Track and deduplicate in-flight requests to `_get_state_for_groups`. (#10870)

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

1 files changed, 133 insertions, 0 deletions

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"}
+        )