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GeezerVison is showing github.com — gh-146452: Fix pickle segfault on concurrent mutation of dict and list by overlorde · Pull Request #146470 · python/cpython

gh-146452: Fix pickle segfault on concurrent mutation of dict and list #146470

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Merged
kumaraditya303 merged 7 commits into python:main from overlorde:fix-issue-146452
May 17, 2026

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Merged

gh-146452: Fix pickle segfault on concurrent mutation of dict and list #146470

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8f55561
gh-146452: Fix pickle segfault when pickling dict with concurrent mut…
overlorde Mar 26, 2026
c4a8ee1
Merge branch 'main' into fix-issue-146452
overlorde Mar 27, 2026
3e57547
Apply suggestion from @kumaraditya303
kumaraditya303 May 17, 2026
6276525
change test to use threading_helper.start_threads
kumaraditya303 May 17, 2026
f83fe04
Merge branch 'main' into fix-issue-146452
kumaraditya303 May 17, 2026
62b5811
fix news
kumaraditya303 May 17, 2026
3d305ca
Merge branch 'fix-issue-146452' of https://github.com/overlorde/cpyth…
kumaraditya303 May 17, 2026
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44 changes: 44 additions & 0 deletions 44 Lib/test/test_free_threading/test_pickle.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,44 @@
import pickle
import threading
import unittest

from test.support import threading_helper


@threading_helper.requires_working_threading()
class TestPickleFreeThreading(unittest.TestCase):

def test_pickle_dumps_with_concurrent_dict_mutation(self):
# gh-146452: Pickling a dict while another thread mutates it
# used to segfault. batch_dict_exact() iterated dict items via
# PyDict_Next() which returns borrowed references, and a
# concurrent pop/replace could free the value before Py_INCREF
# got to it.
shared = {str(i): list(range(20)) for i in range(50)}

def dumper():
for _ in range(1000):
try:
pickle.dumps(shared)
except RuntimeError:
# "dictionary changed size during iteration" is expected
pass

def mutator():
for j in range(1000):
key = str(j % 50)
shared[key] = list(range(j % 20))
if j % 10 == 0:
shared.pop(key, None)
shared[key] = [j]

threads = []
for _ in range(10):
threads.append(threading.Thread(target=dumper))
threads.append(threading.Thread(target=mutator))

with threading_helper.start_threads(threads):
pass

if __name__ == "__main__":
unittest.main()
2 changes: 2 additions & 0 deletions 2 Misc/NEWS.d/next/Library/2026-03-26-09-30-00.gh-issue-146452.Y2N6qZ8J.rst
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Fix segfault in :mod:`pickle` when pickling a dictionary concurrently
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mutated by another thread in the free-threaded build.
14 changes: 13 additions & 1 deletion 14 Modules/_pickle.c
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Expand Up @@ -3452,7 +3452,7 @@ batch_dict(PickleState *state, PicklerObject *self, PyObject *iter, PyObject *or
* Note that this currently doesn't work for protocol 0.
*/
static int
batch_dict_exact(PickleState *state, PicklerObject *self, PyObject *obj)
batch_dict_exact_impl(PickleState *state, PicklerObject *self, PyObject *obj)
{
PyObject *key = NULL, *value = NULL;
int i;
Expand Down Expand Up @@ -3525,6 +3525,18 @@ batch_dict_exact(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)
{
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