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Title: data race between lock free list reads and heapq · Issue #135557 · python/cpython · GitHub

Open Graph Title: data race between lock free list reads and heapq · Issue #135557 · python/cpython

X Title: data race between lock free list reads and heapq · Issue #135557 · python/cpython

Description: Reproducer: import heapq l = [] def writer(): while True: heapq.heappush(l, 1) heapq.heappop(l) def reader(): while True: try: l[0] except IndexError: pass def main(): import threading threads = [] for _ in range(10): t1 = threading.Thre...

Open Graph Description: Reproducer: import heapq l = [] def writer(): while True: heapq.heappush(l, 1) heapq.heappop(l) def reader(): while True: try: l[0] except IndexError: pass def main(): import threading threads = []...

X Description: Reproducer: import heapq l = [] def writer(): while True: heapq.heappush(l, 1) heapq.heappop(l) def reader(): while True: try: l[0] except IndexError: pass def main(): import threading threads = []...

Opengraph URL: https://github.com/python/cpython/issues/135557

X: @github

direct link

Domain: github.com

Hey, it has json ld scripts:

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"data race between lock free list reads and heapq","articleBody":"Reproducer:\n```py\nimport heapq\n\nl = []\n\n\ndef writer():\n    while True:\n        heapq.heappush(l, 1)\n        heapq.heappop(l)\n\ndef reader():\n    while True:\n        try:\n            l[0]\n        except IndexError:\n            pass\n\ndef main():\n    import threading\n\n    threads = []\n    for _ in range(10):\n        t1 = threading.Thread(target=writer)\n        t2 = threading.Thread(target=reader)\n        threads.append(t1)\n        threads.append(t2)\n        t1.start()\n        t2.start()\n\n    for t in threads:\n        t.join()\n\nmain()\n\n```\n\nTSAN data race on current main:\n```console\n==================\nWARNING: ThreadSanitizer: data race (pid=32935)\n  Write of size 8 at 0x7f90240300f8 by thread T3:\n    #0 PyList_SET_ITEM /home/realkumaraditya/cpython/./Include/cpython/listobject.h:47:26 (_heapq.cpython-315t-x86_64-linux-gnu.so+0x43fc) (BuildId: d2d0e185522b78f855f3798fbcc693ee0dba06e3)\n    #1 heappop_internal /home/realkumaraditya/cpython/./Modules/_heapqmodule.c:175:5 (_heapq.cpython-315t-x86_64-linux-gnu.so+0x43fc)\n    #2 _heapq_heappop_impl /home/realkumaraditya/cpython/./Modules/_heapqmodule.c:197:12 (_heapq.cpython-315t-x86_64-linux-gnu.so+0x2ae7) (BuildId: d2d0e185522b78f855f3798fbcc693ee0dba06e3)\n    #3 _heapq_heappop /home/realkumaraditya/cpython/./Modules/clinic/_heapqmodule.c.h:68:20 (_heapq.cpython-315t-x86_64-linux-gnu.so+0x2ae7)\n    #4 cfunction_vectorcall_O /home/realkumaraditya/cpython/Objects/methodobject.c:537:24 (python+0x2a2d0a) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #5 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x1f61b3) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #6 PyObject_Vectorcall /home/realkumaraditya/cpython/Objects/call.c:327:12 (python+0x1f61b3)\n    #7 _PyEval_EvalFrameDefault /home/realkumaraditya/cpython/Python/generated_cases.c.h:1629:35 (python+0x404042) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #8 _PyEval_EvalFrame /home/realkumaraditya/cpython/./Include/internal/pycore_ceval.h:119:16 (python+0x3ff030) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #9 _PyEval_Vector /home/realkumaraditya/cpython/Python/ceval.c:1975:12 (python+0x3ff030)\n    #10 _PyFunction_Vectorcall /home/realkumaraditya/cpython/Objects/call.c (python+0x1f680f) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #11 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x1fb226) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #12 method_vectorcall /home/realkumaraditya/cpython/Objects/classobject.c:72:20 (python+0x1fb226)\n    #13 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x457ac1) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #14 context_run /home/realkumaraditya/cpython/Python/context.c:728:29 (python+0x457ac1)\n    #15 method_vectorcall_FASTCALL_KEYWORDS /home/realkumaraditya/cpython/Objects/descrobject.c:421:24 (python+0x20f499) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #16 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x1f61b3) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #17 PyObject_Vectorcall /home/realkumaraditya/cpython/Objects/call.c:327:12 (python+0x1f61b3)\n    #18 _PyEval_EvalFrameDefault /home/realkumaraditya/cpython/Python/generated_cases.c.h:1629:35 (python+0x404042) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #19 _PyEval_EvalFrame /home/realkumaraditya/cpython/./Include/internal/pycore_ceval.h:119:16 (python+0x3ff030) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #20 _PyEval_Vector /home/realkumaraditya/cpython/Python/ceval.c:1975:12 (python+0x3ff030)\n    #21 _PyFunction_Vectorcall /home/realkumaraditya/cpython/Objects/call.c (python+0x1f680f) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #22 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x1fb226) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #23 method_vectorcall /home/realkumaraditya/cpython/Objects/classobject.c:72:20 (python+0x1fb226)\n    #24 _PyVectorcall_Call /home/realkumaraditya/cpython/Objects/call.c:273:16 (python+0x1f64a9) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #25 _PyObject_Call /home/realkumaraditya/cpython/Objects/call.c:348:16 (python+0x1f64a9)\n    #26 PyObject_Call /home/realkumaraditya/cpython/Objects/call.c:373:12 (python+0x1f6515) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #27 thread_run /home/realkumaraditya/cpython/./Modules/_threadmodule.c:373:21 (python+0x5ad102) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #28 pythread_wrapper /home/realkumaraditya/cpython/Python/thread_pthread.h:232:5 (python+0x5047c7) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n\n  Previous atomic read of size 8 at 0x7f90240300f8 by thread T2:\n    #0 _Py_atomic_load_ptr /home/realkumaraditya/cpython/./Include/cpython/pyatomic_gcc.h:300:18 (python+0x24bbb8) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #1 _Py_TryXGetRef /home/realkumaraditya/cpython/./Include/internal/pycore_object.h:632:23 (python+0x24bbb8)\n    #2 list_get_item_ref /home/realkumaraditya/cpython/Objects/listobject.c:366:22 (python+0x24bbb8)\n    #3 _PyList_GetItemRef /home/realkumaraditya/cpython/Objects/listobject.c:417:12 (python+0x24bd1e) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #4 _PyEval_EvalFrameDefault /home/realkumaraditya/cpython/Python/generated_cases.c.h:736:35 (python+0x401216) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #5 _PyEval_EvalFrame /home/realkumaraditya/cpython/./Include/internal/pycore_ceval.h:119:16 (python+0x3ff030) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #6 _PyEval_Vector /home/realkumaraditya/cpython/Python/ceval.c:1975:12 (python+0x3ff030)\n    #7 _PyFunction_Vectorcall /home/realkumaraditya/cpython/Objects/call.c (python+0x1f680f) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #8 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x1fb226) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #9 method_vectorcall /home/realkumaraditya/cpython/Objects/classobject.c:72:20 (python+0x1fb226)\n    #10 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x457ac1) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #11 context_run /home/realkumaraditya/cpython/Python/context.c:728:29 (python+0x457ac1)\n    #12 method_vectorcall_FASTCALL_KEYWORDS /home/realkumaraditya/cpython/Objects/descrobject.c:421:24 (python+0x20f499) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #13 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x1f61b3) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #14 PyObject_Vectorcall /home/realkumaraditya/cpython/Objects/call.c:327:12 (python+0x1f61b3)\n    #15 _PyEval_EvalFrameDefault /home/realkumaraditya/cpython/Python/generated_cases.c.h:1629:35 (python+0x404042) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #16 _PyEval_EvalFrame /home/realkumaraditya/cpython/./Include/internal/pycore_ceval.h:119:16 (python+0x3ff030) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #17 _PyEval_Vector /home/realkumaraditya/cpython/Python/ceval.c:1975:12 (python+0x3ff030)\n    #18 _PyFunction_Vectorcall /home/realkumaraditya/cpython/Objects/call.c (python+0x1f680f) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #19 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:169:11 (python+0x1fb226) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #20 method_vectorcall /home/realkumaraditya/cpython/Objects/classobject.c:72:20 (python+0x1fb226)\n    #21 _PyVectorcall_Call /home/realkumaraditya/cpython/Objects/call.c:273:16 (python+0x1f64a9) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #22 _PyObject_Call /home/realkumaraditya/cpython/Objects/call.c:348:16 (python+0x1f64a9)\n    #23 PyObject_Call /home/realkumaraditya/cpython/Objects/call.c:373:12 (python+0x1f6515) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #24 thread_run /home/realkumaraditya/cpython/./Modules/_threadmodule.c:373:21 (python+0x5ad102) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #25 pythread_wrapper /home/realkumaraditya/cpython/Python/thread_pthread.h:232:5 (python+0x5047c7) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n\n  Thread T3 'Thread-3 (push)' (tid=32945, running) created by main thread at:\n    #0 pthread_create \u003cnull\u003e (python+0xe21ef) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #1 do_start_joinable_thread /home/realkumaraditya/cpython/Python/thread_pthread.h:279:14 (python+0x5038a8) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #2 PyThread_start_joinable_thread /home/realkumaraditya/cpython/Python/thread_pthread.h:321:9 (python+0x5036ca) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #3 ThreadHandle_start /home/realkumaraditya/cpython/./Modules/_threadmodule.c:459:9 (python+0x5acc97) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #4 do_start_new_thread /home/realkumaraditya/cpython/./Modules/_threadmodule.c:1869:9 (python+0x5acc97)\n    #5 thread_PyThread_start_joinable_thread /home/realkumaraditya/cpython/./Modules/_threadmodule.c:1984:14 (python+0x5aba61) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #6 cfunction_call /home/realkumaraditya/cpython/Objects/methodobject.c:565:18 (python+0x2a34f7) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #7 _PyObject_MakeTpCall /home/realkumaraditya/cpython/Objects/call.c:242:18 (python+0x1f5661) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #8 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:167:16 (python+0x1f6271) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #9 PyObject_Vectorcall /home/realkumaraditya/cpython/Objects/call.c:327:12 (python+0x1f6271)\n    #10 _PyEval_EvalFrameDefault /home/realkumaraditya/cpython/Python/generated_cases.c.h:3245:35 (python+0x40a1e0) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #11 _PyEval_EvalFrame /home/realkumaraditya/cpython/./Include/internal/pycore_ceval.h:119:16 (python+0x3feb6f) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #12 _PyEval_Vector /home/realkumaraditya/cpython/Python/ceval.c:1975:12 (python+0x3feb6f)\n    #13 PyEval_EvalCode /home/realkumaraditya/cpython/Python/ceval.c:866:21 (python+0x3feb6f)\n    #14 run_eval_code_obj /home/realkumaraditya/cpython/Python/pythonrun.c:1365:12 (python+0x4e12dc) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #15 run_mod /home/realkumaraditya/cpython/Python/pythonrun.c:1436:19 (python+0x4e12dc)\n    #16 pyrun_file /home/realkumaraditya/cpython/Python/pythonrun.c:1293:15 (python+0x4dc83e) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #17 _PyRun_SimpleFileObject /home/realkumaraditya/cpython/Python/pythonrun.c:521:13 (python+0x4dc83e)\n    #18 _PyRun_AnyFileObject /home/realkumaraditya/cpython/Python/pythonrun.c:81:15 (python+0x4dbf98) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #19 pymain_run_file_obj /home/realkumaraditya/cpython/Modules/main.c:410:15 (python+0x52215f) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #20 pymain_run_file /home/realkumaraditya/cpython/Modules/main.c:429:15 (python+0x52215f)\n    #21 pymain_run_python /home/realkumaraditya/cpython/Modules/main.c:691:21 (python+0x521488) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #22 Py_RunMain /home/realkumaraditya/cpython/Modules/main.c:772:5 (python+0x521488)\n    #23 pymain_main /home/realkumaraditya/cpython/Modules/main.c:802:12 (python+0x5219f8) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #24 Py_BytesMain /home/realkumaraditya/cpython/Modules/main.c:826:12 (python+0x521a7b) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #25 main /home/realkumaraditya/cpython/./Programs/python.c:15:12 (python+0x1607fb) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n\n  Thread T2 'Thread-2 (pop)' (tid=32944, running) created by main thread at:\n    #0 pthread_create \u003cnull\u003e (python+0xe21ef) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #1 do_start_joinable_thread /home/realkumaraditya/cpython/Python/thread_pthread.h:279:14 (python+0x5038a8) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #2 PyThread_start_joinable_thread /home/realkumaraditya/cpython/Python/thread_pthread.h:321:9 (python+0x5036ca) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #3 ThreadHandle_start /home/realkumaraditya/cpython/./Modules/_threadmodule.c:459:9 (python+0x5acc97) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #4 do_start_new_thread /home/realkumaraditya/cpython/./Modules/_threadmodule.c:1869:9 (python+0x5acc97)\n    #5 thread_PyThread_start_joinable_thread /home/realkumaraditya/cpython/./Modules/_threadmodule.c:1984:14 (python+0x5aba61) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #6 cfunction_call /home/realkumaraditya/cpython/Objects/methodobject.c:565:18 (python+0x2a34f7) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #7 _PyObject_MakeTpCall /home/realkumaraditya/cpython/Objects/call.c:242:18 (python+0x1f5661) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #8 _PyObject_VectorcallTstate /home/realkumaraditya/cpython/./Include/internal/pycore_call.h:167:16 (python+0x1f6271) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #9 PyObject_Vectorcall /home/realkumaraditya/cpython/Objects/call.c:327:12 (python+0x1f6271)\n    #10 _PyEval_EvalFrameDefault /home/realkumaraditya/cpython/Python/generated_cases.c.h:3245:35 (python+0x40a1e0) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #11 _PyEval_EvalFrame /home/realkumaraditya/cpython/./Include/internal/pycore_ceval.h:119:16 (python+0x3feb6f) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #12 _PyEval_Vector /home/realkumaraditya/cpython/Python/ceval.c:1975:12 (python+0x3feb6f)\n    #13 PyEval_EvalCode /home/realkumaraditya/cpython/Python/ceval.c:866:21 (python+0x3feb6f)\n    #14 run_eval_code_obj /home/realkumaraditya/cpython/Python/pythonrun.c:1365:12 (python+0x4e12dc) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #15 run_mod /home/realkumaraditya/cpython/Python/pythonrun.c:1436:19 (python+0x4e12dc)\n    #16 pyrun_file /home/realkumaraditya/cpython/Python/pythonrun.c:1293:15 (python+0x4dc83e) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #17 _PyRun_SimpleFileObject /home/realkumaraditya/cpython/Python/pythonrun.c:521:13 (python+0x4dc83e)\n    #18 _PyRun_AnyFileObject /home/realkumaraditya/cpython/Python/pythonrun.c:81:15 (python+0x4dbf98) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #19 pymain_run_file_obj /home/realkumaraditya/cpython/Modules/main.c:410:15 (python+0x52215f) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #20 pymain_run_file /home/realkumaraditya/cpython/Modules/main.c:429:15 (python+0x52215f)\n    #21 pymain_run_python /home/realkumaraditya/cpython/Modules/main.c:691:21 (python+0x521488) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #22 Py_RunMain /home/realkumaraditya/cpython/Modules/main.c:772:5 (python+0x521488)\n    #23 pymain_main /home/realkumaraditya/cpython/Modules/main.c:802:12 (python+0x5219f8) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #24 Py_BytesMain /home/realkumaraditya/cpython/Modules/main.c:826:12 (python+0x521a7b) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n    #25 main /home/realkumaraditya/cpython/./Programs/python.c:15:12 (python+0x1607fb) (BuildId: 957e91b4c4b85547a8053d7d39b3c2228cdefcde)\n\nSUMMARY: ThreadSanitizer: data race /home/realkumaraditya/cpython/./Include/cpython/listobject.h:47:26 in PyList_SET_ITEM\n==================\n\n```\n\nIn https://github.com/python/cpython/pull/135036/ the heapq module was made thread safe but it still uses non atomic writes so it races with lock free reads in list. \n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-135601\n* gh-135787\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/kumaraditya303","@type":"Person","name":"kumaraditya303"},"datePublished":"2025-06-16T09:40:54.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/135557/cpython/issues/135557"}

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og:image:alt	Reproducer: import heapq l = [] def writer(): while True: heapq.heappush(l, 1) heapq.heappop(l) def reader(): while True: try: l[0] except IndexError: pass def main(): import threading threads = []...
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data race between lock free list reads and heapq	https://github.com/python/cpython/issues/135557#top
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extension-modulesC modules in the Modules dir	https://github.com/python/cpython/issues?q=state%3Aopen%20label%3A%22extension-modules%22
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on Jun 16, 2025	https://github.com/python/cpython/issues/135557#issue-3149281015
#135036	https://github.com/python/cpython/pull/135036
gh-135557: Use atomic writes on heapq operations #135601	https://github.com/python/cpython/pull/135601
[3.14] gh-135557: use atomic stores in heapq operations in free-threading (GH-135601) #135787	https://github.com/python/cpython/pull/135787
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