From 7ff22d6da41cd5ca80db95c18b409aea38e49fcd Mon Sep 17 00:00:00 2001
From: Sean Quah <8349537+squahtx@users.noreply.github.com>
Date: Tue, 30 Nov 2021 16:28:02 +0000
Subject: [PATCH] Fix `LruCache` corruption bug with a `size_callback` that can
 return 0 (#11454)

When all entries in an `LruCache` have a size of 0 according to the
provided `size_callback`, and `drop_from_cache` is called on a cache
node, the node would be unlinked from the LRU linked list but remain in
the cache dictionary. An assertion would be later be tripped due to the
inconsistency.

Avoid unintentionally calling `__len__` and use a strict `is None`
check instead when unwrapping the weak reference.
---
 changelog.d/11454.bugfix        |  1 +
 synapse/util/caches/lrucache.py |  5 ++++-
 tests/util/test_lrucache.py     | 12 ++++++++++++
 3 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 changelog.d/11454.bugfix

diff --git a/changelog.d/11454.bugfix b/changelog.d/11454.bugfix
new file mode 100644
index 000000000000..096265cbc933
--- /dev/null
+++ b/changelog.d/11454.bugfix
@@ -0,0 +1 @@
+Fix an `LruCache` corruption bug, introduced in 1.38.0, that would cause certain requests to fail until the next Synapse restart.
diff --git a/synapse/util/caches/lrucache.py b/synapse/util/caches/lrucache.py
index 05c4dcb062a8..eb96f7e665e6 100644
--- a/synapse/util/caches/lrucache.py
+++ b/synapse/util/caches/lrucache.py
@@ -271,7 +271,10 @@ def drop_from_cache(self) -> None:
         removed from all lists.
         """
         cache = self._cache()
-        if not cache or not cache.pop(self.key, None):
+        if (
+            cache is None
+            or cache.pop(self.key, _Sentinel.sentinel) is _Sentinel.sentinel
+        ):
             # `cache.pop` should call `drop_from_lists()`, unless this Node had
             # already been removed from the cache.
             self.drop_from_lists()
diff --git a/tests/util/test_lrucache.py b/tests/util/test_lrucache.py
index 6578f3411e27..291644eb7dd0 100644
--- a/tests/util/test_lrucache.py
+++ b/tests/util/test_lrucache.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 
+from typing import List
 from unittest.mock import Mock
 
 from synapse.util.caches.lrucache import LruCache, setup_expire_lru_cache_entries
@@ -261,6 +262,17 @@ def test_evict(self):
         self.assertEquals(cache["key4"], [4])
         self.assertEquals(cache["key5"], [5, 6])
 
+    def test_zero_size_drop_from_cache(self) -> None:
+        """Test that `drop_from_cache` works correctly with 0-sized entries."""
+        cache: LruCache[str, List[int]] = LruCache(5, size_callback=lambda x: 0)
+        cache["key1"] = []
+
+        self.assertEqual(len(cache), 0)
+        cache.cache["key1"].drop_from_cache()
+        self.assertIsNone(
+            cache.pop("key1"), "Cache entry should have been evicted but wasn't"
+        )
+
 
 class TimeEvictionTestCase(unittest.HomeserverTestCase):
     """Test that time based eviction works correctly."""