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Fix LruCache corruption bug with a size_callback that can return 0 (#11454)

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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.
This commit is contained in:
Sean Quah 2021-11-30 16:28:02 +00:00 • committed by GitHub
parent 5a0b652d36
commit 7ff22d6da4
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3 changed files with 17 additions and 1 deletions
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5
synapse/util/caches/lrucache.py
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@ -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()