Fix another logcontext leak in _persist_events

We need to run the errback in the sentinel context to avoid losing our own
context.

Also: add logging to runInteraction to help identify where "Starting db
connection from sentinel context" warnings are coming from

1 files changed, 4 insertions, 5 deletions

diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 906a405031..ee9e4d4b65 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -142,15 +142,14 @@ class _EventPeristenceQueue(object):
             try:
                 queue = self._get_drainining_queue(room_id)
                 for item in queue:
-                    # handle_queue_loop runs in the sentinel logcontext, so
-                    # there is no need to preserve_fn when running the
-                    # callbacks on the deferred.
                     try:
                         ret = yield per_item_callback(item)
+                    except Exception:
+                        with PreserveLoggingContext():
+                            item.deferred.errback()
+                    else:
                         with PreserveLoggingContext():
                             item.deferred.callback(ret)
-                    except Exception:
-                        item.deferred.errback()
             finally:
                 queue = self._event_persist_queues.pop(room_id, None)
                 if queue: