data races in decimal module with global context

Reproducer:

import decimal
from threading import Thread, Barrier

context = decimal.Context()
context.prec = 28

barrier = Barrier(10)

def race():
    barrier.wait()
    decimal.Decimal('1.0', context=context)

if __name__ == "__main__":
    while True:
        threads = [Thread(target=race) for _ in range(10)]
        for thread in threads:
            thread.start()
        for thread in threads:
            thread.join()

Data race:

==================
WARNING: ThreadSanitizer: data race (pid=58514)
  Write of size 4 at 0x00010d315cbc by thread T5808:
    #0 dec_addstatus _decimal.c:612 (_decimal.cpython-315t-darwin.so:arm64+0x9a88)
    #1 PyDecType_FromCStringExact _decimal.c:2323 (_decimal.cpython-315t-darwin.so:arm64+0x1a4a4)
    #2 dec_new _decimal.c.h:572 (_decimal.cpython-315t-darwin.so:arm64+0x506c)
    #3 type_call typeobject.c:2432 (python.exe:arm64+0x10017e768)
    #4 _PyObject_MakeTpCall call.c:242 (python.exe:arm64+0x10007eb64)
    #5 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10007f750)
    #6 _PyEval_EvalFrameDefault generated_cases.c.h:1620 (python.exe:arm64+0x100277dec)
    #7 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)
    #8 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)
    #9 method_vectorcall classobject.c:65 (python.exe:arm64+0x100084190)
    #10 _PyObject_Call call.c:348 (python.exe:arm64+0x10007f8b0)
    #11 PyObject_Call call.c:373 (python.exe:arm64+0x10007f9d8)
    #12 _PyEval_EvalFrameDefault generated_cases.c.h:2616 (python.exe:arm64+0x10027bf64)
    #13 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)
    #14 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)
    #15 method_vectorcall classobject.c:73 (python.exe:arm64+0x10008422c)
    #16 context_run context.c:728 (python.exe:arm64+0x1002b53dc)
    #17 _PyEval_EvalFrameDefault generated_cases.c.h:3710 (python.exe:arm64+0x10027ffbc)
    #18 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)
    #19 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)
    #20 method_vectorcall classobject.c:73 (python.exe:arm64+0x10008422c)
    #21 _PyObject_Call call.c:348 (python.exe:arm64+0x10007f964)
    #22 PyObject_Call call.c:373 (python.exe:arm64+0x10007f9d8)
    #23 thread_run _threadmodule.c:387 (python.exe:arm64+0x10041b338)
    #24 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x100358824)

  Previous write of size 4 at 0x00010d315cbc by thread T5806:
    #0 dec_addstatus _decimal.c:612 (_decimal.cpython-315t-darwin.so:arm64+0x9a88)
    #1 PyDecType_FromCStringExact _decimal.c:2323 (_decimal.cpython-315t-darwin.so:arm64+0x1a4a4)
    #2 dec_new _decimal.c.h:572 (_decimal.cpython-315t-darwin.so:arm64+0x506c)
    #3 type_call typeobject.c:2432 (python.exe:arm64+0x10017e768)
    #4 _PyObject_MakeTpCall call.c:242 (python.exe:arm64+0x10007eb64)
    #5 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10007f750)
    #6 _PyEval_EvalFrameDefault generated_cases.c.h:1620 (python.exe:arm64+0x100277dec)
    #7 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)
    #8 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)
    #9 method_vectorcall classobject.c:65 (python.exe:arm64+0x100084190)
    #10 _PyObject_Call call.c:348 (python.exe:arm64+0x10007f8b0)
    #11 PyObject_Call call.c:373 (python.exe:arm64+0x10007f9d8)
    #12 _PyEval_EvalFrameDefault generated_cases.c.h:2616 (python.exe:arm64+0x10027bf64)
    #13 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)
    #14 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)
    #15 method_vectorcall classobject.c:73 (python.exe:arm64+0x10008422c)
    #16 context_run context.c:728 (python.exe:arm64+0x1002b53dc)
    #17 _PyEval_EvalFrameDefault generated_cases.c.h:3710 (python.exe:arm64+0x10027ffbc)
    #18 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)
    #19 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)
    #20 method_vectorcall classobject.c:73 (python.exe:arm64+0x10008422c)
    #21 _PyObject_Call call.c:348 (python.exe:arm64+0x10007f964)
    #22 PyObject_Call call.c:373 (python.exe:arm64+0x10007f9d8)
    #23 thread_run _threadmodule.c:387 (python.exe:arm64+0x10041b338)
    #24 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x100358824)
SUMMARY: ThreadSanitizer: data race _decimal.c:615 in dec_addstatus
==================

I discovered this while adding thread safety support to msgspec. Running its test suite under pytest-run-parallel leads to similar data races as present above from the minimal reproducer.

See https://gist.github.com/kumaraditya303/3c26f31eaf0beea39d7eeb539b662846 for full tsan report

Linked PRs

• gh-146482
• gh-151953

_decimal.c、_decimal.c.h、context.c にある報告された呼び出しパスから始め、次に ThreadSanitizer で最小の再現プログラムを実行します。動作を、リンクされている PR gh-146482 および gh-151953 と比較します。再現プログラムと同等の並列ワークロードでこのデータ競合が報告されなくなり、適切な箇所に回帰テストのカバレッジが追加されていれば完了です。