Fix error when out of servers to sync partial state with (matrix-org#13432)

so that we raise the intended error instead.

Signed-off-by: Sean Quah <[email protected]>

Author: squahtx

changelog.d/13432.bugfix

@@ -0,0 +1 @@
+Faster room joins: Fix error when running out of servers to sync partial state with, so that Synapse raises the intended error instead.

synapse/handlers/federation.py

@@ -1543,15 +1543,16 @@ async def _sync_partial_state_room(
 
         # 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
-            destination_iter = itertools.cycle(destinations)
         else:
-            destination_iter = itertools.cycle(other_destinations)
+            destinations = other_destinations
+        destination_iter = itertools.cycle(destinations)
 
         # `destination` is the current remote homeserver we're pulling from.
         destination = next(destination_iter)