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: 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.
---
 synapse/util/caches/lrucache.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

(limited to 'synapse/util/caches/lrucache.py')

diff --git a/synapse/util/caches/lrucache.py b/synapse/util/caches/lrucache.py
index 05c4dcb062..eb96f7e665 100644
--- a/synapse/util/caches/lrucache.py
+++ b/synapse/util/caches/lrucache.py
@@ -271,7 +271,10 @@ class _Node(Generic[KT, VT]):
         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()
-- 
cgit 1.4.1