Data race between `sys._current_exceptions()` and a concurrently attaching thread

Bug description:

In the free-threaded build, sys._current_exceptions() iterates every thread state under stop-the-world + HEAD_LOCK and reads t->thread_id.

https://github.com/python/cpython/blob/ecdef1773006529b0fea6639d0effeecbb41679c/Python/pystate.c#L2808-L2817

But a newly started thread sets tstate->thread_id in bind_tstate() without holding HEAD_LOCK. Its PyThreadState is already on the interpreter's thread list by this point.

https://github.com/python/cpython/blob/ecdef1773006529b0fea6639d0effeecbb41679c/Python/pystate.c#L164-L178

A thread that is still attaching is not yet stop-the-world-stoppable. _PyEval_StopTheWorldAll() therefore does not pause it, and its unlocked write to tstate->thread_id races the stop-the-world reader in _PyThread_CurrentExceptions.

Reproducer:

import sys
import threading

stop = threading.Event()

def reader():                      # keep stop-the-world almost always active
    for _ in range(20000):
        sys._current_exceptions()

def churn():                       # continuously attach new threads (bind_tstate)
    for _ in range(20000):
        t = threading.Thread(target=lambda: None)
        t.start()
        t.join()

readers  = [threading.Thread(target=reader) for _ in range(8)]
churners = [threading.Thread(target=churn)  for _ in range(4)]
for t in readers + churners: t.start()
for t in readers + churners: t.join()

TSAN Report:

WARNING: ThreadSanitizer: data race (pid=94157)
  Read of size 8 at 0x0001081180b0 by thread T1:
    #0 _PyThread_CurrentExceptions pystate.c:2817
    #1 sys__current_exceptions sysmodule.c.h:1180
    #2 cfunction_vectorcall_NOARGS methodobject.c:508
    #3 PyObject_Vectorcall call.c:327
    #4 _Py_VectorCallInstrumentation_StackRefSteal ceval.c:768
    #5 _PyEval_EvalFrameDefault generated_cases.c.h:1906
...
  Previous write of size 8 by thread T9:
    #0 bind_tstate pystate.c:178
    #1 _PyThreadState_Bind pystate.c:2660
    #2 thread_run _threadmodule.c:384
    #3 pythread_wrapper thread_pthread.h:234

SUMMARY: ThreadSanitizer: data race pystate.c:2817 in _PyThread_CurrentExceptions

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-152755
• gh-156202

Python/pystate.c の bind_tstate() と _PyThread_CurrentExceptions から始め、続いてリンクされている PR gh-152755 と gh-156202 を確認してください。提供されている free-threaded の再現プログラムを ThreadSanitizer で実行し、thread_id の書き込みと読み取りの間で報告された競合が、新たな失敗を引き起こすことなく解消されていることを確認してください。