Reduce event lookups during room creation by passing known event IDs (#13210)

Inspired by the room batch handler, this uses previous event inserts to
pre-populate prev events during room creation, reducing the number of
queries required to create a room.

Signed off by Nick @ Beeper (@Fizzadar)

changelog.d/13210.misc

@@ -0,0 +1 @@
+Reduce number of events queried during room creation. Contributed by Nick @ Beeper (@fizzadar).

synapse/handlers/room.py

@@ -1019,6 +1019,8 @@ class RoomCreationHandler:
 
         event_keys = {"room_id": room_id, "sender": creator_id, "state_key": ""}
 
+        last_sent_event_id: Optional[str] = None
+
         def create(etype: str, content: JsonDict, **kwargs: Any) -> JsonDict:
             e = {"type": etype, "content": content}
 
@@ -1028,19 +1030,27 @@ class RoomCreationHandler:
             return e
 
         async def send(etype: str, content: JsonDict, **kwargs: Any) -> int:
+            nonlocal last_sent_event_id
+
             event = create(etype, content, **kwargs)
             logger.debug("Sending %s in new room", etype)
             # Allow these events to be sent even if the user is shadow-banned to
             # allow the room creation to complete.
             (
-                _,
+                sent_event,
                 last_stream_id,
             ) = await self.event_creation_handler.create_and_send_nonmember_event(
                 creator,
                 event,
                 ratelimit=False,
                 ignore_shadow_ban=True,
+                # Note: we don't pass state_event_ids here because this triggers
+                # an additional query per event to look them up from the events table.
+                prev_event_ids=[last_sent_event_id] if last_sent_event_id else [],
             )
+
+            last_sent_event_id = sent_event.event_id
+
             return last_stream_id
 
         try:
@@ -1054,7 +1064,9 @@ class RoomCreationHandler:
         await send(etype=EventTypes.Create, content=creation_content)
 
         logger.debug("Sending %s in new room", EventTypes.Member)
-        await self.room_member_handler.update_membership(
+        # Room create event must exist at this point
+        assert last_sent_event_id is not None
+        member_event_id, _ = await self.room_member_handler.update_membership(
             creator,
             creator.user,
             room_id,
@@ -1062,7 +1074,9 @@ class RoomCreationHandler:
             ratelimit=ratelimit,
             content=creator_join_profile,
             new_room=True,
+            prev_event_ids=[last_sent_event_id],
         )
+        last_sent_event_id = member_event_id
 
         # We treat the power levels override specially as this needs to be one
         # of the first events that get sent into a room.

tests/rest/client/test_rooms.py

@@ -708,6 +708,21 @@ class RoomsCreateTestCase(RoomBase):
 
         self.assertEqual(200, channel.code, channel.result)
         self.assertTrue("room_id" in channel.json_body)
+        assert channel.resource_usage is not None
+        self.assertEqual(33, channel.resource_usage.db_txn_count)
+
+    def test_post_room_initial_state(self) -> None:
+        # POST with initial_state config key, expect new room id
+        channel = self.make_request(
+            "POST",
+            "/createRoom",
+            b'{"initial_state":[{"type": "m.bridge", "content": {}}]}',
+        )
+
+        self.assertEqual(200, channel.code, channel.result)
+        self.assertTrue("room_id" in channel.json_body)
+        assert channel.resource_usage is not None
+        self.assertEqual(37, channel.resource_usage.db_txn_count)
 
     def test_post_room_visibility_key(self) -> None:
         # POST with visibility config key, expect new room id