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gh-148268: Avoid debug assertion in specialized list compares #148309

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gh-148268: Avoid debug assertion in specialized list compares
officialasishkumar Apr 9, 2026
338abd4
Merge branch 'main' into gh-148268-list-sort-debug-assert
officialasishkumar Apr 10, 2026
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gh-148268: Avoid debug assertion in specialized list compares 
Handle recursion errors raised by the debug-only cross-check in the specialized latin, compact-int, and float comparison helpers used by list sorting. When the verification path hits the C stack overflow guard, clear that exception so the optimized comparison remains side-effect free.\n\nThe change preserves the consistency assertion when the fallback comparison succeeds and adds a NEWS entry for the fix.
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officialasishkumar committed Apr 9, 2026
commit 25cb72c70fb073ba80c4c7421154cb26dda3e07d
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-04-09-18-30-00.gh-issue-148268.7cF3yM.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Avoid a debug-build assertion in list sorting when its specialized comparison
cross-check hits the C stack overflow guard.
26 changes: 23 additions & 3 deletions Objects/listobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2810,6 +2810,26 @@ unsafe_object_compare(PyObject *v, PyObject *w, MergeState *ms)
return res;
}

#ifndef NDEBUG
static void
assert_compare_consistent(PyObject *v, PyObject *w, int res)
{
int cmp = PyObject_RichCompareBool(v, w, Py_LT);
if (cmp < 0) {
/* This debug-only cross-check can hit the C stack guard even when
* the specialized comparison itself is safe. Keep the optimized path
* side-effect free by discarding that exception.
*/
assert(PyErr_ExceptionMatches(PyExc_RecursionError));
PyErr_Clear();
return;
}
assert(res == cmp);
}
#else
# define assert_compare_consistent(v, w, res) ((void)0)
#endif

/* Latin string compare: safe for any two latin (one byte per char) strings. */
static int
unsafe_latin_compare(PyObject *v, PyObject *w, MergeState *ms)
Expand All @@ -2830,7 +2850,7 @@ unsafe_latin_compare(PyObject *v, PyObject *w, MergeState *ms)
res < 0 :
PyUnicode_GET_LENGTH(v) < PyUnicode_GET_LENGTH(w));

assert(res == PyObject_RichCompareBool(v, w, Py_LT));;
assert_compare_consistent(v, w, res);
return res;
}

Expand All @@ -2855,7 +2875,7 @@ unsafe_long_compare(PyObject *v, PyObject *w, MergeState *ms)
w0 = _PyLong_CompactValue(wl);

res = v0 < w0;
assert(res == PyObject_RichCompareBool(v, w, Py_LT));
assert_compare_consistent(v, w, res);
return res;
}

Expand All @@ -2870,7 +2890,7 @@ unsafe_float_compare(PyObject *v, PyObject *w, MergeState *ms)
assert(Py_IS_TYPE(w, &PyFloat_Type));

res = PyFloat_AS_DOUBLE(v) < PyFloat_AS_DOUBLE(w);
assert(res == PyObject_RichCompareBool(v, w, Py_LT));
assert_compare_consistent(v, w, res);
return res;
}

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