atexit.unregister() can remove wrong callback

[serhiy-storchaka]

Bug report

atexit.unregister() can call the user __eq__() when it searches the callback(s) to remove. If __eq__() returns true, the callback is removed by index. But __eq__() can call arbitrary code, it can release the GIL, and atexit.unregister() can be called in other thread. If it removes the callback at the current index or before current index, the current index will no point to the compared callable, and the following deletion will delete wrong callable.

This issue is similar to #112127, but causes different symptoms.

Linked PRs

• gh-142881: Fix concurrent and reentrant call of atexit.unregister() #142901

[johnslavik]

@ZeroIntensity extension-modules