diff --git a/tests/util/test_async_helpers.py b/tests/util/test_async_helpers.py
new file mode 100644
index 0000000000..ab89cab812
--- /dev/null
+++ b/tests/util/test_async_helpers.py
@@ -0,0 +1,173 @@
+# Copyright 2019 New Vector Ltd
+#
+# 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 twisted.internet import defer
+from twisted.internet.defer import CancelledError, Deferred
+from twisted.internet.task import Clock
+
+from synapse.logging.context import (
+ SENTINEL_CONTEXT,
+ LoggingContext,
+ PreserveLoggingContext,
+ current_context,
+)
+from synapse.util.async_helpers import ObservableDeferred, timeout_deferred
+
+from tests.unittest import TestCase
+
+
+class ObservableDeferredTest(TestCase):
+ def test_succeed(self):
+ origin_d = Deferred()
+ observable = ObservableDeferred(origin_d)
+
+ observer1 = observable.observe()
+ observer2 = observable.observe()
+
+ self.assertFalse(observer1.called)
+ self.assertFalse(observer2.called)
+
+ # check the first observer is called first
+ def check_called_first(res):
+ self.assertFalse(observer2.called)
+ return res
+
+ observer1.addBoth(check_called_first)
+
+ # store the results
+ results = [None, None]
+
+ def check_val(res, idx):
+ results[idx] = res
+ return res
+
+ observer1.addCallback(check_val, 0)
+ observer2.addCallback(check_val, 1)
+
+ origin_d.callback(123)
+ self.assertEqual(results[0], 123, "observer 1 callback result")
+ self.assertEqual(results[1], 123, "observer 2 callback result")
+
+ def test_failure(self):
+ origin_d = Deferred()
+ observable = ObservableDeferred(origin_d, consumeErrors=True)
+
+ observer1 = observable.observe()
+ observer2 = observable.observe()
+
+ self.assertFalse(observer1.called)
+ self.assertFalse(observer2.called)
+
+ # check the first observer is called first
+ def check_called_first(res):
+ self.assertFalse(observer2.called)
+ return res
+
+ observer1.addBoth(check_called_first)
+
+ # store the results
+ results = [None, None]
+
+ def check_val(res, idx):
+ results[idx] = res
+ return None
+
+ observer1.addErrback(check_val, 0)
+ observer2.addErrback(check_val, 1)
+
+ try:
+ raise Exception("gah!")
+ except Exception as e:
+ origin_d.errback(e)
+ self.assertEqual(str(results[0].value), "gah!", "observer 1 errback result")
+ self.assertEqual(str(results[1].value), "gah!", "observer 2 errback result")
+
+
+class TimeoutDeferredTest(TestCase):
+ def setUp(self):
+ self.clock = Clock()
+
+ def test_times_out(self):
+ """Basic test case that checks that the original deferred is cancelled and that
+ the timing-out deferred is errbacked
+ """
+ cancelled = [False]
+
+ def canceller(_d):
+ cancelled[0] = True
+
+ non_completing_d = Deferred(canceller)
+ timing_out_d = timeout_deferred(non_completing_d, 1.0, self.clock)
+
+ self.assertNoResult(timing_out_d)
+ self.assertFalse(cancelled[0], "deferred was cancelled prematurely")
+
+ self.clock.pump((1.0,))
+
+ self.assertTrue(cancelled[0], "deferred was not cancelled by timeout")
+ self.failureResultOf(timing_out_d, defer.TimeoutError)
+
+ def test_times_out_when_canceller_throws(self):
+ """Test that we have successfully worked around
+ https://twistedmatrix.com/trac/ticket/9534"""
+
+ def canceller(_d):
+ raise Exception("can't cancel this deferred")
+
+ non_completing_d = Deferred(canceller)
+ timing_out_d = timeout_deferred(non_completing_d, 1.0, self.clock)
+
+ self.assertNoResult(timing_out_d)
+
+ self.clock.pump((1.0,))
+
+ self.failureResultOf(timing_out_d, defer.TimeoutError)
+
+ def test_logcontext_is_preserved_on_cancellation(self):
+ blocking_was_cancelled = [False]
+
+ @defer.inlineCallbacks
+ def blocking():
+ non_completing_d = Deferred()
+ with PreserveLoggingContext():
+ try:
+ yield non_completing_d
+ except CancelledError:
+ blocking_was_cancelled[0] = True
+ raise
+
+ with LoggingContext("one") as context_one:
+ # the errbacks should be run in the test logcontext
+ def errback(res, deferred_name):
+ self.assertIs(
+ current_context(),
+ context_one,
+ "errback %s run in unexpected logcontext %s"
+ % (deferred_name, current_context()),
+ )
+ return res
+
+ original_deferred = blocking()
+ original_deferred.addErrback(errback, "orig")
+ timing_out_d = timeout_deferred(original_deferred, 1.0, self.clock)
+ self.assertNoResult(timing_out_d)
+ self.assertIs(current_context(), SENTINEL_CONTEXT)
+ timing_out_d.addErrback(errback, "timingout")
+
+ self.clock.pump((1.0,))
+
+ self.assertTrue(
+ blocking_was_cancelled[0], "non-completing deferred was not cancelled"
+ )
+ self.failureResultOf(timing_out_d, defer.TimeoutError)
+ self.assertIs(current_context(), context_one)
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