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gh-116738: Make _suggestions module thread-safe #140321

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f9cef6b
gh-116738: Make _suggestions module thread-safe
yoney Oct 19, 2025
b6937c2
gh-116738: Remove module reference from NEWS
yoney Oct 19, 2025
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24 changes: 24 additions & 0 deletions Lib/test/test_free_threading/test_suggestions.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,24 @@
import unittest

from test.support import import_helper, threading_helper
from test.support.threading_helper import run_concurrently

suggestions = import_helper.import_module("_suggestions")

NTHREADS = 10


@threading_helper.requires_working_threading()
class SuggestionsTests(unittest.TestCase):
def test_generate_suggestions(self):
candidates = [str(i) for i in range(100)]

def worker():
_ = suggestions._generate_suggestions(candidates, "42")
candidates.clear()

run_concurrently(worker_func=worker, nthreads=NTHREADS)


if __name__ == "__main__":
unittest.main()
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-10-18-19-52-20.gh-issue-116738.NLJW0L.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Make _suggestions module thread-safe on the :term:`free threaded <free
threading>` build.
5 changes: 3 additions & 2 deletions Modules/_suggestions.c
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@ module _suggestions
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=e58d81fafad5637b]*/

/*[clinic input]
@critical_section candidates
_suggestions._generate_suggestions
candidates: object
item: unicode
Expand All @@ -18,7 +19,7 @@ Returns the candidate in candidates that's closest to item
static PyObject *
_suggestions__generate_suggestions_impl(PyObject *module,
PyObject *candidates, PyObject *item)
/*[clinic end generated code: output=79be7b653ae5e7ca input=ba2a8dddc654e33a]*/
/*[clinic end generated code: output=79be7b653ae5e7ca input=92861a6c9bd8f667]*/
{
// Check if dir is a list
if (!PyList_CheckExact(candidates)) {
Expand All @@ -29,7 +30,7 @@ _suggestions__generate_suggestions_impl(PyObject *module,
// Check if all elements in the list are Unicode
Py_ssize_t size = PyList_Size(candidates);
for (Py_ssize_t i = 0; i < size; ++i) {
PyObject *elem = PyList_GetItem(candidates, i);
PyObject *elem = PyList_GET_ITEM(candidates, i);
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How about using PyList_GetItemRef with this change.

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@corona10, Thank you so much for your comment!

Just to make sure I understand your suggestion correctly, did you mean:

  1. To use PyList_GetItemRef() instead of PyList_GET_ITEM() within the current critical_section?
  2. Or to remove the critical_section and use PyList_GetItemRef() in both loops I mentioned in the description?

Regarding 1: I replaced PyList_GetItem() with PyList_GET_ITEM() because, in the with-GIL build, it’s already assumed that PyList_GetItem() cannot fail, so the returned value isn't checked. Within the critical_section, the assumption is that the candidates list cannot be mutated, and PyList_GET_ITEM() avoids extra atomic operations.

Regarding 2: I think this could be implemented with PyList_GetItemRef() and no critical_section, as long as errors returned from PyList_GetItemRef() are properly handled. However, in my opinion, using a critical_section makes the code a bit easier to read and reason about, since the candidates list cannot be mutated in this context. This means we don’t need to consider update cases between blocks or iterations.

What do you think, would it be better to use PyList_GetItemRef() here?

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@corona10 corona10 Oct 20, 2025 •
edited
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@yoney
Hmm, even if you use PyList_GET_ITEM, it won’t check whether the returned value is NULL, so it won’t actually fix the UB you mentioned. What do you think?

cpython/Include/cpython/listobject.h

Line 40 in f11ec6e

#define PyList_GET_ITEM(op, index) (_PyList_CAST(op)->ob_item[(index)])

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@corona10
Sorry, my previous comment wasn't clear before. I was referring to cases where PyList_GetItem() could fail due to PyList_Check() or valid_index(), particularly in the free-threading build. I was trying to explain why we need a critical_section.

cpython/Objects/listobject.c

Lines 384 to 397 in f11ec6e

PyObject *
PyList_GetItem(PyObject *op, Py_ssize_t i)
{
if (!PyList_Check(op)) {
PyErr_BadInternalCall();
return NULL;
}
if (!valid_index(i, Py_SIZE(op))) {
_Py_DECLARE_STR(list_err, "list index out of range");
PyErr_SetObject(PyExc_IndexError, &_Py_STR(list_err));
return NULL;
}
return ((PyListObject *)op) -> ob_item[i];
}

These properties are checked in _suggestions__generate_suggestions_impl() earlier, before calling PyList_GetItem(), and the GIL or the critical_section "protects" them. That's why I replaced PyList_GetItem() with PyList_GET_ITEM(). However, the actual element of the list could still be NULL. I had assumed this wasn’t possible, but should we check if the element is NULL?

if (!PyUnicode_Check(elem)) {
PyErr_SetString(PyExc_TypeError, "all elements in 'candidates' must be strings");
return NULL;
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5 changes: 4 additions & 1 deletion Modules/clinic/_suggestions.c.h
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