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gh-153296: Fix thread-safety data race in io.StringIO iterator #153368

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gh-153296: Fix thread-safety data race in io.StringIO iterator #153368

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21fafe9
gh-153296: Fix thread-safety data race in io.StringIO iterator
Twix1288 Jul 8, 2026
8ba4fe6
gh-153296: Fix flaky test_rollover_at_midnight on Windows
Twix1288 Jul 8, 2026
1592fc9
Revert "gh-153296: Fix flaky test_rollover_at_midnight on Windows"
Twix1288 Jul 8, 2026
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2026-07-08-15-26-00.gh-issue-153296.Twix12.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix a data race and use-after-free when iterating over an :class:`io.StringIO` object while it is being concurrently mutated. The ``__next__`` method now properly acquires the object's lock.
14 changes: 13 additions & 1 deletion Modules/_io/stringio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -407,8 +407,10 @@ _io_StringIO_readline_impl(stringio *self, Py_ssize_t size)
}

static PyObject *
stringio_iternext(PyObject *op)
stringio_iternext_lock_held(PyObject *op)
{
_Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op);

PyObject *line;
stringio *self = stringio_CAST(op);

Expand Down Expand Up @@ -444,6 +446,16 @@ stringio_iternext(PyObject *op)
return line;
}

static PyObject *
stringio_iternext(PyObject *op)
{
PyObject *ret;
Py_BEGIN_CRITICAL_SECTION(op);
ret = stringio_iternext_lock_held(op);
Py_END_CRITICAL_SECTION();
return ret;
}

/*[clinic input]
@critical_section
_io.StringIO.truncate
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