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gh-154916: Fix data races in GenericAlias using critical sections #155634

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gh-154916: Fix data races in GenericAlias using critical sections #155634

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28 changes: 28 additions & 0 deletions Lib/test/test_free_threading/test_types.py
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,34 @@ def refresh():
*[refresh for _ in range(2)],
])

def test_getitem_parameters_race(self):
# gh-153298: ga_getitem() lazily initializes __parameters__;
# racing subscriptions must not race on the write or leak.
T = TypeVar('T')
for _ in range(100):
alias = list[T]

def subscribe():
self.assertEqual(alias[int], list[int])

threading_helper.run_concurrently(subscribe, nthreads=8)

def test_iter_next_reduce_race(self):
# gh-154916: next() clears the iterator's reference to the alias
# while __reduce__() reads it; the alias must not be freed in
# between (the iterator can hold the last reference).
def use(it):
it.__reduce__()
try:
next(it)
except StopIteration:
pass
it.__reduce__()

for _ in range(100):
it = iter(list[int])
threading_helper.run_concurrently(use, nthreads=8, args=(it,))


if __name__ == "__main__":
unittest.main()
43 changes: 29 additions & 14 deletions Objects/genericaliasobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@

#include "Python.h"
#include "pycore_ceval.h" // _PyEval_GetBuiltin()
#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION()
#include "pycore_modsupport.h" // _PyArg_NoKeywords()
#include "pycore_object.h"
#include "pycore_typevarobject.h" // _Py_typing_type_repr
Expand Down Expand Up @@ -578,19 +579,24 @@ PyDoc_STRVAR(genericalias__doc__,
"For example, for t = list[int], t.__origin__ is list and t.__args__\n"
"is (int,).");

static PyObject *
ga_parameters_lock_held(PyObject *self);

static PyObject *
ga_getitem(PyObject *self, PyObject *item)
{
gaobject *alias = (gaobject *)self;
// Populate __parameters__ if needed.
if (alias->parameters == NULL) {
alias->parameters = _Py_make_parameters(alias->args);
if (alias->parameters == NULL) {
return NULL;
}
PyObject *parameters;
Py_BEGIN_CRITICAL_SECTION(self);
parameters = ga_parameters_lock_held(self);
Py_END_CRITICAL_SECTION();
if (parameters == NULL) {
return NULL;
}

PyObject *newargs = _Py_subs_parameters(self, alias->args, alias->parameters, item);
PyObject *newargs = _Py_subs_parameters(self, alias->args, parameters, item);
Py_DECREF(parameters);
if (newargs == NULL) {
return NULL;
}
Expand Down Expand Up @@ -846,6 +852,7 @@ static PyMemberDef ga_members[] = {
static PyObject *
ga_parameters_lock_held(PyObject *self)
{
_Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
gaobject *alias = (gaobject *)self;
if (alias->parameters == NULL) {
alias->parameters = _Py_make_parameters(alias->args);
Expand Down Expand Up @@ -942,12 +949,11 @@ static PyObject *
ga_iternext(PyObject *op)
{
gaiterobject *gi = (gaiterobject*)op;
#ifdef Py_GIL_DISABLED
PyObject *obj = _Py_atomic_exchange_ptr(&gi->obj, NULL);
#else
PyObject* obj = gi->obj;
PyObject *obj;
Py_BEGIN_CRITICAL_SECTION(gi);
obj = gi->obj;
gi->obj = NULL;
#endif
Py_END_CRITICAL_SECTION();
if (obj == NULL) {
PyErr_SetNone(PyExc_StopIteration);
return NULL;
Expand Down Expand Up @@ -997,10 +1003,19 @@ ga_iter_reduce(PyObject *self, PyObject *Py_UNUSED(ignored))
* call must be before access of iterator pointers.
* see issue #101765 */

if (gi->obj)
return Py_BuildValue("N(O)", iter, gi->obj);
else
PyObject *obj;
Py_BEGIN_CRITICAL_SECTION(gi);
obj = Py_XNewRef(gi->obj);
Py_END_CRITICAL_SECTION();

if (obj) {
PyObject *result = Py_BuildValue("N(O)", iter, obj);
Py_DECREF(obj);
return result;
}
else {
return Py_BuildValue("N(())", iter);
}
}

static PyMethodDef ga_iter_methods[] = {
Expand Down
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