From 6c48aa0256ce7dfd62e72a77b03f5bb2222473ba Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 19 Sep 2018 11:04:42 +0100
Subject: [PATCH] Run canceller first to allow it to generate correct error

---
 synapse/util/async_helpers.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
index 2e12bcda9..2b2e85ecb 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -421,11 +421,14 @@ def timeout_deferred(deferred, timeout, reactor, on_timeout_cancel=None):
     def time_it_out():
         timed_out[0] = True
 
+        try:
+            deferred.cancel()
+        except:   # noqa: E722, if we throw any exception it'll break time outs
+            logger.exception("Canceller failed during timeout")
+
         if not new_d.called:
             new_d.errback(DeferredTimeoutError(timeout, "Deferred"))
 
-        deferred.cancel()
-
     delayed_call = reactor.callLater(timeout, time_it_out)
 
     def convert_cancelled(value):