diff --git a/tests/util/caches/test_descriptors.py b/tests/util/caches/test_descriptors.py
index fc2663c02d..2ad08f541b 100644
--- a/tests/util/caches/test_descriptors.py
+++ b/tests/util/caches/test_descriptors.py
@@ -14,6 +14,7 @@
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
+from typing import Set
import mock
@@ -130,6 +131,57 @@ class DescriptorTestCase(unittest.TestCase):
d = obj.fn(1)
self.failureResultOf(d, SynapseError)
+ def test_cache_with_async_exception(self):
+ """The wrapped function returns a failure
+ """
+
+ class Cls:
+ result = None
+ call_count = 0
+
+ @cached()
+ def fn(self, arg1):
+ self.call_count += 1
+ return self.result
+
+ obj = Cls()
+ callbacks = set() # type: Set[str]
+
+ # set off an asynchronous request
+ obj.result = origin_d = defer.Deferred()
+
+ d1 = obj.fn(1, on_invalidate=lambda: callbacks.add("d1"))
+ self.assertFalse(d1.called)
+
+ # a second request should also return a deferred, but should not call the
+ # function itself.
+ d2 = obj.fn(1, on_invalidate=lambda: callbacks.add("d2"))
+ self.assertFalse(d2.called)
+ self.assertEqual(obj.call_count, 1)
+
+ # no callbacks yet
+ self.assertEqual(callbacks, set())
+
+ # the original request fails
+ e = Exception("bzz")
+ origin_d.errback(e)
+
+ # ... which should cause the lookups to fail similarly
+ self.assertIs(self.failureResultOf(d1, Exception).value, e)
+ self.assertIs(self.failureResultOf(d2, Exception).value, e)
+
+ # ... and the callbacks to have been, uh, called.
+ self.assertEqual(callbacks, {"d1", "d2"})
+
+ # ... leaving the cache empty
+ self.assertEqual(len(obj.fn.cache.cache), 0)
+
+ # and a second call should work as normal
+ obj.result = defer.succeed(100)
+ d3 = obj.fn(1)
+ self.assertEqual(self.successResultOf(d3), 100)
+ self.assertEqual(obj.call_count, 2)
+
def test_cache_logcontexts(self):
"""Check that logcontexts are set and restored correctly when
using the cache."""
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