Fix logcontext leak in HttpTransactionCache

ONE DAY I WILL PURGE THE WORLD OF THIS EVIL

tests/rest/client/test_transactions.py

@@ -3,6 +3,7 @@ from synapse.rest.client.transactions import CLEANUP_PERIOD_MS
 from twisted.internet import defer
 from mock import Mock, call
 
+from synapse.util import async
 from synapse.util.logcontext import LoggingContext
 from tests import unittest
 from tests.utils import MockClock
@@ -41,6 +42,26 @@ class HttpTransactionCacheTestCase(unittest.TestCase):
         # expect only a single call to do the work
         cb.assert_called_once_with("some_arg", keyword="arg", changing_args=0)
 
+    @defer.inlineCallbacks
+    def test_logcontexts_with_async_result(self):
+        @defer.inlineCallbacks
+        def cb():
+            yield async.sleep(0)
+            defer.returnValue("yay")
+
+        @defer.inlineCallbacks
+        def test():
+            with LoggingContext("c") as c1:
+                res = yield self.cache.fetch_or_execute(self.mock_key, cb)
+                self.assertIs(LoggingContext.current_context(), c1)
+                self.assertEqual(res, "yay")
+
+        # run the test twice in parallel
+        d = defer.gatherResults([test(), test()])
+        self.assertIs(LoggingContext.current_context(), LoggingContext.sentinel)
+        yield d
+        self.assertIs(LoggingContext.current_context(), LoggingContext.sentinel)
+
     @defer.inlineCallbacks
     def test_does_not_cache_exceptions(self):
         """Checks that, if the callback throws an exception, it is called again