From d7ba685afd134221881c79f6991e6639fdd0ab89 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 22 Jan 2021 17:45:12 +0000
Subject: [PATCH 1/2] Fix chain cover update to handle events with duplicate
 auth events

---
 synapse/util/iterutils.py    |  2 +-
 tests/util/test_itertools.py | 12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/synapse/util/iterutils.py b/synapse/util/iterutils.py
index 6ef2b008a4d1..8d2411513fd7 100644
--- a/synapse/util/iterutils.py
+++ b/synapse/util/iterutils.py
@@ -78,7 +78,7 @@ def sorted_topologically(
         if node not in degree_map:
             continue
 
-        for edge in edges:
+        for edge in set(edges):
             if edge in degree_map:
                 degree_map[node] += 1
 
diff --git a/tests/util/test_itertools.py b/tests/util/test_itertools.py
index 522c8061f973..1ef0af8e8f03 100644
--- a/tests/util/test_itertools.py
+++ b/tests/util/test_itertools.py
@@ -92,3 +92,15 @@ def test_fork(self):
         # Valid orderings are `[1, 3, 2, 4]` or `[1, 2, 3, 4]`, but we should
         # always get the same one.
         self.assertEqual(list(sorted_topologically([4, 3, 2, 1], graph)), [1, 2, 3, 4])
+
+    def test_duplicates(self):
+        "Test that a graph with duplicate edges work"
+        graph = {1: [], 2: [1, 1], 3: [2, 2], 4: [3]}  # type: Dict[int, List[int]]
+
+        self.assertEqual(list(sorted_topologically([4, 3, 2, 1], graph)), [1, 2, 3, 4])
+
+    def test_multiple_paths(self):
+        "Test that a graph with multiple paths between two nodes work"
+        graph = {1: [], 2: [1], 3: [2], 4: [3, 2, 1]}  # type: Dict[int, List[int]]
+
+        self.assertEqual(list(sorted_topologically([4, 3, 2, 1], graph)), [1, 2, 3, 4])

From 7d956eb5589f27f5485734cba213312ab715ec2d Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 22 Jan 2021 17:53:16 +0000
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/9210.bugfix | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/9210.bugfix

diff --git a/changelog.d/9210.bugfix b/changelog.d/9210.bugfix
new file mode 100644
index 000000000000..f9e076557065
--- /dev/null
+++ b/changelog.d/9210.bugfix
@@ -0,0 +1 @@
+Fix chain cover update to handle events with duplicate auth events. Introduced in v1.26.0rc1.