From 7639c3d9e53cdb6222df6a8e1b12bc2a40612367 Mon Sep 17 00:00:00 2001
From: Mark Haines <mark.haines@matrix.org>
Date: Tue, 13 Oct 2015 17:13:04 +0100
Subject: [PATCH] Bounce all deferreds through the reactor to make debugging
 easier.

If all deferreds wait a reactor tick before resolving then there is
always a chance to add an errback to the deferred so that stacktraces
get reported, rather than being discarded.
---
 synapse/app/homeserver.py |  2 ++
 synapse/util/debug.py     | 68 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+)
 create mode 100644 synapse/util/debug.py

diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index af53acb369..1c84242aa3 100755
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -33,6 +33,8 @@ if __name__ == '__main__':
         sys.stderr.writelines(message)
         sys.exit(1)
 
+    from synapse.util.debug import debug_deferreds
+    debug_deferreds()
 
 from synapse.storage.engines import create_engine, IncorrectDatabaseSetup
 from synapse.storage import are_all_users_on_domain
diff --git a/synapse/util/debug.py b/synapse/util/debug.py
new file mode 100644
index 0000000000..66ac12c291
--- /dev/null
+++ b/synapse/util/debug.py
@@ -0,0 +1,68 @@
+# -*- coding: utf-8 -*-
+# Copyright 2015 OpenMarket 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, reactor
+from functools import wraps
+from synapse.util.logcontext import LoggingContext, PreserveLoggingContext
+
+def with_logging_context(fn):
+    context = LoggingContext.current_context()
+    def restore_context_callback(x):
+        with PreserveLoggingContext():
+            LoggingContext.thread_local.current_context = context
+            return fn(x)
+    return restore_context_callback
+
+def debug_deferreds():
+    """Cause all deferreds to wait for a reactor tick before running their
+    callbacks. This increases the chance of getting a stack trace out of
+    a defer.inlineCallback since the code waiting on the deferred will get
+    a chance to add an errback before the deferred runs."""
+
+    # We are going to modify the __init__ method of defer.Deferred so we
+    # need to get a copy of the old method so we can still call it.
+    old__init__ = defer.Deferred.__init__
+
+    # We need to create a deferred to bounce the callbacks through the reactor
+    # but we don't want to add a callback when we create that deferred so we
+    # we create a new type of deferred that uses the old __init__ method.
+    # This is safe as long as the old __init__ method doesn't invoke an
+    # __init__ using super.
+    class Bouncer(defer.Deferred):
+        __init__ = old__init__
+
+    # We'll add this as a callback to all Deferreds. Twisted will wait until
+    # the bouncer deferred resolves before calling the callbacks of the
+    # original deferred.
+    def bounce_callback(x):
+        bouncer = Bouncer()
+        reactor.callLater(0, with_logging_context(bouncer.callback), x)
+        return bouncer
+
+    # We'll add this as an errback to all Deferreds. Twisted will wait until
+    # the bouncer deferred resolves before calling the errbacks of the
+    # original deferred.
+    def bounce_errback(x):
+        bouncer = Bouncer()
+        reactor.callLater(0, with_logging_context(bouncer.errback), x)
+        return bouncer
+
+    @wraps(old__init__)
+    def new__init__(self, *args, **kargs):
+        old__init__(self, *args, **kargs)
+        self.addCallbacks(bounce_callback, bounce_errback)
+
+    defer.Deferred.__init__ = new__init__
+