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gh-155606: Increment the managed buffer export count atomically #155882

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gh-155606: Increment the managed buffer export count atomically #155882

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36 changes: 36 additions & 0 deletions Lib/test/test_free_threading/test_memoryview.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,36 @@
import threading
import unittest

from test.support import threading_helper


@threading_helper.requires_working_threading()
class TestMemoryViewSliceRace(unittest.TestCase):
def test_concurrent_slicing_keeps_export_count(self):
# gh-155606: slicing registers a new view on the shared managed buffer,
# and mbuf_add_view() bumped that buffer's export count with a plain
# ++. Concurrent slices of a single memoryview therefore lost
# increments, the count reached zero while views were still alive, and
# the underlying buffer was released early.
#
# The slices are created concurrently but only dropped afterwards, on
# one thread, so this covers the increment on its own.
mv = memoryview(bytes(2 ** 16))
slices = []
lock = threading.Lock()

def make_slices():
local = [mv[0:64] for _ in range(2000)]
with lock:
slices.extend(local)

threading_helper.run_concurrently(make_slices, nthreads=8)
del slices

# An early release makes this raise "operation forbidden on released
# memoryview object".
self.assertEqual(bytes(mv[0:4]), b"\x00" * 4)


if __name__ == "__main__":
unittest.main()
5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-08-15-16-20-11.gh-issue-155606.Kv3Nqp.rst
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@@ -0,0 +1,5 @@
Fix a data race on free-threaded builds where slicing a shared
:class:`memoryview` from several threads could release the underlying buffer
while views were still alive, raising ``ValueError: operation forbidden on
released memoryview object``. The managed buffer's export count is now
incremented atomically.
4 changes: 2 additions & 2 deletions Objects/memoryobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -699,7 +699,7 @@ mbuf_add_view(_PyManagedBufferObject *mbuf, const Py_buffer *src)
init_flags(mv);

mv->mbuf = (_PyManagedBufferObject*)Py_NewRef(mbuf);
mbuf->exports++;
FT_ATOMIC_ADD_SSIZE(mbuf->exports, 1);

return (PyObject *)mv;
}
Expand Down Expand Up @@ -729,7 +729,7 @@ mbuf_add_incomplete_view(_PyManagedBufferObject *mbuf, const Py_buffer *src,
init_shared_values(dest, src);

mv->mbuf = (_PyManagedBufferObject*)Py_NewRef(mbuf);
mbuf->exports++;
FT_ATOMIC_ADD_SSIZE(mbuf->exports, 1);

return (PyObject *)mv;
}
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