestCase):
             etype=EventTypes.Name, name=name, content={"name": name}, depth=1
         )
 
-        state = yield self.store.get_current_state(room_id=self.room.to_string())
+        state = yield defer.ensureDeferred(
+            self.store.get_current_state(room_id=self.room.to_string())
+        )
 
         self.assertEquals(1, len(state))
         self.assertObjectHasAttributes(
@@ -125,7 +127,9 @@ class RoomEventsStoreTestCase(unittest.TestCase):
             etype=EventTypes.Topic, topic=topic, content={"topic": topic}, depth=1
         )
 
-        state = yield self.store.get_current_state(room_id=self.room.to_string())
+        state = yield defer.ensureDeferred(
+            self.store.get_current_state(room_id=self.room.to_string())
+        )
 
         self.assertEquals(1, len(state))
         self.assertObjectHasAttributes(
diff --git a/tests/test_state.py b/tests/test_state.py
index 66f22f6813..4858e8fc59 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -97,17 +97,19 @@ class StateGroupStore(object):
 
         self._group_to_state[state_group] = dict(current_state_ids)
 
-        return state_group
+        return defer.succeed(state_group)
 
     def get_events(self, event_ids, **kwargs):
-        return {
-            e_id: self._event_id_to_event[e_id]
-            for e_id in event_ids
-            if e_id in self._event_id_to_event
-        }
+        return defer.succeed(
+            {
+                e_id: self._event_id_to_event[e_id]
+                for e_id in event_ids
+                if e_id in self._event_id_to_event
+            }
+        )
 
     def get_state_group_delta(self, name):
-        return None, None
+        return defer.succeed((None, None))
 
     def register_events(self, events):
         for e in events:
@@ -120,7 +122,7 @@ class StateGroupStore(object):
         self._event_to_state_group[event_id] = state_group
 
     def get_room_version_id(self, room_id):
-        return RoomVersions.V1.identifier
+        return defer.succeed(RoomVersions.V1.identifier)
 
 
 class DictObj(dict):
@@ -202,7 +204,9 @@ class StateTestCase(unittest.TestCase):
         context_store = {}  # type: dict[str, EventContext]
 
         for event in graph.walk():
-            context = yield self.state.compute_event_context(event)
+            context = yield defer.ensureDeferred(
+                self.state.compute_event_context(event)
+            )
             self.store.register_event_context(event, context)
             context_store[event.event_id] = context
 
@@ -244,7 +248,9 @@ class StateTestCase(unittest.TestCase):
         context_store = {}
 
         for event in graph.walk():
-            context = yield self.state.compute_event_context(event)
+            context = yield defer.ensureDeferred(
+                self.state.compute_event_context(event)
+            )
             self.store.register_event_context(event, context)
             context_store[event.event_id] = context
 
@@ -300,7 +306,9 @@ class StateTestCase(unittest.TestCase):
         context_store = {}
 
         for event in graph.walk():
-            context = yield self.state.compute_event_context(event)
+            context = yield defer.ensureDeferred(
+                self.state.compute_event_context(event)
+            )
             self.store.register_event_context(event, context)
             context_store[event.event_id] = context
 
@@ -373,7 +381,9 @@ class StateTestCase(unittest.TestCase):
         context_store = {}
 
         for event in graph.walk():
-            context = yield self.state.compute_event_context(event)
+            context = yield defer.ensureDeferred(
+                self.state.compute_event_context(event)
+            )
             self.store.register_event_context(event, context)
             context_store[event.event_id] = context
 
@@ -411,12 +421,14 @@ class StateTestCase(unittest.TestCase):
             create_event(type="test2", state_key=""),
         ]
 
-        context = yield self.state.compute_event_context(event, old_state=old_state)
+        context = yield defer.ensureDeferred(
+            self.state.compute_event_context(event, old_state=old_state)
+        )
 
         prev_state_ids = yield context.get_prev_state_ids()
         self.assertCountEqual((e.event_id for e in old_state), prev_state_ids.values())
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
         self.assertCountEqual(
             (e.event_id for e in old_state), current_state_ids.values()
         )
@@ -434,12 +446,14 @@ class StateTestCase(unittest.TestCase):
             create_event(type="test2", state_key=""),
         ]
 
