Parallelize calls to fetch bundled aggregations. (#14510)

The bundled aggregations for annotations, references, and edits
can be parallelized.

changelog.d/14510.feature

@@ -0,0 +1 @@
+Reduce database load of [Client-Server endpoints](https://spec.matrix.org/v1.4/client-server-api/#aggregations) which return bundled aggregations.

synapse/handlers/relations.py

@@ -20,10 +20,12 @@ import attr
 from synapse.api.constants import EventTypes, RelationTypes
 from synapse.api.errors import SynapseError
 from synapse.events import EventBase, relation_from_event
+from synapse.logging.context import make_deferred_yieldable, run_in_background
 from synapse.logging.opentracing import trace
 from synapse.storage.databases.main.relations import ThreadsNextBatch, _RelatedEvent
 from synapse.streams.config import PaginationConfig
 from synapse.types import JsonDict, Requester, UserID
+from synapse.util.async_helpers import gather_results
 from synapse.visibility import filter_events_for_client
 
 if TYPE_CHECKING:
@@ -525,7 +527,8 @@ class RelationsHandler:
                 # (as that is what makes it part of the thread).
                 relations_by_id[latest_thread_event.event_id] = RelationTypes.THREAD
 
-        # Fetch any annotations (ie, reactions) to bundle with this event.
+        async def _fetch_annotations() -> None:
+            """Fetch any annotations (ie, reactions) to bundle with this event."""
             annotations_by_event_id = await self.get_annotations_for_events(
                 events_by_id.keys(), ignored_users=ignored_users
             )
@@ -535,7 +538,8 @@ class RelationsHandler:
                         "chunk": annotations
                     }
 
-        # Fetch any references to bundle with this event.
+        async def _fetch_references() -> None:
+            """Fetch any references to bundle with this event."""
             references_by_event_id = await self.get_references_for_events(
                 events_by_id.keys(), ignored_users=ignored_users
             )
@@ -545,10 +549,13 @@ class RelationsHandler:
                         "chunk": [{"event_id": ev.event_id} for ev in references]
                     }
 
-        # Fetch any edits (but not for redacted events).
-        #
-        # Note that there is no use in limiting edits by ignored users since the
-        # parent event should be ignored in the first place if the user is ignored.
+        async def _fetch_edits() -> None:
+            """
+            Fetch any edits (but not for redacted events).
+
+            Note that there is no use in limiting edits by ignored users since the
+            parent event should be ignored in the first place if the user is ignored.
+            """
             edits = await self._main_store.get_applicable_edits(
                 [
                     event_id
@@ -559,6 +566,18 @@ class RelationsHandler:
             for event_id, edit in edits.items():
                 results.setdefault(event_id, BundledAggregations()).replace = edit
 
+        # Parallelize the calls for annotations, references, and edits since they
+        # are unrelated.
+        await make_deferred_yieldable(
+            gather_results(
+                (
+                    run_in_background(_fetch_annotations),
+                    run_in_background(_fetch_references),
+                    run_in_background(_fetch_edits),
+                )
+            )
+        )
+
         return results
 
     async def get_threads(