Bug report

Bug description:

class C: pass

f = C()
# Reader code:
f.foo
# Writer code:
f.__dict__ = {}

When there's a thread reading from an object and another thread replacing the dictionary we can crash. This is because the read has a borrowed reference to the dictionary which gets simultaneously decref'd when the writer replaces it.

Expected behavior: No crashes

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-124470: Fix crash when reading from object instance dictionary while replacing it #122489