Sliding Sync: Use `stream_ordering` based timeline pagination for incremental sync (#17510)

Use `stream_ordering` based `timeline` pagination for incremental
`/sync` in Sliding Sync. Previously, we were always using a
`topological_ordering` but we should only be using that for historical
scenarios (initial `/sync`, newly joined, or haven't sent the room down
the connection before).

This is slightly different than what the [spec
suggests](https://spec.matrix.org/v1.10/client-server-api/#syncing)

> Events are ordered in this API according to the arrival time of the
event on the homeserver. This can conflict with other APIs which order
events based on their partial ordering in the event graph. This can
result in duplicate events being received (once per distinct API
called). Clients SHOULD de-duplicate events based on the event ID when
this happens.

But we've had a [discussion below in this
PR](https://github.com/element-hq/synapse/pull/17510#discussion_r1699105569)
and this matches what Sync v2 already does and seems like it makes
sense. Created a spec issue
https://github.com/matrix-org/matrix-spec/issues/1917 to clarify this.

Related issues:

 - https://github.com/matrix-org/matrix-spec/issues/1917
 - https://github.com/matrix-org/matrix-spec/issues/852
 - https://github.com/matrix-org/matrix-spec-proposals/pull/4033

1 files changed, 18 insertions, 14 deletions

diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index 872c85fbad..6fd7afa280 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -507,13 +507,15 @@ class PaginationHandler:
 
         # Initially fetch the events from the database. With any luck, we can return
         # these without blocking on backfill (handled below).
-        events, next_key = await self.store.paginate_room_events(
-            room_id=room_id,
-            from_key=from_token.room_key,
-            to_key=to_room_key,
-            direction=pagin_config.direction,
-            limit=pagin_config.limit,
-            event_filter=event_filter,
+        events, next_key = (
+            await self.store.paginate_room_events_by_topological_ordering(
+                room_id=room_id,
+                from_key=from_token.room_key,
+                to_key=to_room_key,
+                direction=pagin_config.direction,
+                limit=pagin_config.limit,
+                event_filter=event_filter,
+            )
         )
 
         if pagin_config.direction == Direction.BACKWARDS:
@@ -582,13 +584,15 @@ class PaginationHandler:
                 # If we did backfill something, refetch the events from the database to
                 # catch anything new that might have been added since we last fetched.
                 if did_backfill:
-                    events, next_key = await self.store.paginate_room_events(
-                        room_id=room_id,
-                        from_key=from_token.room_key,
-                        to_key=to_room_key,
-                        direction=pagin_config.direction,
-                        limit=pagin_config.limit,
-                        event_filter=event_filter,
+                    events, next_key = (
+                        await self.store.paginate_room_events_by_topological_ordering(
+                            room_id=room_id,
+                            from_key=from_token.room_key,
+                            to_key=to_room_key,
+                            direction=pagin_config.direction,
+                            limit=pagin_config.limit,
+                            event_filter=event_filter,
+                        )
                     )
             else:
                 # Otherwise, we can backfill in the background for eventual