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Add PyUnstable_TryIncref and PyUnstable_EnableTryIncRef.#159

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Merged
vstinner merged 4 commits intopython:mainfromcolesbury:try_incref
Nov 5, 2025
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75 changes: 75 additions & 0 deletionspythoncapi_compat.h
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Original file line numberDiff line numberDiff line change
Expand Up@@ -2231,6 +2231,81 @@ static inline int PyUnstable_Object_IsUniquelyReferenced(PyObject *obj)
}
#endif

// gh-128926 added PyUnstable_TryIncRef() and PyUnstable_EnableTryIncRef() to
// Python 3.14.0a5. Adapted from _Py_TryIncref() and _PyObject_SetMaybeWeakref().
#if PY_VERSION_HEX < 0x030E00A5
static inline int PyUnstable_TryIncRef(PyObject *op)
{
#ifndef Py_GIL_DISABLED
if (Py_REFCNT(op) > 0) {
Py_INCREF(op);
return 1;
}
return 0;
#else
// _Py_TryIncrefFast()
uint32_t local = _Py_atomic_load_uint32_relaxed(&op->ob_ref_local);
local += 1;
if (local == 0) {
// immortal
return 1;
}
if (_Py_IsOwnedByCurrentThread(op)) {
_Py_INCREF_STAT_INC();
_Py_atomic_store_uint32_relaxed(&op->ob_ref_local, local);
#ifdef Py_REF_DEBUG
_Py_INCREF_IncRefTotal();
#endif
return 1;
}

// _Py_TryIncRefShared()
Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&op->ob_ref_shared);
for (;;) {
// If the shared refcount is zero and the object is either merged
// or may not have weak references, then we cannot incref it.
if (shared == 0 || shared == _Py_REF_MERGED) {
return 0;
}

if (_Py_atomic_compare_exchange_ssize(
&op->ob_ref_shared,
&shared,
shared + (1 << _Py_REF_SHARED_SHIFT))) {
#ifdef Py_REF_DEBUG
_Py_INCREF_IncRefTotal();
#endif
_Py_INCREF_STAT_INC();
return 1;
}
}
#endif
}

static inline void PyUnstable_EnableTryIncRef(PyObject *op)
{
#ifdef Py_GIL_DISABLED
// _PyObject_SetMaybeWeakref()
if (_Py_IsImmortal(op)) {
return;
}
for (;;) {
Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&op->ob_ref_shared);
if ((shared & _Py_REF_SHARED_FLAG_MASK) != 0) {
// Nothing to do if it's in WEAKREFS, QUEUED, or MERGED states.
return;
}
if (_Py_atomic_compare_exchange_ssize(
&op->ob_ref_shared, &shared, shared | _Py_REF_MAYBE_WEAKREF)) {
return;
}
}
#else
(void)op; // unused argument
#endif
}
#endif


#if PY_VERSION_HEX < 0x030F0000
static inline PyObject*
Expand Down
48 changes: 48 additions & 0 deletionstests/test_pythoncapi_compat_cext.c
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Original file line numberDiff line numberDiff line change
Expand Up@@ -2448,6 +2448,46 @@ test_tuple(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
return test_tuple_fromarray();
}

// Test adapted from CPython's _testcapi/object.c
static int TryIncref_dealloc_called = 0;

static void
TryIncref_dealloc(PyObject *op)
{
// PyUnstable_TryIncRef should return 0 if object is being deallocated
assert(Py_REFCNT(op) == 0);
assert(!PyUnstable_TryIncRef(op));
assert(Py_REFCNT(op) == 0);

TryIncref_dealloc_called++;
Py_TYPE(op)->tp_free(op);
}

static PyTypeObject TryIncrefType;

static PyObject*
test_try_incref(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
{
TryIncref_dealloc_called = 0;

PyObject *obj = PyObject_New(PyObject, &TryIncrefType);
if (obj == _Py_NULL) {
return _Py_NULL;
}

PyUnstable_EnableTryIncRef(obj);

Py_ssize_t refcount = Py_REFCNT(obj);
assert(PyUnstable_TryIncRef(obj));
assert(Py_REFCNT(obj) == refcount + 1);

Py_DECREF(obj);
Py_DECREF(obj);

assert(TryIncref_dealloc_called == 1);
Py_RETURN_NONE;
}


static struct PyMethodDef methods[] = {
{"test_object", test_object, METH_NOARGS, _Py_NULL},
Expand DownExpand Up@@ -2504,6 +2544,7 @@ static struct PyMethodDef methods[] = {
{"test_uniquely_referenced", test_uniquely_referenced, METH_NOARGS, _Py_NULL},
{"test_byteswriter", test_byteswriter, METH_NOARGS, _Py_NULL},
{"test_tuple", test_tuple, METH_NOARGS, _Py_NULL},
{"test_try_incref", test_try_incref, METH_NOARGS, _Py_NULL},
{_Py_NULL, _Py_NULL, 0, _Py_NULL}
};

Expand DownExpand Up@@ -2532,6 +2573,13 @@ module_exec(PyObject *module)
return -1;
}
#endif
TryIncrefType.tp_name = "TryIncrefType";
TryIncrefType.tp_basicsize = sizeof(PyObject);
TryIncrefType.tp_dealloc = TryIncref_dealloc;
TryIncrefType.tp_free = PyObject_Del;
if (PyType_Ready(&TryIncrefType) < 0) {
return -1;
}
return 0;
}

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