Fix event chain bg update. (#9118)

We passed in a graph to `sorted_topologically` which didn't have an
entry for each node (as we dropped nodes with no edges).

1 files changed, 1 insertions, 1 deletions

diff --git a/synapse/util/iterutils.py b/synapse/util/iterutils.py

index f7b4857a84..6ef2b008a4 100644
--- a/synapse/util/iterutils.py
+++ b/synapse/util/iterutils.py
@@ -92,7 +92,7 @@ def sorted_topologically(
         node = heapq.heappop(zero_degree)
         yield node
 
-        for edge in reverse_graph[node]:
+        for edge in reverse_graph.get(node, []):
             if edge in degree_map:
                 degree_map[edge] -= 1
                 if degree_map[edge] == 0: