list iterator and list reverse iterator are not thread safe

[kumaraditya303]

While reviewing #130096, I checked the thread safety of list object and there at some places we are missing atomic stores of index.

Specifically the following:

• cpython/Objects/listobject.c

  Lines 3994 to 3997 in 97d0011

  	else if (index > PyList_GET_SIZE(it->it_seq))
  	index = PyList_GET_SIZE(it->it_seq); /* iterator exhausted */
  	it->it_index = index;
  	}

• cpython/Objects/listobject.c

  Lines 4146 to 4149 in 97d0011

  	else if (index > PyList_GET_SIZE(it->it_seq) - 1)
  	index = PyList_GET_SIZE(it->it_seq) - 1;
  	it->it_index = index;
  	}

They should use atomic relaxed stores.

Linked PRs

• gh-130263: fix list iterators to use atomics for storing index #130266

[kumaraditya303]

Fixed by 388e1ca