From eb8c841e14e28a21a99599aa0a2190dbf5b29334 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Thu, 4 Nov 2021 12:31:40 +0000
Subject: [PATCH 1/2] Additional test for `cachedList`

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                |  1 +
 tests/util/caches/test_descriptors.py | 43 +++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 changelog.d/11246.misc

diff --git a/changelog.d/11246.misc b/changelog.d/11246.misc
new file mode 100644
index 000000000000..e5e912c1b0d6
--- /dev/null
+++ b/changelog.d/11246.misc
@@ -0,0 +1 @@
+Add an additional test for the `cachedList` method decorator.
diff --git a/tests/util/caches/test_descriptors.py b/tests/util/caches/test_descriptors.py
index 39947a166b9b..dc747e0220c9 100644
--- a/tests/util/caches/test_descriptors.py
+++ b/tests/util/caches/test_descriptors.py
@@ -17,6 +17,7 @@
 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 @@ async def list_fn(self, args1, arg2):
             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."""

From 6ba5af5768466fe07f41bfe7b1b08b5a4d140e7a Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Thu, 4 Nov 2021 13:12:57 +0000
Subject: [PATCH 2/2] fix olddeps

---
 tests/util/caches/test_descriptors.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/util/caches/test_descriptors.py b/tests/util/caches/test_descriptors.py
index dc747e0220c9..ced3efd93f8b 100644
--- a/tests/util/caches/test_descriptors.py
+++ b/tests/util/caches/test_descriptors.py
@@ -716,7 +716,7 @@ def fn(self, arg1):
                 pass
 
             @descriptors.cachedList("fn", "args1")
-            def list_fn(self, args1) -> Deferred[dict]:
+            def list_fn(self, args1) -> "Deferred[dict]":
                 return self.mock(args1)
 
         obj = Cls()