python / python/cpython

Data race in set iterator length_hint under no-gil

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interpreter-core topic-free-threading type-bug
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Description

Bug report

Build info
$ cd cpython-tsan/bin/
$ ./python3.15
Python 3.15.0a5+ free-threading build (heads/main:96e4cd698a, Jan 31 2026, 11:45:37) [Clang 18.1.3 (1ubuntu1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
Bug description

Under Py_GIL_DISABLED, setiter_len() reads so->used non-atomically while concurrent set mutations update it atomically.

This triggers a data race detectable by TSAN when one thread repeatedly calls iterator.__length_hint__() while another mutates the same set

Python script
import threading, time

s = set(range(2000))
it = iter(s)
stop = threading.Event()

def reader():
    while not stop.is_set():
        it.__length_hint__()

def writer():
    i = 0
    while not stop.is_set():
        s.add(i)
        s.discard(i - 1)
        i += 1

threads = [threading.Thread(target=reader) for _ in range(4)]
threads.append(threading.Thread(target=writer))

for t in threads: t.start()
time.sleep(3)
stop.set()
for t in threads: t.join()
TSAN result
$ TSAN_OPTIONS="halt_on_error=1:history_size=7:report_signal_unsafe=0:verbosity=1" ./python3.15 ./tsan_setiter_len_race.py 
==1703386==WARNING: ThreadSanitizer: memory layout is incompatible, possibly due to high-entropy ASLR.
Re-execing with fixed virtual address space.
N.B. reducing ASLR entropy is preferable.
==1703386==Installed the sigaction for signal 11
==1703386==Installed the sigaction for signal 7
==1703386==Installed the sigaction for signal 8
***** Running under ThreadSanitizer v3 (pid 1703386) *****
==================
WARNING: ThreadSanitizer: data race (pid=1703386)
  Atomic write of size 8 at 0x7fffb4395ce8 by thread T5:
    #0 _Py_atomic_store_ssize_relaxed /home/hyongtao/worksapce/cpython/./Include/cpython/pyatomic_gcc.h:513:3 (python3.15+0x30378c) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #1 set_discard_entry /home/hyongtao/worksapce/cpython/Objects/setobject.c:594:5 (python3.15+0x30378c)
    #2 set_discard_key /home/hyongtao/worksapce/cpython/Objects/setobject.c:630:12 (python3.15+0x30378c)
    #3 set_discard_impl /home/hyongtao/worksapce/cpython/Objects/setobject.c:2616:10 (python3.15+0x30b3c3) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #4 set_discard /home/hyongtao/worksapce/cpython/Objects/clinic/setobject.c.h:502:20 (python3.15+0x30b3c3)
    #5 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:4033:35 (python3.15+0x42abe4) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #6 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f890) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #7 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f890)
    #8 _PyFunction_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c (python3.15+0x20b85f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #9 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20ff36) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #10 method_vectorcall /home/hyongtao/worksapce/cpython/Objects/classobject.c:73:20 (python3.15+0x20ff36)
    #11 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x472d31) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #12 context_run /home/hyongtao/worksapce/cpython/Python/context.c:727:29 (python3.15+0x472d31)
    #13 method_vectorcall_FASTCALL_KEYWORDS /home/hyongtao/worksapce/cpython/Objects/descrobject.c:421:24 (python3.15+0x224ab7) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #14 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20b1e3) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #15 PyObject_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c:327:12 (python3.15+0x20b1e3)
    #16 _Py_VectorCallInstrumentation_StackRefSteal /home/hyongtao/worksapce/cpython/Python/ceval.c:762:11 (python3.15+0x4204bc) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #17 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:1809:35 (python3.15+0x426376) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #18 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f890) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #19 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f890)
    #20 _PyFunction_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c (python3.15+0x20b85f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #21 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20ff36) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #22 method_vectorcall /home/hyongtao/worksapce/cpython/Objects/classobject.c:73:20 (python3.15+0x20ff36)
    #23 _PyVectorcall_Call /home/hyongtao/worksapce/cpython/Objects/call.c:273:16 (python3.15+0x20b4eb) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #24 _PyObject_Call /home/hyongtao/worksapce/cpython/Objects/call.c:348:16 (python3.15+0x20b4eb)
    #25 PyObject_Call /home/hyongtao/worksapce/cpython/Objects/call.c:373:12 (python3.15+0x20b555) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #26 thread_run /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:387:21 (python3.15+0x5f12e2) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #27 pythread_wrapper /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:234:5 (python3.15+0x522317) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)

