`groupby_next` data race on free-threaded builds

Bug report

Bug description:

Two threads calling next on the same groupby object concurrently in a free-threaded build race on the currgrouper field, corrupting the iterator's internal state and producing AttributeError on slot accesses of live objects.

groupby_next has no Py_BEGIN_CRITICAL_SECTION guard. The first thing it does is write gbo->currgrouper = NULL:

https://github.com/python/cpython/blob/c5516e7e371f7b273eb37c7b65f14ef14ee81f11/Modules/itertoolsmodule.c#L531-L540

Later, after the loop exits, it calls _grouper_create, which writes parent->currgrouper = igo:

https://github.com/python/cpython/blob/c5516e7e371f7b273eb37c7b65f14ef14ee81f11/Modules/itertoolsmodule.c#L624-L637

If Thread A is past line 633 (just stored the new grouper pointer) while Thread B is at line 537 (about to store NULL), Thread B overwrites the pointer Thread A just wrote. Both are plain pointer stores with no synchronisation.

---

The following script reproduces the condition under which this happens.

import itertools, threading

class K:
    __slots__ = ("v",)
    def __init__(self, v): self.v = v
    def __eq__(self, o): return isinstance(o, K) and self.v == o.v
    def __hash__(self): return hash(self.v)

def consume(g):
    try:
        while True:
            _, _ = next(g)
    except StopIteration:
        pass

keys = [K(i) for i in range(500_000)]
g = itertools.groupby(keys)
threads = [threading.Thread(target=consume, args=(g,)) for _ in range(8)]
for t in threads: t.start()
for t in threads: t.join()

On a free-threaded build, this results in the following Python logs...

Exception in Thread-15 (consume):
  File "<python-input-2>", line 6, in __eq__
    def __eq__(self, o): return isinstance(o, K) and self.v == o.v
                                                     ^^^^^^
AttributeError: 'K' object has no attribute 'v'

... and the following TSan logs.

WARNING: ThreadSanitizer: data race (pid=20464)
  Write of size 8 at 0x0003026e4a88 by thread T2:
    #0 groupby_next itertoolsmodule.c:537 (python.exe:arm64+0x10042a8b8)
    #1 builtin_next bltinmodule.c:1770

  Previous write of size 8 at 0x0003026e4a88 by thread T1:
    #0 _grouper_create itertoolsmodule.c:633 (python.exe:arm64+0x10042ac5c)
    #1 groupby_next itertoolsmodule.c:570 (python.exe:arm64+0x10042ac5c)
    #2 builtin_next bltinmodule.c:1770

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-150792

Start in Modules/itertoolsmodule.c at groupby_next and _grouper_create, then run the provided concurrent reproducer on a free-threaded build with ThreadSanitizer. Done means the reproducer no longer reports the currgrouper race or produces the described AttributeError.