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[3.14] gh-146452: Improve locking granularity in pickle's batch_dict_… #150062

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[3.14] gh-146452: Improve locking granularity in pickle's batch_dict_… #150062

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-05-18-15-30-34.gh-issue-146452.RM0EVJ.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix race condition when pickling dictionaries in free threaded builds. Also
reduce critical section cover.
48 changes: 29 additions & 19 deletions Modules/_pickle.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3351,9 +3351,12 @@ batch_dict(PickleState *state, PicklerObject *self, PyObject *iter, PyObject *or
* Returns 0 on success, -1 on error.
*
* Note that this currently doesn't work for protocol 0.

* gh-146452: Wrap the dict iteration in a critical sections to prevent
* concurrent mutation from invalidating PyDict_Next() iteration state.
*/
static int
batch_dict_exact_impl(PickleState *state, PicklerObject *self, PyObject *obj)
batch_dict_exact(PickleState *state, PicklerObject *self, PyObject *obj)
{
PyObject *key = NULL, *value = NULL;
int i;
Expand All @@ -3370,9 +3373,19 @@ batch_dict_exact_impl(PickleState *state, PicklerObject *self, PyObject *obj)

/* Special-case len(d) == 1 to save space. */
if (dict_size == 1) {
PyDict_Next(obj, &ppos, &key, &value);
Py_INCREF(key);
Py_INCREF(value);
int next;
Py_BEGIN_CRITICAL_SECTION(obj);
next = PyDict_Next(obj, &ppos, &key, &value);
if (next) {
Py_INCREF(key);
Py_INCREF(value);
}
Py_END_CRITICAL_SECTION();
if (!next) {
PyErr_SetString(PyExc_RuntimeError,
"dictionary changed size during iteration");
goto error;
}
if (save(state, self, key, 0) < 0) {
goto error;
}
Expand All @@ -3392,9 +3405,18 @@ batch_dict_exact_impl(PickleState *state, PicklerObject *self, PyObject *obj)
i = 0;
if (_Pickler_Write(self, &mark_op, 1) < 0)
return -1;
while (PyDict_Next(obj, &ppos, &key, &value)) {
Py_INCREF(key);
Py_INCREF(value);
int next;
while (1) {
Py_BEGIN_CRITICAL_SECTION(obj);
next = PyDict_Next(obj, &ppos, &key, &value);
if (next) {
Py_INCREF(key);
Py_INCREF(value);
}
Py_END_CRITICAL_SECTION();
if (!next) {
break;
}
if (save(state, self, key, 0) < 0) {
goto error;
}
Expand Down Expand Up @@ -3424,18 +3446,6 @@ batch_dict_exact_impl(PickleState *state, PicklerObject *self, PyObject *obj)
return -1;
}

/* gh-146452: Wrap the dict iteration in a critical section to prevent
concurrent mutation from invalidating PyDict_Next() iteration state. */
static int
batch_dict_exact(PickleState *state, PicklerObject *self, PyObject *obj)
{
int ret;
Py_BEGIN_CRITICAL_SECTION(obj);
ret = batch_dict_exact_impl(state, self, obj);
Py_END_CRITICAL_SECTION();
return ret;
}

static int
save_dict(PickleState *state, PicklerObject *self, PyObject *obj)
{
Expand Down
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