Improve exception handling for background processes

There were a bunch of places where we fire off a process to happen in the
background, but don't have any exception handling on it - instead relying on
the unhandled error being logged when the relevent deferred gets
garbage-collected.

This is unsatisfactory for a number of reasons:
 - logging on garbage collection is best-effort and may happen some time after
   the error, if at all
 - it can be hard to figure out where the error actually happened.
 - it is logged as a scary CRITICAL error which (a) I always forget to grep for
   and (b) it's not really CRITICAL if a background process we don't care about
   fails.

So this is an attempt to add exception handling to everything we fire off into
the background.

1 files changed, 17 insertions, 14 deletions

diff --git a/synapse/app/pusher.py b/synapse/app/pusher.py
index d5c3a85195..739d113ad5 100644
--- a/synapse/app/pusher.py
+++ b/synapse/app/pusher.py
@@ -144,20 +144,23 @@ class PusherReplicationHandler(ReplicationClientHandler):
 
     @defer.inlineCallbacks
     def poke_pushers(self, stream_name, token, rows):
-        if stream_name == "pushers":
-            for row in rows:
-                if row.deleted:
-                    yield self.stop_pusher(row.user_id, row.app_id, row.pushkey)
-                else:
-                    yield self.start_pusher(row.user_id, row.app_id, row.pushkey)
-        elif stream_name == "events":
-            yield self.pusher_pool.on_new_notifications(
-                token, token,
-            )
-        elif stream_name == "receipts":
-            yield self.pusher_pool.on_new_receipts(
-                token, token, set(row.room_id for row in rows)
-            )
+        try:
+            if stream_name == "pushers":
+                for row in rows:
+                    if row.deleted:
+                        yield self.stop_pusher(row.user_id, row.app_id, row.pushkey)
+                    else:
+                        yield self.start_pusher(row.user_id, row.app_id, row.pushkey)
+            elif stream_name == "events":
+                yield self.pusher_pool.on_new_notifications(
+                    token, token,
+                )
+            elif stream_name == "receipts":
+                yield self.pusher_pool.on_new_receipts(
+                    token, token, set(row.room_id for row in rows)
+                )
+        except Exception:
+            logger.exception("Error poking pushers")
 
     def stop_pusher(self, user_id, app_id, pushkey):
         key = "%s:%s" % (app_id, pushkey)