From f30c4ed2bc2255dc7182bd026fb6437afec735a5 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Wed, 11 Oct 2017 17:26:17 +0100
Subject: [PATCH] logformatter: fix AttributeError

make sure we have the relevant fields before we try to log them.
---
 synapse/util/logformatter.py | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/synapse/util/logformatter.py b/synapse/util/logformatter.py
index 60504162e..cdbc4bffd 100644
--- a/synapse/util/logformatter.py
+++ b/synapse/util/logformatter.py
@@ -33,9 +33,17 @@ class LogFormatter(logging.Formatter):
 
     def formatException(self, ei):
         sio = StringIO.StringIO()
-        sio.write("Capture point (most recent call last):\n")
-        traceback.print_stack(ei[2].tb_frame.f_back, None, sio)
-        traceback.print_exception(ei[0], ei[1], ei[2], None, sio)
+        (typ, val, tb) = ei
+
+        # log the stack above the exception capture point if possible, but
+        # check that we actually have an f_back attribute to work around
+        # https://twistedmatrix.com/trac/ticket/9305
+
+        if tb and hasattr(tb.tb_frame, 'f_back'):
+            sio.write("Capture point (most recent call last):\n")
+            traceback.print_stack(tb.tb_frame.f_back, None, sio)
+
+        traceback.print_exception(typ, val, tb, None, sio)
         s = sio.getvalue()
         sio.close()
         if s[-1:] == "\n":