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gh-151640: avoid sharing BytesIO buffer in free-threaded builds #151651

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gh-151640: avoid sharing BytesIO buffer in free-threaded builds #151651

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aff39e2
gh-151640: avoid sharing BytesIO buffer in free-threaded builds
pedramkarimii Jun 18, 2026
96dfcb7
gh-151640: add NEWS entry
pedramkarimii Jun 18, 2026
918fb1c
gh-151640: fix NEWS entry filename
pedramkarimii Jun 18, 2026
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27 changes: 27 additions & 0 deletions Lib/test/test_free_threading/test_io.py
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Original file line number Diff line number Diff line change
Expand Up @@ -117,6 +117,33 @@ def sizeof(barrier, b, *ignore):
class CBytesIOTest(ThreadSafetyMixin, TestCase):
ioclass = io.BytesIO

@threading_helper.requires_working_threading()
@threading_helper.reap_threads
def test_concurrent_whole_buffer_read_and_resize(self):
shared = self.ioclass(b"x" * 64)
writers = 2
readers = 8
loops = 2000
barrier = threading.Barrier(writers + readers)

def writer():
barrier.wait()
for i in range(loops):
shared.seek(0)
shared.write(b"a" * (64 + (i & 63)))

def reader():
barrier.wait()
for _ in range(loops):
shared.seek(0)
shared.read()
shared.getvalue()

threads = [threading.Thread(target=writer) for _ in range(writers)]
threads += [threading.Thread(target=reader) for _ in range(readers)]
with threading_helper.start_threads(threads):
pass

class PyBytesIOTest(ThreadSafetyMixin, TestCase):
ioclass = pyio.BytesIO

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-06-18-12-48-03.gh-issue-151640.R4c3Fx.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix a data race in :class:`io.BytesIO` in free-threaded builds when
whole-buffer reads or :meth:`~io.BytesIO.getvalue` share the internal buffer
with concurrent writes.
7 changes: 7 additions & 0 deletions Modules/_io/bytesio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -371,6 +371,10 @@ _io_BytesIO_getvalue_impl(bytesio *self)
/*[clinic end generated code: output=b3f6a3233c8fd628 input=c91bff398df0c352]*/
{
CHECK_CLOSED(self);
#ifdef Py_GIL_DISABLED
return PyBytes_FromStringAndSize(PyBytes_AS_STRING(self->buf),
self->string_size);
#else
if (self->string_size <= 1 || FT_ATOMIC_LOAD_SSIZE_RELAXED(self->exports) > 0)
return PyBytes_FromStringAndSize(PyBytes_AS_STRING(self->buf),
self->string_size);
Expand All @@ -386,6 +390,7 @@ _io_BytesIO_getvalue_impl(bytesio *self)
}
}
return Py_NewRef(self->buf);
#endif
}

/*[clinic input]
Expand Down Expand Up @@ -429,12 +434,14 @@ read_bytes_lock_held(bytesio *self, Py_ssize_t size)

assert(self->buf != NULL);
assert(size <= self->string_size);
#ifndef Py_GIL_DISABLED
if (size > 1 &&
self->pos == 0 && size == PyBytes_GET_SIZE(self->buf) &&
FT_ATOMIC_LOAD_SSIZE_RELAXED(self->exports) == 0) {
self->pos += size;
return Py_NewRef(self->buf);
}
#endif

/* gh-141311: Avoid undefined behavior when self->pos (limit PY_SSIZE_T_MAX)
is beyond the size of self->buf. Assert above validates size is always in
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