Stop preserve_fn leaking context into the reactor

Fix a bug in ``logcontext.preserve_fn`` which made it leak context into the
reactor, and add a test for it.

Also, get rid of ``logcontext.reset_context_after_deferred``, which tried to do
the same thing but had its own, different, set of bugs.

tests/util/test_log_context.py

@@ -1,8 +1,10 @@
+import twisted.python.failure
 from twisted.internet import defer
 from twisted.internet import reactor
 from .. import unittest
 
 from synapse.util.async import sleep
+from synapse.util import logcontext
 from synapse.util.logcontext import LoggingContext
 
 
@@ -33,3 +35,62 @@ class LoggingContextTestCase(unittest.TestCase):
             context_one.test_key = "one"
             yield sleep(0)
             self._check_test_key("one")
+
+    def _test_preserve_fn(self, function):
+        sentinel_context = LoggingContext.current_context()
+
+        callback_completed = [False]
+
+        @defer.inlineCallbacks
+        def cb():
+            context_one.test_key = "one"
+            yield function()
+            self._check_test_key("one")
+
+            callback_completed[0] = True
+
+        with LoggingContext() as context_one:
+            context_one.test_key = "one"
+
+            # fire off function, but don't wait on it.
+            logcontext.preserve_fn(cb)()
+
+            self._check_test_key("one")
+
+        # now wait for the function under test to have run, and check that
+        # the logcontext is left in a sane state.
+        d2 = defer.Deferred()
+
+        def check_logcontext():
+            if not callback_completed[0]:
+                reactor.callLater(0.01, check_logcontext)
+                return
+
+            # make sure that the context was reset before it got thrown back
+            # into the reactor
+            try:
+                self.assertIs(LoggingContext.current_context(),
+                              sentinel_context)
+                d2.callback(None)
+            except BaseException:
+                d2.errback(twisted.python.failure.Failure())
+
+        reactor.callLater(0.01, check_logcontext)
+
+        # test is done once d2 finishes
+        return d2
+
+    def test_preserve_fn_with_blocking_fn(self):
+        @defer.inlineCallbacks
+        def blocking_function():
+            yield sleep(0)
+
+        return self._test_preserve_fn(blocking_function)
+
+    def test_preserve_fn_with_non_blocking_fn(self):
+        @defer.inlineCallbacks
+        def nonblocking_function():
+            with logcontext.PreserveLoggingContext():
+                yield defer.succeed(None)
+
+        return self._test_preserve_fn(nonblocking_function)