Add `rooms.bump_stamp` to Sliding Sync `/sync` for easier client-side sorting (#17395)

`bump_stamp` corresponds to the `stream_ordering` of the latest `DEFAULT_BUMP_EVENT_TYPES` in the room. This helps clients sort more readily without them needing to pull in a bunch of the timeline to determine the last activity. `bump_event_types` is a thing because for example, we don't want display name changes to mark the room as unread and bump it to the top. For encrypted rooms, we just have to consider any activity as a bump because we can't see the content and the client has to figure it out for themselves.

Outside of Synapse, `bump_stamp` is just a free-form counter so other implementations could use `received_ts`or `origin_server_ts` (see the [*Security considerations* section in MSC3575 about the potential pitfalls of using `origin_server_ts`](https://github.com/matrix-org/matrix-spec-proposals/blob/kegan/sync-v3/proposals/3575-sync.md#security-considerations)). It doesn't have any guarantee about always going up. In the Synapse case, it could go down if an event was redacted/removed (or purged in cases of retention policies).

In the future, we could add `bump_event_types` as [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) mentions if people need to customize the event types.

---

In the Sliding Sync proxy, a similar [`timestamp` field was added](https://github.com/matrix-org/sliding-sync/pull/247) for the same purpose but the name is not obvious what it pertains to or what it's for.

The `timestamp` field was also added to Ruma in https://github.com/ruma/ruma/pull/1622

1 files changed, 64 insertions, 4 deletions

diff --git a/tests/handlers/test_sliding_sync.py b/tests/handlers/test_sliding_sync.py
index 5f83b637c5..9dd2363adc 100644
--- a/tests/handlers/test_sliding_sync.py
+++ b/tests/handlers/test_sliding_sync.py
@@ -2844,7 +2844,7 @@ class SortRoomsTestCase(HomeserverTestCase):
         )
 
         # Sort the rooms (what we're testing)
-        sorted_room_info = self.get_success(
+        sorted_sync_rooms = self.get_success(
             self.sliding_sync_handler.sort_rooms(
                 sync_room_map=sync_room_map,
                 to_token=after_rooms_token,
@@ -2852,7 +2852,7 @@ class SortRoomsTestCase(HomeserverTestCase):
         )
 
         self.assertEqual(
-            [room_id for room_id, _ in sorted_room_info],
+            [room_membership.room_id for room_membership in sorted_sync_rooms],
             [room_id2, room_id1],
         )
 
@@ -2927,7 +2927,7 @@ class SortRoomsTestCase(HomeserverTestCase):
         )
 
         # Sort the rooms (what we're testing)
-        sorted_room_info = self.get_success(
+        sorted_sync_rooms = self.get_success(
             self.sliding_sync_handler.sort_rooms(
                 sync_room_map=sync_room_map,
                 to_token=after_rooms_token,
@@ -2935,7 +2935,7 @@ class SortRoomsTestCase(HomeserverTestCase):
         )
 
         self.assertEqual(
-            [room_id for room_id, _ in sorted_room_info],
+            [room_membership.room_id for room_membership in sorted_sync_rooms],
             [room_id2, room_id1, room_id3],
             "Corresponding map to disambiguate the opaque room IDs: "
             + str(
@@ -2946,3 +2946,63 @@ class SortRoomsTestCase(HomeserverTestCase):
                 }
             ),
         )
+
+    def test_default_bump_event_types(self) -> None:
+        """
+        Test that we only consider the *latest* event in the room when sorting (not
+        `bump_event_types`).
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        room_id1 = self.helper.create_room_as(
+            user1_id,
+            tok=user1_tok,
+        )
+        message_response = self.helper.send(room_id1, "message in room1", tok=user1_tok)
+        room_id2 = self.helper.create_room_as(
+            user1_id,
+            tok=user1_tok,
+        )
+        self.helper.send(room_id2, "message in room2", tok=user1_tok)
+
+        # Send a reaction in room1 which isn't in `DEFAULT_BUMP_EVENT_TYPES` but we only
+        # care about sorting by the *latest* event in the room.
+        self.helper.send_event(
+            room_id1,
+            type=EventTypes.Reaction,
+            content={
+                "m.relates_to": {
+                    "event_id": message_response["event_id"],
+                    "key": "👍",
+                    "rel_type": "m.annotation",
+                }
+            },
+            tok=user1_tok,
+        )
+
+        after_rooms_token = self.event_sources.get_current_token()
+
+        # Get the rooms the user should be syncing with
+        sync_room_map = self.get_success(
+            self.sliding_sync_handler.get_sync_room_ids_for_user(
+                UserID.from_string(user1_id),
+                from_token=None,
+                to_token=after_rooms_token,
+            )
+        )
+
+        # Sort the rooms (what we're testing)
+        sorted_sync_rooms = self.get_success(
+            self.sliding_sync_handler.sort_rooms(
+                sync_room_map=sync_room_map,
+                to_token=after_rooms_token,
+            )
+        )
+
+        self.assertEqual(
+            [room_membership.room_id for room_membership in sorted_sync_rooms],
+            # room1 sorts before room2 because it has the latest event (the reaction).
+            # We only care about the *latest* event in the room.
+            [room_id1, room_id2],
+        )