When restarting a partial join resync, prioritise the server which actioned a partial join (#14126)

synapse/handlers/federation.py

@@ -632,6 +632,7 @@ class FederationHandler:
                     room_id=room_id,
                     servers=ret.servers_in_room,
                     device_lists_stream_id=self.store.get_device_stream_token(),
+                    joined_via=origin,
                 )
 
             try:
@@ -1615,13 +1616,13 @@ class FederationHandler:
         """Resumes resyncing of all partial-state rooms after a restart."""
         assert not self.config.worker.worker_app
 
-        partial_state_rooms = await self.store.get_partial_state_rooms_and_servers()
-        for room_id, servers_in_room in partial_state_rooms.items():
+        partial_state_rooms = await self.store.get_partial_state_room_resync_info()
+        for room_id, resync_info in partial_state_rooms.items():
             run_as_background_process(
                 desc="sync_partial_state_room",
                 func=self._sync_partial_state_room,
-                initial_destination=None,
-                other_destinations=servers_in_room,
+                initial_destination=resync_info.joined_via,
+                other_destinations=resync_info.servers_in_room,
                 room_id=room_id,
             )
 
@@ -1650,28 +1651,12 @@ class FederationHandler:
         #   really leave, that might mean we have difficulty getting the room state over
         #   federation.
         #   https://github.com/matrix-org/synapse/issues/12802
-        #
-        # TODO(faster_joins): we need some way of prioritising which homeservers in
-        #   `other_destinations` to try first, otherwise we'll spend ages trying dead
-        #   homeservers for large rooms.
-        #   https://github.com/matrix-org/synapse/issues/12999
-
-        if initial_destination is None and len(other_destinations) == 0:
-            raise ValueError(
-                f"Cannot resync state of {room_id}: no destinations provided"
-            )
 
         # Make an infinite iterator of destinations to try. Once we find a working
         # destination, we'll stick with it until it flakes.
-        destinations: Collection[str]
-        if initial_destination is not None:
-            # Move `initial_destination` to the front of the list.
-            destinations = list(other_destinations)
-            if initial_destination in destinations:
-                destinations.remove(initial_destination)
-            destinations = [initial_destination] + destinations
-        else:
-            destinations = other_destinations
+        destinations = _prioritise_destinations_for_partial_state_resync(
+            initial_destination, other_destinations, room_id
+        )
         destination_iter = itertools.cycle(destinations)
 
         # `destination` is the current remote homeserver we're pulling from.
@@ -1769,3 +1754,29 @@ class FederationHandler:
                             room_id,
                             destination,
                         )
+
+
+def _prioritise_destinations_for_partial_state_resync(
+    initial_destination: Optional[str],
+    other_destinations: Collection[str],
+    room_id: str,
+) -> Collection[str]:
+    """Work out the order in which we should ask servers to resync events.
+
+    If an `initial_destination` is given, it takes top priority. Otherwise
+    all servers are treated equally.
+
+    :raises ValueError: if no destination is provided at all.
+    """
+    if initial_destination is None and len(other_destinations) == 0:
+        raise ValueError(f"Cannot resync state of {room_id}: no destinations provided")
+
+    if initial_destination is None:
+        return other_destinations
+
+    # Move `initial_destination` to the front of the list.
+    destinations = list(other_destinations)
+    if initial_destination in destinations:
+        destinations.remove(initial_destination)
+    destinations = [initial_destination] + destinations
+    return destinations