From 8a1817f0d29308a233783d43cbf1ad27891120c1 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Mon, 8 Oct 2018 13:26:54 +0100
Subject: [PATCH] Use errback pattern and catch async failures

---
 synapse/handlers/appservice.py |  7 +++++-
 synapse/util/__init__.py       | 43 +++++++++++++++++++++++-----------
 2 files changed, 35 insertions(+), 15 deletions(-)

diff --git a/synapse/handlers/appservice.py b/synapse/handlers/appservice.py
index f0f89af7dc..16b897eb18 100644
--- a/synapse/handlers/appservice.py
+++ b/synapse/handlers/appservice.py
@@ -28,6 +28,7 @@ from synapse.metrics import (
     event_processing_loop_room_count,
 )
 from synapse.metrics.background_process_metrics import run_as_background_process
+from synapse.util import make_log_failure_errback
 from synapse.util.logcontext import make_deferred_yieldable, run_in_background
 from synapse.util.metrics import Measure
 
@@ -112,7 +113,11 @@ class ApplicationServicesHandler(object):
 
                         if not self.started_scheduler:
                             def start_scheduler():
-                                return self.scheduler.start().addErrback(log_failure)
+                                return self.scheduler.start().addErrback(
+                                    make_log_failure_errback(
+                                        "Application Services Failure",
+                                    )
+                                )
                             run_as_background_process("as_scheduler", start_scheduler)
                             self.started_scheduler = True
 
diff --git a/synapse/util/__init__.py b/synapse/util/__init__.py
index c237d003bc..964078aed4 100644
--- a/synapse/util/__init__.py
+++ b/synapse/util/__init__.py
@@ -13,7 +13,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import functools
 import logging
 from itertools import islice
 
@@ -67,9 +66,12 @@ class Clock(object):
             f(function): The function to call repeatedly.
             msec(float): How long to wait between calls in milliseconds.
         """
-        call = task.LoopingCall(_log_exception_wrapper(f))
+        call = task.LoopingCall(f)
         call.clock = self._reactor
-        call.start(msec / 1000.0, now=False)
+        d = call.start(msec / 1000.0, now=False)
+        d.addErrback(make_log_failure_errback(
+            "Looping call died", consumeErrors=False,
+        ))
         return call
 
     def call_later(self, delay, callback, *args, **kwargs):
@@ -112,17 +114,30 @@ def batch_iter(iterable, size):
     return iter(lambda: tuple(islice(sourceiter, size)), ())
 
 
-def _log_exception_wrapper(f):
-    """Used to wrap looping calls to log loudly if they get killed
+def make_log_failure_errback(msg, consumeErrors=True):
+    """Creates a function suitable for passing to `Deferred.addErrback` that
+    logs any failures that occur.
+
+    Args:
+        msg (str): Message to log
+        consumeErrors (bool): If true consumes the failure, otherwise passes
+            on down the callback chain
+
+    Returns:
+        func(Failure)
     """
 
-    @functools.wraps(f)
-    def wrap(*args, **kwargs):
-        try:
-            logger.info("Running looping call")
-            return f(*args, **kwargs)
-        except:  # noqa: E722, as we reraise the exception this is fine.
-            logger.exception("Looping called died")
-            raise
+    def log_failure(failure):
+        logger.error(
+            msg,
+            exc_info=(
+                failure.type,
+                failure.value,
+                failure.getTracebackObject()
+            )
+        )
 
-    return wrap
+        if not consumeErrors:
+            return failure
+
+    return log_failure