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

Start with bind_tstate() and _PyThread_CurrentExceptions in Python/pystate.c, then review the linked PRs gh-152755 and gh-156202. Run the supplied free-threaded reproducer under ThreadSanitizer and verify that the reported race between the thread_id write and read is resolved without introducing new failures.