Refactor recursive code so we can wrap just the redaction part

See #13489 (comment)

synapse/storage/databases/main/events_worker.py

@@ -54,6 +54,7 @@
     current_context,
     make_deferred_yieldable,
 )
+from synapse.logging.opentracing import start_active_span, tag_args, trace
 from synapse.metrics.background_process_metrics import (
     run_as_background_process,
     wrap_as_background_process,
@@ -430,6 +431,8 @@ async def get_events(
 
         return {e.event_id: e for e in events}
 
+    @trace
+    @tag_args
     async def get_events_as_list(
         self,
         event_ids: Collection[str],
@@ -1090,15 +1093,11 @@ async def _get_events_from_db(
         """
         fetched_event_ids: Set[str] = set()
         fetched_events: Dict[str, _EventRow] = {}
-        events_to_fetch = event_ids
-
-        while events_to_fetch:
-            row_map = await self._enqueue_events(events_to_fetch)
 
+        async def _recursively_fetch_redactions(row_map: Dict[str, _EventRow]) -> None:
             # we need to recursively fetch any redactions of those events
             redaction_ids: Set[str] = set()
-            for event_id in events_to_fetch:
-                row = row_map.get(event_id)
+            for event_id, row in row_map.items():
                 fetched_event_ids.add(event_id)
                 if row:
                     fetched_events[event_id] = row
@@ -1107,6 +1106,14 @@ async def _get_events_from_db(
             events_to_fetch = redaction_ids.difference(fetched_event_ids)
             if events_to_fetch:
                 logger.debug("Also fetching redaction events %s", events_to_fetch)
+                row_map = await self._enqueue_events(events_to_fetch)
+                await _recursively_fetch_redactions(row_map)
+
+        events_to_fetch = event_ids
+        row_map = await self._enqueue_events(events_to_fetch)
+
+        with start_active_span("recursively fetching redactions"):
+            await _recursively_fetch_redactions(row_map)
 
         # build a map from event_id to EventBase
         event_map: Dict[str, EventBase] = {}
@@ -1424,6 +1431,8 @@ async def have_events_in_timeline(self, event_ids: Iterable[str]) -> Set[str]:
 
         return {r["event_id"] for r in rows}
 
+    @trace
+    @tag_args
     async def have_seen_events(
         self, room_id: str, event_ids: Iterable[str]
     ) -> Set[str]: