René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"data races in decimal module with global context","articleBody":"Reproducer:\n```py\nimport decimal\nfrom threading import Thread, Barrier\n\ncontext = decimal.Context()\ncontext.prec = 28\n\nbarrier = Barrier(10)\n\ndef race():\n    barrier.wait()\n    decimal.Decimal('1.0', context=context)\n\nif __name__ == \"__main__\":\n    while True:\n        threads = [Thread(target=race) for _ in range(10)]\n        for thread in threads:\n            thread.start()\n        for thread in threads:\n            thread.join()\n```\n\nData race:\n```console\n==================\nWARNING: ThreadSanitizer: data race (pid=58514)\n  Write of size 4 at 0x00010d315cbc by thread T5808:\n    #0 dec_addstatus _decimal.c:612 (_decimal.cpython-315t-darwin.so:arm64+0x9a88)\n    #1 PyDecType_FromCStringExact _decimal.c:2323 (_decimal.cpython-315t-darwin.so:arm64+0x1a4a4)\n    #2 dec_new _decimal.c.h:572 (_decimal.cpython-315t-darwin.so:arm64+0x506c)\n    #3 type_call typeobject.c:2432 (python.exe:arm64+0x10017e768)\n    #4 _PyObject_MakeTpCall call.c:242 (python.exe:arm64+0x10007eb64)\n    #5 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10007f750)\n    #6 _PyEval_EvalFrameDefault generated_cases.c.h:1620 (python.exe:arm64+0x100277dec)\n    #7 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)\n    #8 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)\n    #9 method_vectorcall classobject.c:65 (python.exe:arm64+0x100084190)\n    #10 _PyObject_Call call.c:348 (python.exe:arm64+0x10007f8b0)\n    #11 PyObject_Call call.c:373 (python.exe:arm64+0x10007f9d8)\n    #12 _PyEval_EvalFrameDefault generated_cases.c.h:2616 (python.exe:arm64+0x10027bf64)\n    #13 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)\n    #14 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)\n    #15 method_vectorcall classobject.c:73 (python.exe:arm64+0x10008422c)\n    #16 context_run context.c:728 (python.exe:arm64+0x1002b53dc)\n    #17 _PyEval_EvalFrameDefault generated_cases.c.h:3710 (python.exe:arm64+0x10027ffbc)\n    #18 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)\n    #19 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)\n    #20 method_vectorcall classobject.c:73 (python.exe:arm64+0x10008422c)\n    #21 _PyObject_Call call.c:348 (python.exe:arm64+0x10007f964)\n    #22 PyObject_Call call.c:373 (python.exe:arm64+0x10007f9d8)\n    #23 thread_run _threadmodule.c:387 (python.exe:arm64+0x10041b338)\n    #24 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x100358824)\n\n  Previous write of size 4 at 0x00010d315cbc by thread T5806:\n    #0 dec_addstatus _decimal.c:612 (_decimal.cpython-315t-darwin.so:arm64+0x9a88)\n    #1 PyDecType_FromCStringExact _decimal.c:2323 (_decimal.cpython-315t-darwin.so:arm64+0x1a4a4)\n    #2 dec_new _decimal.c.h:572 (_decimal.cpython-315t-darwin.so:arm64+0x506c)\n    #3 type_call typeobject.c:2432 (python.exe:arm64+0x10017e768)\n    #4 _PyObject_MakeTpCall call.c:242 (python.exe:arm64+0x10007eb64)\n    #5 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10007f750)\n    #6 _PyEval_EvalFrameDefault generated_cases.c.h:1620 (python.exe:arm64+0x100277dec)\n    #7 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)\n    #8 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)\n    #9 method_vectorcall classobject.c:65 (python.exe:arm64+0x100084190)\n    #10 _PyObject_Call call.c:348 (python.exe:arm64+0x10007f8b0)\n    #11 PyObject_Call call.c:373 (python.exe:arm64+0x10007f9d8)\n    #12 _PyEval_EvalFrameDefault generated_cases.c.h:2616 (python.exe:arm64+0x10027bf64)\n    #13 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)\n    #14 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)\n    #15 method_vectorcall classobject.c:73 (python.exe:arm64+0x10008422c)\n    #16 context_run context.c:728 (python.exe:arm64+0x1002b53dc)\n    #17 _PyEval_EvalFrameDefault generated_cases.c.h:3710 (python.exe:arm64+0x10027ffbc)\n    #18 _PyEval_Vector ceval.c:1989 (python.exe:arm64+0x10026ad88)\n    #19 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10007fcec)\n    #20 method_vectorcall classobject.c:73 (python.exe:arm64+0x10008422c)\n    #21 _PyObject_Call call.c:348 (python.exe:arm64+0x10007f964)\n    #22 PyObject_Call call.c:373 (python.exe:arm64+0x10007f9d8)\n    #23 thread_run _threadmodule.c:387 (python.exe:arm64+0x10041b338)\n    #24 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x100358824)\nSUMMARY: ThreadSanitizer: data race _decimal.c:615 in dec_addstatus\n==================\n```\n\nI discovered this while adding thread safety support to [msgspec](https://github.com/jcrist/msgspec). Running its test suite under pytest-run-parallel leads to similar data races as present above from the minimal reproducer. \n\nSee https://gist.github.com/kumaraditya303/3c26f31eaf0beea39d7eeb539b662846 for full tsan report\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-146482\n* gh-151953\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/kumaraditya303","@type":"Person","name":"kumaraditya303"},"datePublished":"2025-11-06T14:54:31.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":16},"url":"https://github.com/141148/cpython/issues/141148"}