diff --git a/tests/storage/databases/test_state_store.py b/tests/storage/databases/test_state_store.py
new file mode 100644
index 0000000000..cf126ee62d
--- /dev/null
+++ b/tests/storage/databases/test_state_store.py
@@ -0,0 +1,133 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import typing
+from typing import Dict, List, Sequence, Tuple
+from unittest.mock import patch
+
+from twisted.internet.defer import Deferred, ensureDeferred
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.storage.state import StateFilter
+from synapse.types import MutableStateMap, StateMap
+from synapse.util import Clock
+
+from tests.unittest import HomeserverTestCase
+
+if typing.TYPE_CHECKING:
+ from synapse.server import HomeServer
+
+
+class StateGroupInflightCachingTestCase(HomeserverTestCase):
+ def prepare(
+ self, reactor: MemoryReactor, clock: Clock, homeserver: "HomeServer"
+ ) -> None:
+ self.state_storage = homeserver.get_storage().state
+ self.state_datastore = homeserver.get_datastores().state
+ # Patch out the `_get_state_groups_from_groups`.
+ # This is useful because it lets us pretend we have a slow database.
+ get_state_groups_patch = patch.object(
+ self.state_datastore,
+ "_get_state_groups_from_groups",
+ self._fake_get_state_groups_from_groups,
+ )
+ get_state_groups_patch.start()
+
+ self.addCleanup(get_state_groups_patch.stop)
+ self.get_state_group_calls: List[
+ Tuple[Tuple[int, ...], StateFilter, Deferred[Dict[int, StateMap[str]]]]
+ ] = []
+
+ def _fake_get_state_groups_from_groups(
+ self, groups: Sequence[int], state_filter: StateFilter
+ ) -> "Deferred[Dict[int, StateMap[str]]]":
+ d: Deferred[Dict[int, StateMap[str]]] = Deferred()
+ self.get_state_group_calls.append((tuple(groups), state_filter, d))
+ return d
+
+ def _complete_request_fake(
+ self,
+ groups: Tuple[int, ...],
+ state_filter: StateFilter,
+ d: "Deferred[Dict[int, StateMap[str]]]",
+ ) -> None:
+ """
+ Assemble a fake database response and complete the database request.
+ """
+
+ result: Dict[int, StateMap[str]] = {}
+
+ for group in groups:
+ group_result: MutableStateMap[str] = {}
+ result[group] = group_result
+
+ for state_type, state_keys in state_filter.types.items():
+ if state_keys is None:
+ group_result[(state_type, "a")] = "xyz"
+ group_result[(state_type, "b")] = "xyz"
+ else:
+ for state_key in state_keys:
+ group_result[(state_type, state_key)] = "abc"
+
+ if state_filter.include_others:
+ group_result[("other.event.type", "state.key")] = "123"
+
+ d.callback(result)
+
+ def test_duplicate_requests_deduplicated(self) -> None:
+ """
+ Tests that duplicate requests for state are deduplicated.
+
+ This test:
+ - requests some state (state group 42, 'all' state filter)
+ - requests it again, before the first request finishes
+ - checks to see that only one database query was made
+ - completes the database query
+ - checks that both requests see the same retrieved state
+ """
+ req1 = ensureDeferred(
+ self.state_datastore._get_state_for_group_using_inflight_cache(
+ 42, StateFilter.all()
+ )
+ )
+ self.pump(by=0.1)
+
+ # This should have gone to the database
+ self.assertEqual(len(self.get_state_group_calls), 1)
+ self.assertFalse(req1.called)
+
+ req2 = ensureDeferred(
+ self.state_datastore._get_state_for_group_using_inflight_cache(
+ 42, StateFilter.all()
+ )
+ )
+ self.pump(by=0.1)
+
+ # No more calls should have gone to the database
+ self.assertEqual(len(self.get_state_group_calls), 1)
+ self.assertFalse(req1.called)
+ self.assertFalse(req2.called)
+
+ groups, sf, d = self.get_state_group_calls[0]
+ self.assertEqual(groups, (42,))
+ self.assertEqual(sf, StateFilter.all())
+
+ # Now we can complete the request
+ self._complete_request_fake(groups, sf, d)
+
+ self.assertEqual(
+ self.get_success(req1), {("other.event.type", "state.key"): "123"}
+ )
+ self.assertEqual(
+ self.get_success(req2), {("other.event.type", "state.key"): "123"}
+ )
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