gh-135552: Add tests that check if weakref for tp_subclasses cleared after finalization

[sergey-miryanov]

Tests for #136189

• Issue: Segmentation fault, possibly due to a GC issue (tp_subclasses) #135552

[sergey-miryanov]

I believe this PR should skip news

[nascheme]

This looks okay to me. Some suggested improvements. The new test might fit better into test_weakref.py but that's minor.

I don't think it's required to run these in a separate Python process. That makes the test run slower and it's not supposed to crash. Generally a separate process is used for the tests that exercise the interpreter shutdown logic and we are not testing that. So you could just do:

 def test_clearing_weakrefs_in_gc(self):
# This test checks that:
# 1. weakrefs for types with callbacks are cleared before the
# finalizer is called
# 2. weakrefs for types without callbacks are cleared after the
# finalizer is called
# 3. other weakrefs cleared before the finalizer is called
errors = []
class Class:
def __init__(self):
self._self = self
self._1 = weakref.ref(Class, lambda x: None)
self._2 = weakref.ref(Class)
self._3 = weakref.ref(self)
def __del__(self):
if self._1() is not None:
errors.append("Type weakref with callback is not None as expected")
if self._2() is not Class:
errors.append("Type weakref is not Class as expected")
if self._3() is not None:
errors.append("Instance weakref is not None as expected")
Class()
gc.collect()
self.assertEqual(errors, [])

I find the variable naming a bit esoteric. I would have just called them wr1, wr2 and wr3.

[sergey-miryanov]

@nascheme Thanks for review! Fixed.

[sergey-miryanov]

@kumaraditya303 Thanks for review!

[sergey-miryanov]

Thanks everyone!