diff --git a/tests/util/caches/test_deferred_cache.py b/tests/util/caches/test_deferred_cache.py
new file mode 100644
index 0000000000..9717be56b6
--- /dev/null
+++ b/tests/util/caches/test_deferred_cache.py
@@ -0,0 +1,137 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 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 unittest
+from functools import partial
+
+from twisted.internet import defer
+
+from synapse.util.caches.deferred_cache import DeferredCache
+
+
+class DeferredCacheTestCase(unittest.TestCase):
+ def test_empty(self):
+ cache = DeferredCache("test")
+ failed = False
+ try:
+ cache.get("foo")
+ except KeyError:
+ failed = True
+
+ self.assertTrue(failed)
+
+ def test_hit(self):
+ cache = DeferredCache("test")
+ cache.prefill("foo", 123)
+
+ self.assertEquals(cache.get("foo"), 123)
+
+ def test_invalidate(self):
+ cache = DeferredCache("test")
+ cache.prefill(("foo",), 123)
+ cache.invalidate(("foo",))
+
+ failed = False
+ try:
+ cache.get(("foo",))
+ except KeyError:
+ failed = True
+
+ self.assertTrue(failed)
+
+ def test_invalidate_all(self):
+ cache = DeferredCache("testcache")
+
+ callback_record = [False, False]
+
+ def record_callback(idx):
+ callback_record[idx] = True
+
+ # add a couple of pending entries
+ d1 = defer.Deferred()
+ cache.set("key1", d1, partial(record_callback, 0))
+
+ d2 = defer.Deferred()
+ cache.set("key2", d2, partial(record_callback, 1))
+
+ # lookup should return observable deferreds
+ self.assertFalse(cache.get("key1").has_called())
+ self.assertFalse(cache.get("key2").has_called())
+
+ # let one of the lookups complete
+ d2.callback("result2")
+
+ # for now at least, the cache will return real results rather than an
+ # observabledeferred
+ self.assertEqual(cache.get("key2"), "result2")
+
+ # now do the invalidation
+ cache.invalidate_all()
+
+ # lookup should return none
+ self.assertIsNone(cache.get("key1", None))
+ self.assertIsNone(cache.get("key2", None))
+
+ # both callbacks should have been callbacked
+ self.assertTrue(callback_record[0], "Invalidation callback for key1 not called")
+ self.assertTrue(callback_record[1], "Invalidation callback for key2 not called")
+
+ # letting the other lookup complete should do nothing
+ d1.callback("result1")
+ self.assertIsNone(cache.get("key1", None))
+
+ def test_eviction(self):
+ cache = DeferredCache(
+ "test", max_entries=2, apply_cache_factor_from_config=False
+ )
+
+ cache.prefill(1, "one")
+ cache.prefill(2, "two")
+ cache.prefill(3, "three") # 1 will be evicted
+
+ failed = False
+ try:
+ cache.get(1)
+ except KeyError:
+ failed = True
+
+ self.assertTrue(failed)
+
+ cache.get(2)
+ cache.get(3)
+
+ def test_eviction_lru(self):
+ cache = DeferredCache(
+ "test", max_entries=2, apply_cache_factor_from_config=False
+ )
+
+ cache.prefill(1, "one")
+ cache.prefill(2, "two")
+
+ # Now access 1 again, thus causing 2 to be least-recently used
+ cache.get(1)
+
+ cache.prefill(3, "three")
+
+ failed = False
+ try:
+ cache.get(2)
+ except KeyError:
+ failed = True
+
+ self.assertTrue(failed)
+
+ cache.get(1)
+ cache.get(3)
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