Limit amount of replication we send (#17358)

Fixes up #17333, where we failed to actually send less data (the
`DISTINCT` didn't work due to `stream_id` being different).

We fix this by making it so that every device list outbound poke for a
given user ID has the same stream ID. We can't change the query to only
return e.g. max stream ID as the receivers look up the destinations to
send to by doing `SELECT WHERE stream_id = ?`

Author: erikjohnston

changelog.d/17358.misc

@@ -0,0 +1 @@
+Handle device lists notifications for large accounts more efficiently in worker mode.

synapse/storage/databases/main/devices.py

@@ -2131,19 +2131,18 @@ def _add_device_outbound_poke_to_stream_txn(
         user_id: str,
         device_id: str,
         hosts: Collection[str],
-        stream_ids: List[int],
+        stream_id: int,
         context: Optional[Dict[str, str]],
     ) -> None:
         if self._device_list_federation_stream_cache:
             for host in hosts:
                 txn.call_after(
                     self._device_list_federation_stream_cache.entity_has_changed,
                     host,
-                    stream_ids[-1],
+                    stream_id,
                 )
 
         now = self._clock.time_msec()
-        stream_id_iterator = iter(stream_ids)
 
         encoded_context = json_encoder.encode(context)
         mark_sent = not self.hs.is_mine_id(user_id)
@@ -2152,7 +2151,7 @@ def _add_device_outbound_poke_to_stream_txn(
             (
                 destination,
                 self._instance_name,
-                next(stream_id_iterator),
+                stream_id,
                 user_id,
                 device_id,
                 mark_sent,
@@ -2337,22 +2336,22 @@ async def add_device_list_outbound_pokes(
             return
 
         def add_device_list_outbound_pokes_txn(
-            txn: LoggingTransaction, stream_ids: List[int]
+            txn: LoggingTransaction, stream_id: int
         ) -> None:
             self._add_device_outbound_poke_to_stream_txn(
                 txn,
                 user_id=user_id,
                 device_id=device_id,
                 hosts=hosts,
-                stream_ids=stream_ids,
+                stream_id=stream_id,
                 context=context,
             )
 
-        async with self._device_list_id_gen.get_next_mult(len(hosts)) as stream_ids:
+        async with self._device_list_id_gen.get_next() as stream_id:
             return await self.db_pool.runInteraction(
                 "add_device_list_outbound_pokes",
                 add_device_list_outbound_pokes_txn,
-                stream_ids,
+                stream_id,
             )
 
     async def add_remote_device_list_to_pending(