1 files changed, 27 insertions, 15 deletions
diff --git a/synapse/storage/events_worker.py b/synapse/storage/events_worker.py
index 79680ee856..5b606bec7c 100644
--- a/synapse/storage/events_worker.py
+++ b/synapse/storage/events_worker.py
@@ -17,6 +17,7 @@ from __future__ import division
 
 import itertools
 import logging
+import operator
 from collections import namedtuple
 
 from canonicaljson import json
@@ -421,28 +422,28 @@ class EventsWorkerStore(SQLBaseStore):
                 The fetch requests. Each entry consists of a list of event
                 ids to be fetched, and a deferred to be completed once the
                 events have been fetched.
+
+                The deferreds are callbacked with a dictionary mapping from event id
+                to event row. Note that it may well contain additional events that
+                were not part of this request.
         """
         with Measure(self._clock, "_fetch_event_list"):
             try:
-                event_id_lists = list(zip(*event_list))[0]
-                event_ids = [item for sublist in event_id_lists for item in sublist]
+                events_to_fetch = set(
+                    event_id for events, _ in event_list for event_id in events
+                )
 
                 row_dict = self._new_transaction(
-                    conn, "do_fetch", [], [], self._fetch_event_rows, event_ids
+                    conn, "do_fetch", [], [], self._fetch_event_rows, events_to_fetch
                 )
 
                 # We only want to resolve deferreds from the main thread
-                def fire(lst, res):
-                    for ids, d in lst:
-                        if not d.called:
-                            try:
-                                with PreserveLoggingContext():
-                                    d.callback([res[i] for i in ids if i in res])
-                            except Exception:
-                                logger.exception("Failed to callback")
+                def fire():
+                    for _, d in event_list:
+                        d.callback(row_dict)
 
                 with PreserveLoggingContext():
-                    self.hs.get_reactor().callFromThread(fire, event_list, row_dict)
+                    self.hs.get_reactor().callFromThread(fire)
             except Exception as e:
                 logger.exception("do_fetch")
 
@@ -461,6 +462,12 @@ class EventsWorkerStore(SQLBaseStore):
         """Fetches events from the database using the _event_fetch_list. This
         allows batch and bulk fetching of events - it allows us to fetch events
         without having to create a new transaction for each request for events.
+
+        Args:
+            events (Iterable[str]): events to be fetched.
+
+        Returns:
+            Deferred[Dict[str, _EventCacheEntry]]: map from event id to result.
         """
         if not events:
             return {}
@@ -484,11 +491,16 @@ class EventsWorkerStore(SQLBaseStore):
 
         logger.debug("Loading %d events", len(events))
         with PreserveLoggingContext():
-            rows = yield events_d
-        logger.debug("Loaded %d events (%d rows)", len(events), len(rows))
+            row_map = yield events_d
+        logger.debug("Loaded %d events (%d rows)", len(events), len(row_map))
+
+        rows = (row_map.get(event_id) for event_id in events)
+
+        # filter out absent rows
+        rows = filter(operator.truth, rows)
 
         if not allow_rejected:
-            rows[:] = [r for r in rows if r["rejected_reason"] is None]
+            rows = (r for r in rows if r["rejected_reason"] is None)
 
         res = yield make_deferred_yieldable(
             defer.gatherResults(