diff --git a/tests/util/caches/test_cached_call.py b/tests/util/caches/test_cached_call.py
new file mode 100644
index 0000000000..f349b5ced0
--- /dev/null
+++ b/tests/util/caches/test_cached_call.py
@@ -0,0 +1,161 @@
+# -*- coding: utf-8 -*-
+# Copyright 2021 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.
+from unittest.mock import Mock
+
+from twisted.internet import defer
+from twisted.internet.defer import Deferred
+
+from synapse.util.caches.cached_call import CachedCall, RetryOnExceptionCachedCall
+
+from tests.test_utils import get_awaitable_result
+from tests.unittest import TestCase
+
+
+class CachedCallTestCase(TestCase):
+ def test_get(self):
+ """
+ Happy-path test case: makes a couple of calls and makes sure they behave
+ correctly
+ """
+ d = Deferred()
+
+ async def f():
+ return await d
+
+ slow_call = Mock(side_effect=f)
+
+ cached_call = CachedCall(slow_call)
+
+ # the mock should not yet have been called
+ slow_call.assert_not_called()
+
+ # now fire off a couple of calls
+ completed_results = []
+
+ async def r():
+ res = await cached_call.get()
+ completed_results.append(res)
+
+ r1 = defer.ensureDeferred(r())
+ r2 = defer.ensureDeferred(r())
+
+ # neither result should be complete yet
+ self.assertNoResult(r1)
+ self.assertNoResult(r2)
+
+ # and the mock should have been called *once*, with no params
+ slow_call.assert_called_once_with()
+
+ # allow the deferred to complete, which should complete both the pending results
+ d.callback(123)
+ self.assertEqual(completed_results, [123, 123])
+ self.successResultOf(r1)
+ self.successResultOf(r2)
+
+ # another call to the getter should complete immediately
+ slow_call.reset_mock()
+ r3 = get_awaitable_result(cached_call.get())
+ self.assertEqual(r3, 123)
+ slow_call.assert_not_called()
+
+ def test_fast_call(self):
+ """
+ Test the behaviour when the underlying function completes immediately
+ """
+
+ async def f():
+ return 12
+
+ fast_call = Mock(side_effect=f)
+ cached_call = CachedCall(fast_call)
+
+ # the mock should not yet have been called
+ fast_call.assert_not_called()
+
+ # run the call a couple of times, which should complete immediately
+ self.assertEqual(get_awaitable_result(cached_call.get()), 12)
+ self.assertEqual(get_awaitable_result(cached_call.get()), 12)
+
+ # the mock should have been called once
+ fast_call.assert_called_once_with()
+
+
+class RetryOnExceptionCachedCallTestCase(TestCase):
+ def test_get(self):
+ # set up the RetryOnExceptionCachedCall around a function which will fail
+ # (after a while)
+ d = Deferred()
+
+ async def f1():
+ await d
+ raise ValueError("moo")
+
+ slow_call = Mock(side_effect=f1)
+ cached_call = RetryOnExceptionCachedCall(slow_call)
+
+ # the mock should not yet have been called
+ slow_call.assert_not_called()
+
+ # now fire off a couple of calls
+ completed_results = []
+
+ async def r():
+ try:
+ await cached_call.get()
+ except Exception as e1:
+ completed_results.append(e1)
+
+ r1 = defer.ensureDeferred(r())
+ r2 = defer.ensureDeferred(r())
+
+ # neither result should be complete yet
+ self.assertNoResult(r1)
+ self.assertNoResult(r2)
+
+ # and the mock should have been called *once*, with no params
+ slow_call.assert_called_once_with()
+
+ # complete the deferred, which should make the pending calls fail
+ d.callback(0)
+ self.assertEqual(len(completed_results), 2)
+ for e in completed_results:
+ self.assertIsInstance(e, ValueError)
+ self.assertEqual(e.args, ("moo",))
+
+ # reset the mock to return a successful result, and make another pair of calls
+ # to the getter
+ d = Deferred()
+
+ async def f2():
+ return await d
+
+ slow_call.reset_mock()
+ slow_call.side_effect = f2
+ r3 = defer.ensureDeferred(cached_call.get())
+ r4 = defer.ensureDeferred(cached_call.get())
+
+ self.assertNoResult(r3)
+ self.assertNoResult(r4)
+ slow_call.assert_called_once_with()
+
+ # let that call complete, and check the results
+ d.callback(123)
+ self.assertEqual(self.successResultOf(r3), 123)
+ self.assertEqual(self.successResultOf(r4), 123)
+
+ # and now more calls to the getter should complete immediately
+ slow_call.reset_mock()
+ self.assertEqual(get_awaitable_result(cached_call.get()), 123)
+ slow_call.assert_not_called()
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