  Previous read of size 8 at 0x7fffb4395ce8 by thread T3:
    #0 setiter_len /home/hyongtao/worksapce/cpython/Objects/setobject.c:1059:58 (python3.15+0x304ce9) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #1 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:3935:35 (python3.15+0x42a9d4) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #2 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f890) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #3 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f890)
    #4 _PyFunction_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c (python3.15+0x20b85f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #5 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20ff36) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #6 method_vectorcall /home/hyongtao/worksapce/cpython/Objects/classobject.c:73:20 (python3.15+0x20ff36)
    #7 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x472d31) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #8 context_run /home/hyongtao/worksapce/cpython/Python/context.c:727:29 (python3.15+0x472d31)
    #9 method_vectorcall_FASTCALL_KEYWORDS /home/hyongtao/worksapce/cpython/Objects/descrobject.c:421:24 (python3.15+0x224ab7) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #10 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20b1e3) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #11 PyObject_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c:327:12 (python3.15+0x20b1e3)
    #12 _Py_VectorCallInstrumentation_StackRefSteal /home/hyongtao/worksapce/cpython/Python/ceval.c:762:11 (python3.15+0x4204bc) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #13 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:1809:35 (python3.15+0x426376) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #14 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f890) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #15 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f890)
    #16 _PyFunction_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c (python3.15+0x20b85f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #17 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20ff36) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #18 method_vectorcall /home/hyongtao/worksapce/cpython/Objects/classobject.c:73:20 (python3.15+0x20ff36)
    #19 _PyVectorcall_Call /home/hyongtao/worksapce/cpython/Objects/call.c:273:16 (python3.15+0x20b4eb) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #20 _PyObject_Call /home/hyongtao/worksapce/cpython/Objects/call.c:348:16 (python3.15+0x20b4eb)
    #21 PyObject_Call /home/hyongtao/worksapce/cpython/Objects/call.c:373:12 (python3.15+0x20b555) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #22 thread_run /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:387:21 (python3.15+0x5f12e2) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #23 pythread_wrapper /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:234:5 (python3.15+0x522317) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)

  Thread T5 'Thread-5 (write' (tid=1703392, running) created by main thread at:
    #0 pthread_create <null> (python3.15+0xf421f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #1 do_start_joinable_thread /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:281:14 (python3.15+0x521424) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #2 PyThread_start_joinable_thread /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:323:9 (python3.15+0x52123a) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #3 ThreadHandle_start /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:474:9 (python3.15+0x5f109a) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #4 do_start_new_thread /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:1920:9 (python3.15+0x5f0b3f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #5 thread_PyThread_start_joinable_thread /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:2043:14 (python3.15+0x5efc01) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #6 cfunction_call /home/hyongtao/worksapce/cpython/Objects/methodobject.c:564:18 (python3.15+0x2bab87) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #7 _PyObject_MakeTpCall /home/hyongtao/worksapce/cpython/Objects/call.c:242:18 (python3.15+0x20a68f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #8 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:134:16 (python3.15+0x20b2a7) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #9 PyObject_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c:327:12 (python3.15+0x20b2a7)
    #10 _Py_VectorCall_StackRefSteal /home/hyongtao/worksapce/cpython/Python/ceval.c:720:11 (python3.15+0x41fce9) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #11 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:3387:35 (python3.15+0x429c79) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #12 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f3b0) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #13 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f3b0)
    #14 PyEval_EvalCode /home/hyongtao/worksapce/cpython/Python/ceval.c:673:21 (python3.15+0x41f3b0)
    #15 run_eval_code_obj /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1366:12 (python3.15+0x4fe967) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #16 run_mod /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1469:19 (python3.15+0x4fe967)
    #17 pyrun_file /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1294:15 (python3.15+0x4f9d13) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #18 _PyRun_SimpleFileObject /home/hyongtao/worksapce/cpython/Python/pythonrun.c:518:13 (python3.15+0x4f9d13)
    #19 _PyRun_AnyFileObject /home/hyongtao/worksapce/cpython/Python/pythonrun.c:81:15 (python3.15+0x4f9498) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #20 pymain_run_file_obj /home/hyongtao/worksapce/cpython/Modules/main.c:410:15 (python3.15+0x53bedf) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #21 pymain_run_file /home/hyongtao/worksapce/cpython/Modules/main.c:429:15 (python3.15+0x53bedf)
    #22 pymain_run_python /home/hyongtao/worksapce/cpython/Modules/main.c:691:21 (python3.15+0x53b20b) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #23 Py_RunMain /home/hyongtao/worksapce/cpython/Modules/main.c:772:5 (python3.15+0x53b20b)
    #24 pymain_main /home/hyongtao/worksapce/cpython/Modules/main.c:802:12 (python3.15+0x53b778) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #25 Py_BytesMain /home/hyongtao/worksapce/cpython/Modules/main.c:826:12 (python3.15+0x53b7fb) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #26 main /home/hyongtao/worksapce/cpython/./Programs/python.c:15:12 (python3.15+0x17282b) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)

  Thread T3 'Thread-3 (reade' (tid=1703390, running) created by main thread at:
    #0 pthread_create <null> (python3.15+0xf421f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #1 do_start_joinable_thread /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:281:14 (python3.15+0x521424) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #2 PyThread_start_joinable_thread /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:323:9 (python3.15+0x52123a) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #3 ThreadHandle_start /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:474:9 (python3.15+0x5f109a) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #4 do_start_new_thread /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:1920:9 (python3.15+0x5f0b3f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #5 thread_PyThread_start_joinable_thread /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:2043:14 (python3.15+0x5efc01) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #6 cfunction_call /home/hyongtao/worksapce/cpython/Objects/methodobject.c:564:18 (python3.15+0x2bab87) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #7 _PyObject_MakeTpCall /home/hyongtao/worksapce/cpython/Objects/call.c:242:18 (python3.15+0x20a68f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #8 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:134:16 (python3.15+0x20b2a7) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #9 PyObject_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c:327:12 (python3.15+0x20b2a7)
    #10 _Py_VectorCall_StackRefSteal /home/hyongtao/worksapce/cpython/Python/ceval.c:720:11 (python3.15+0x41fce9) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #11 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:3387:35 (python3.15+0x429c79) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #12 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f3b0) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #13 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f3b0)
    #14 PyEval_EvalCode /home/hyongtao/worksapce/cpython/Python/ceval.c:673:21 (python3.15+0x41f3b0)
    #15 run_eval_code_obj /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1366:12 (python3.15+0x4fe967) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #16 run_mod /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1469:19 (python3.15+0x4fe967)
    #17 pyrun_file /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1294:15 (python3.15+0x4f9d13) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #18 _PyRun_SimpleFileObject /home/hyongtao/worksapce/cpython/Python/pythonrun.c:518:13 (python3.15+0x4f9d13)
    #19 _PyRun_AnyFileObject /home/hyongtao/worksapce/cpython/Python/pythonrun.c:81:15 (python3.15+0x4f9498) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #20 pymain_run_file_obj /home/hyongtao/worksapce/cpython/Modules/main.c:410:15 (python3.15+0x53bedf) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #21 pymain_run_file /home/hyongtao/worksapce/cpython/Modules/main.c:429:15 (python3.15+0x53bedf)
    #22 pymain_run_python /home/hyongtao/worksapce/cpython/Modules/main.c:691:21 (python3.15+0x53b20b) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #23 Py_RunMain /home/hyongtao/worksapce/cpython/Modules/main.c:772:5 (python3.15+0x53b20b)
    #24 pymain_main /home/hyongtao/worksapce/cpython/Modules/main.c:802:12 (python3.15+0x53b778) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #25 Py_BytesMain /home/hyongtao/worksapce/cpython/Modules/main.c:826:12 (python3.15+0x53b7fb) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #26 main /home/hyongtao/worksapce/cpython/./Programs/python.c:15:12 (python3.15+0x17282b) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)

SUMMARY: ThreadSanitizer: data race /home/hyongtao/worksapce/cpython/./Include/cpython/pyatomic_gcc.h:513:3 in _Py_atomic_store_ssize_relaxed
==================
CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs
  • gh-144357

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start in Objects/setobject.c at setiter_len() around line 1059, then compare it with the atomic update in set_discard_entry() around line 594. Run the provided tsan_setiter_len_race.py script with the free-threading ThreadSanitizer build; done means the concurrent length-hint and set-mutation workload no longer reports this race.

Written by the indexing model from the issue text.

Assessment

Tech stack
c, python
Domain
backend, operating-systems
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
52/100

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