-        context = yield self.state.compute_event_context(event, old_state=old_state)
+        context = yield defer.ensureDeferred(
+            self.state.compute_event_context(event, old_state=old_state)
+        )
 
         prev_state_ids = yield context.get_prev_state_ids()
         self.assertCountEqual((e.event_id for e in old_state), prev_state_ids.values())
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
         self.assertCountEqual(
             (e.event_id for e in old_state + [event]), current_state_ids.values()
         )
@@ -462,7 +476,7 @@ class StateTestCase(unittest.TestCase):
             create_event(type="test2", state_key=""),
         ]
 
-        group_name = self.store.store_state_group(
+        group_name = yield self.store.store_state_group(
             prev_event_id,
             event.room_id,
             None,
@@ -471,9 +485,9 @@ class StateTestCase(unittest.TestCase):
         )
         self.store.register_event_id_state_group(prev_event_id, group_name)
 
-        context = yield self.state.compute_event_context(event)
+        context = yield defer.ensureDeferred(self.state.compute_event_context(event))
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
 
         self.assertEqual(
             {e.event_id for e in old_state}, set(current_state_ids.values())
@@ -494,7 +508,7 @@ class StateTestCase(unittest.TestCase):
             create_event(type="test2", state_key=""),
         ]
 
-        group_name = self.store.store_state_group(
+        group_name = yield self.store.store_state_group(
             prev_event_id,
             event.room_id,
             None,
@@ -503,7 +517,7 @@ class StateTestCase(unittest.TestCase):
         )
         self.store.register_event_id_state_group(prev_event_id, group_name)
 
-        context = yield self.state.compute_event_context(event)
+        context = yield defer.ensureDeferred(self.state.compute_event_context(event))
 
         prev_state_ids = yield context.get_prev_state_ids()
 
@@ -544,7 +558,7 @@ class StateTestCase(unittest.TestCase):
             event, prev_event_id1, old_state_1, prev_event_id2, old_state_2
         )
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
 
         self.assertEqual(len(current_state_ids), 6)
 
@@ -586,7 +600,7 @@ class StateTestCase(unittest.TestCase):
             event, prev_event_id1, old_state_1, prev_event_id2, old_state_2
         )
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
 
         self.assertEqual(len(current_state_ids), 6)
 
@@ -641,7 +655,7 @@ class StateTestCase(unittest.TestCase):
             event, prev_event_id1, old_state_1, prev_event_id2, old_state_2
         )
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
 
         self.assertEqual(old_state_2[3].event_id, current_state_ids[("test1", "1")])
 
@@ -669,14 +683,15 @@ class StateTestCase(unittest.TestCase):
             event, prev_event_id1, old_state_1, prev_event_id2, old_state_2
         )
 
-        current_state_ids = yield context.get_current_state_ids()
+        current_state_ids = yield defer.ensureDeferred(context.get_current_state_ids())
 
         self.assertEqual(old_state_1[3].event_id, current_state_ids[("test1", "1")])
 
+    @defer.inlineCallbacks
     def _get_context(
         self, event, prev_event_id_1, old_state_1, prev_event_id_2, old_state_2
     ):
-        sg1 = self.store.store_state_group(
+        sg1 = yield self.store.store_state_group(
             prev_event_id_1,
             event.room_id,
             None,
@@ -685,7 +700,7 @@ class StateTestCase(unittest.TestCase):
         )
         self.store.register_event_id_state_group(prev_event_id_1, sg1)
 
-        sg2 = self.store.store_state_group(
+        sg2 = yield self.store.store_state_group(
             prev_event_id_2,
             event.room_id,
             None,
@@ -694,4 +709,5 @@ class StateTestCase(unittest.TestCase):
         )
         self.store.register_event_id_state_group(prev_event_id_2, sg2)
 
-        return self.state.compute_event_context(event)
+        result = yield defer.ensureDeferred(self.state.compute_event_context(event))
+        return result
diff --git a/tests/test_utils/__init__.py b/tests/test_utils/__init__.py
index 7b345b03bb..508aeba078 100644
--- a/tests/test_utils/__init__.py
+++ b/tests/test_utils/__init__.py
@@ -17,7 +17,7 @@
 """
 Utilities for running the unit tests
 """
-from typing import Awaitable, TypeVar
+from typing import Any, Awaitable, TypeVar
 
 TV = TypeVar("TV")
 
@@ -36,3 +36,8 @@ def get_awaitable_result(awaitable: Awaitable[TV]) -> TV:
 
     # if next didn't raise, the awaitable hasn't completed.
     raise Exception("awaitable has not yet completed")
+
+
+async def make_awaitable(result: Any):
+    """Create an awaitable that just returns a result."""
+    return result