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gh-146452: Improve locking granularity in pickle's batch_dict_exact #150025

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Merged
JelleZijlstra merged 1 commit into from
May 18, 2026
+31 −20
Merged

gh-146452: Improve locking granularity in pickle's batch_dict_exact #150025

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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.
49 changes: 29 additions & 20 deletions Modules/_pickle.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3450,9 +3450,12 @@ 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 @@ -3466,15 +3469,24 @@ batch_dict_exact_impl(PickleState *state, PicklerObject *self, PyObject *obj)
assert(self->proto > 0);

dict_size = PyDict_GET_SIZE(obj);
assert(dict_size);
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@JelleZijlstra JelleZijlstra May 18, 2026

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We discussed this in person. This assertion could fail if the dictionary gets mutated by another thread between the size check in the caller and this check. The code should work correctly in this case (it will just emit a SETITEMS that sets 0 items), so removing the assertion is the right change.


/* Write in batches of BATCHSIZE. */
Py_ssize_t total = 0;
do {
if (dict_size - total == 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 @@ -3492,9 +3504,18 @@ 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 @@ -3525,18 +3546,6 @@ 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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