Use state before join to determine if we `_should_perform_remote_join` (#13270)

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

1 files changed, 13 insertions, 8 deletions

diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py

index 3a65bd0849..56606e9afb 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -153,22 +153,27 @@ class StateHandler:
             ReplicationUpdateCurrentStateRestServlet.make_client(hs)
         )
 
-    async def get_current_state_ids(
+    async def compute_state_after_events(
         self,
         room_id: str,
-        latest_event_ids: Collection[str],
+        event_ids: Collection[str],
     ) -> StateMap[str]:
-        """Get the current state, or the state at a set of events, for a room
+        """Fetch the state after each of the given event IDs. Resolve them and return.
+
+        This is typically used where `event_ids` is a collection of forward extremities
+        in a room, intended to become the `prev_events` of a new event E. If so, the
+        return value of this function represents the state before E.
 
         Args:
-            room_id:
-            latest_event_ids: The forward extremities to resolve.
+            room_id: the room_id containing the given events.
+            event_ids: the events whose state should be fetched and resolved.
 
         Returns:
-            the state dict, mapping from (event_type, state_key) -> event_id
+            the state dict (a mapping from (event_type, state_key) -> event_id) which
+            holds the resolution of the states after the given event IDs.
         """
-        logger.debug("calling resolve_state_groups from get_current_state_ids")
-        ret = await self.resolve_state_groups_for_events(room_id, latest_event_ids)
+        logger.debug("calling resolve_state_groups from compute_state_after_events")
+        ret = await self.resolve_state_groups_for_events(room_id, event_ids)
         return await ret.get_state(self._state_storage_controller, StateFilter.all())
 
     async def get_current_users_in_room(