New submission from Anders Kaseorg <ande...@mit.edu>:
Because WeakKeyDictionary unconditionally maintains strong references to its
values, the garbage collector fails to collect a reference cycle from a
WeakKeyDictionary value to its key. For example, the following program
unexpectedly leaks memory:
from weakref import WeakKeyDictionary
class C: pass
d = WeakKeyDictionary()
while True:
c = C()
d[c] = [c]
I would expect a WeakKeyDictionary value to be marked live _if_ its key is
marked live, not unconditionally. This could be implemented with garbage
collector support for ephemerons
(https://www.researchgate.net/publication/221320677_Ephemerons_A_New_Finalization_Mechanism).
To motivate this issue, a typical use of WeakKeyDictionary is as a hygienic
replacement for patching extra properties into third-party objects:
# before:
obj._extra_state = ExtraState(obj)
# after:
extra_states = WeakKeyDictionary()
extra_states[o] = ExtraState(obj)
However, such a conversion will introduce this memory leak if ExtraState(obj)
has any transitive references to obj.
This leak does not occur in JavaScript:
class C {}
const d = new WeakMap();
while (true) {
const c = new C();
d[c] = [c];
}
----------
components: Library (Lib)
messages: 397841
nosy: andersk
priority: normal
severity: normal
status: open
title: Reference cycles from a WeakKeyDictionary value to its key aren’t
collected
type: resource usage
versions: Python 3.9
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue44680>
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