Check external event cache within deferred

Still maintains local in memory lookup optimisation, but does any external
lookup as part of the deferred that prevents duplicate lookups for the same
event at once. This makes the assumption that fetching from an external
cache is a non-zero load operation.

synapse/storage/databases/main/events_worker.py

@@ -600,7 +600,10 @@ async def _get_events_from_cache_or_db(
         Returns:
             map from event id to result
         """
-        event_entry_map = await self._get_events_from_cache(
+        # Shortcut: check if we have any events in the *in memory* cache - this function
+        # may be called repeatedly for the same event so at this point we cannot reach
+        # out to any external cache for performance reasons.
+        event_entry_map = self._get_events_from_local_cache(
             event_ids,
         )
 
@@ -632,7 +635,7 @@ async def _get_events_from_cache_or_db(
 
         if missing_events_ids:
 
-            async def get_missing_events_from_db() -> Dict[str, EventCacheEntry]:
+            async def get_missing_events_from_cache_or_db() -> Dict[str, EventCacheEntry]:
                 """Fetches the events in `missing_event_ids` from the database.
 
                 Also creates entries in `self._current_event_fetches` to allow
@@ -657,10 +660,18 @@ async def get_missing_events_from_db() -> Dict[str, EventCacheEntry]:
                 # the events have been redacted, and if so pulling the redaction event
                 # out of the database to check it.
                 #
+                missing_events = {}
                 try:
-                    missing_events = await self._get_events_from_db(
+                    # First fetch from the cache - including any external caches
+                    cache_missing_events = await self._get_events_from_cache(
                         missing_events_ids,
                     )
+                    missing_events.update(cache_missing_events)
+                    # Now actually fetch any remaining events from the DB
+                    db_missing_events = await self._get_events_from_db(
+                        missing_events_ids - set(cache_missing_events.keys()),
+                    )
+                    missing_events.update(db_missing_events)
                 except Exception as e:
                     with PreserveLoggingContext():
                         fetching_deferred.errback(e)
@@ -679,7 +690,7 @@ async def get_missing_events_from_db() -> Dict[str, EventCacheEntry]:
             # cancellations, since multiple `_get_events_from_cache_or_db` calls can
             # reuse the same fetch.
             missing_events: Dict[str, EventCacheEntry] = await delay_cancellation(
-                get_missing_events_from_db()
+                get_missing_events_from_cache_or_db()
             )
             event_entry_map.update(missing_events)