Yield during large v2 state res. (#7735)

State res v2 across large data sets can be very CPU intensive, and if
all the relevant events are in the cache the algorithm will run from
start to finish within a single reactor tick. This can result in
blocking the reactor tick for several seconds, which can have major
repercussions on other requests.

To fix this we simply add the occaisonal `sleep(0)` during iterations to
yield execution until the next reactor tick. The aim is to only do this
for large data sets so that we don't impact otherwise quick resolutions.=

1 files changed, 9 insertions, 0 deletions

diff --git a/tests/state/test_v2.py b/tests/state/test_v2.py
index cdc347bc53..38f9b423ef 100644
--- a/tests/state/test_v2.py
+++ b/tests/state/test_v2.py
@@ -17,6 +17,8 @@ import itertools
 
 import attr
 
+from twisted.internet import defer
+
 from synapse.api.constants import EventTypes, JoinRules, Membership
 from synapse.api.room_versions import RoomVersions
 from synapse.event_auth import auth_types_for_event
@@ -41,6 +43,11 @@ MEMBERSHIP_CONTENT_BAN = {"membership": Membership.BAN}
 ORIGIN_SERVER_TS = 0
 
 
+class FakeClock:
+    def sleep(self, msec):
+        return defer.succeed(None)
+
+
 class FakeEvent(object):
     """A fake event we use as a convenience.
 
@@ -417,6 +424,7 @@ class StateTestCase(unittest.TestCase):
                 state_before = dict(state_at_event[prev_events[0]])
             else:
                 state_d = resolve_events_with_store(
+                    FakeClock(),
                     ROOM_ID,
                     RoomVersions.V2.identifier,
                     [state_at_event[n] for n in prev_events],
@@ -565,6 +573,7 @@ class SimpleParamStateTestCase(unittest.TestCase):
         # Test that we correctly handle passing `None` as the event_map
 
         state_d = resolve_events_with_store(
+            FakeClock(),
             ROOM_ID,
             RoomVersions.V2.identifier,
             [self.state_at_bob, self.state_at_charlie],