Additional test for cachedList (#11246)

I was trying to understand how `cachedList` works, and ended up writing this
extra test. I figure we may as well keep it.

changelog.d/11246.misc

@@ -0,0 +1 @@
+Add an additional test for the `cachedList` method decorator.

tests/util/caches/test_descriptors.py

@@ -17,6 +17,7 @@ from typing import Set
 from unittest import mock
 
 from twisted.internet import defer, reactor
+from twisted.internet.defer import Deferred
 
 from synapse.api.errors import SynapseError
 from synapse.logging.context import (
@@ -703,6 +704,48 @@ class CachedListDescriptorTestCase(unittest.TestCase):
             obj.mock.assert_called_once_with((40,), 2)
             self.assertEqual(r, {10: "fish", 40: "gravy"})
 
+    def test_concurrent_lookups(self):
+        """All concurrent lookups should get the same result"""
+
+        class Cls:
+            def __init__(self):
+                self.mock = mock.Mock()
+
+            @descriptors.cached()
+            def fn(self, arg1):
+                pass
+
+            @descriptors.cachedList("fn", "args1")
+            def list_fn(self, args1) -> "Deferred[dict]":
+                return self.mock(args1)
+
+        obj = Cls()
+        deferred_result = Deferred()
+        obj.mock.return_value = deferred_result
+
+        # start off several concurrent lookups of the same key
+        d1 = obj.list_fn([10])
+        d2 = obj.list_fn([10])
+        d3 = obj.list_fn([10])
+
+        # the mock should have been called exactly once
+        obj.mock.assert_called_once_with((10,))
+        obj.mock.reset_mock()
+
+        # ... and none of the calls should yet be complete
+        self.assertFalse(d1.called)
+        self.assertFalse(d2.called)
+        self.assertFalse(d3.called)
+
+        # complete the lookup. @cachedList functions need to complete with a map
+        # of input->result
+        deferred_result.callback({10: "peas"})
+
+        # ... which should give the right result to all the callers
+        self.assertEqual(self.successResultOf(d1), {10: "peas"})
+        self.assertEqual(self.successResultOf(d2), {10: "peas"})
+        self.assertEqual(self.successResultOf(d3), {10: "peas"})
+
     @defer.inlineCallbacks
     def test_invalidate(self):
         """Make sure that invalidation callbacks are called."""