Refactor getting replication updates from database. (#7636)

The aim here is to make it easier to reason about when streams are limited and when they're not, by moving the logic into the database functions themselves. This should mean we can kill of `db_query_to_update_function` function.

synapse/handlers/presence.py

@@ -25,7 +25,7 @@ The methods that define policy are:
 import abc
 import logging
 from contextlib import contextmanager
-from typing import Dict, Iterable, List, Set
+from typing import Dict, Iterable, List, Set, Tuple
 
 from prometheus_client import Counter
 from typing_extensions import ContextManager
@@ -773,7 +773,9 @@ class PresenceHandler(BasePresenceHandler):
 
         return False
 
-    async def get_all_presence_updates(self, last_id, current_id, limit):
+    async def get_all_presence_updates(
+        self, instance_name: str, last_id: int, current_id: int, limit: int
+    ) -> Tuple[List[Tuple[int, list]], int, bool]:
         """
         Gets a list of presence update rows from between the given stream ids.
         Each row has:
@@ -785,10 +787,31 @@ class PresenceHandler(BasePresenceHandler):
         - last_user_sync_ts(int)
         - status_msg(int)
         - currently_active(int)
+
+        Args:
+            instance_name: The writer we want to fetch updates from. Unused
+                here since there is only ever one writer.
+            last_id: The token to fetch updates from. Exclusive.
+            current_id: The token to fetch updates up to. Inclusive.
+            limit: The requested limit for the number of rows to return. The
+                function may return more or fewer rows.
+
+        Returns:
+            A tuple consisting of: the updates, a token to use to fetch
+            subsequent updates, and whether we returned fewer rows than exists
+            between the requested tokens due to the limit.
+
+            The token returned can be used in a subsequent call to this
+            function to get further updatees.
+
+            The updates are a list of 2-tuples of stream ID and the row data
         """
+
         # TODO(markjh): replicate the unpersisted changes.
         # This could use the in-memory stores for recent changes.
-        rows = await self.store.get_all_presence_updates(last_id, current_id, limit)
+        rows = await self.store.get_all_presence_updates(
+            instance_name, last_id, current_id, limit
+        )
         return rows
 
     def notify_new_event(self):