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, 8 insertions, 0 deletions

diff --git a/tests/util/test_itertools.py b/tests/util/test_itertools.py
index 1184cea5a3..522c8061f9 100644
--- a/tests/util/test_itertools.py
+++ b/tests/util/test_itertools.py
@@ -56,6 +56,14 @@ class SortTopologically(TestCase):
         graph = {}  # type: Dict[int, List[int]]
         self.assertEqual(list(sorted_topologically([], graph)), [])
 
+    def test_handle_empty_graph(self):
+        "Test that a graph where a node doesn't have an entry is treated as empty"
+
+        graph = {}  # type: Dict[int, List[int]]
+
+        # For disconnected nodes the output is simply sorted.
+        self.assertEqual(list(sorted_topologically([1, 2], graph)), [1, 2])
+
     def test_disconnected(self):
         "Test that a graph with no edges work"