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Sharing a memoryview iterator across threads can abort free-threaded CPython #159760
Description
On a free-threaded debug build, concurrently exhausting a memoryview iterator can abort with a negative refcount. The exporter is not mutated or explicitly released.
import array
import threading
N = 20_000
shared = iter(memoryview(array.array("I", range(N))))
results = [[] for _ in range(4)]
start = threading.Barrier(4)
def worker(i):
start.wait()
results[i].extend(shared)
threads = [threading.Thread(target=worker, args=(i,)) for i in range(4)]
for thread in threads:
thread.start()
for thread in threads:
thread.join()
values = [v for result in results for v in result]
print("count:", len(values), "distinct:", len(set(values)))Run PYTHON_GIL=0 ./python.exe repro.py repeatedly to trigger the following crash:
Python/brc.c:69: _Py_NegativeRefcount: Assertion failed: object has negative ref count Enable tracemalloc to get the memory block allocation traceback object address : 0x2000296d670 object refcount : 0 object type : 0x1038bc060 object type name: memoryview object repr : <refcnt 0 at 0x2000296d670> Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed Python runtime state: initialized Stack (most recent call first): File "/Users/goldbaum/Documents/test.py", line 11 in worker File "/Users/goldbaum/.pyenv/versions/3.16t-dev-debug/lib/python3.16t/threading.py", line 1160 in run File "/Users/goldbaum/.pyenv/versions/3.16t-dev-debug/lib/python3.16t/threading.py", line 1218 in _bootstrap_inner File "/Users/goldbaum/.pyenv/versions/3.16t-dev-debug/lib/python3.16t/threading.py", line 1180 in _bootstrap [1] 61221 abort python -Xfaulthandler test.py
With PYTHON_GIL=1 on the same binary I'm unable to trigger any crashes.
memoryiter_next() reads it_seq before clearing and decrementing it at exhaustion, so two threads can drop the iterator's single owning reference. @devdanzin reported this race earlier in a comment, with a separate reproducer and sanitizer results.
CPython versions tested on:
CPython main branch
Operating systems tested on:
macOS
Output from running 'python -VV' on the command line:
Python 3.16.0a1+dev free-threading build (main, Oct 19 2026, 11:39:38) [Clang 21.0.0 (clang-2100.3.34.2)]
Activity
ZeroIntensity commented on Oct 20, 2026
Possibly a duplicate of #127716?
ngoldbaum commented on Oct 20, 2026
I guess that's fair but I was hoping we could merge Neil's PR more or less as-is and leave this behind as a followup.