Re-entering the __next__() method of itertools.pairwise leaks references, because old hold a borrowed reference when the __next__() method of the underlying iterator is called. It may potentially lead to the use of a freed memory and a crash, but I only have reproducer for leaks.

Even if the re-entrant call is not very meaningful, it should correctly count references. There are several ways to fix this issue. I choose the simplest one which matches the current results (only without leaks) in most of simple tests, even if there is no other reasons to prefer these results.

cc @rhettinger

Linked PRs

• gh-109786: Fix leaks when re-enter itertools.pairwise.__next__() #109788
• [3.12] gh-109786: Fix leaks and crash when re-enter itertools.pairwise.__next__() (GH-109788) #112699
• [3.11] gh-109786: Fix leaks and crash when re-enter itertools.pairwise.__next__() (GH-109788) #112700