gh-140061: Use _PyObject_IsUniquelyReferenced() to check if objects are uniquely referenced (gh-140062)

The previous `Py_REFCNT(x) == 1` checks can have data races in the free
threaded build. `_PyObject_IsUniquelyReferenced(x)` is a more conservative
check that is safe in the free threaded build and is identical to
`Py_REFCNT(x) == 1` in the default GIL-enabled build.

Author: sergey-miryanov

Misc/NEWS.d/next/Core_and_Builtins/2025-10-15-00-21-40.gh-issue-140061.J0XeDV.rst

@@ -0,0 +1,2 @@
+Fixing the checking of whether an object is uniquely referenced to ensure
+free-threaded compatibility. Patch by Sergey Miryanov.

Modules/_elementtree.c

@@ -912,7 +912,7 @@ deepcopy(elementtreestate *st, PyObject *object, PyObject *memo)
         return Py_NewRef(object);
     }
 
-    if (Py_REFCNT(object) == 1) {
+    if (_PyObject_IsUniquelyReferenced(object)) {
         if (PyDict_CheckExact(object)) {
             PyObject *key, *value;
             Py_ssize_t pos = 0;
@@ -2794,8 +2794,9 @@ treebuilder_handle_data(TreeBuilderObject* self, PyObject* data)
         self->data = Py_NewRef(data);
     } else {
         /* more than one item; use a list to collect items */
-        if (PyBytes_CheckExact(self->data) && Py_REFCNT(self->data) == 1 &&
-            PyBytes_CheckExact(data) && PyBytes_GET_SIZE(data) == 1) {
+        if (PyBytes_CheckExact(self->data)
+            && _PyObject_IsUniquelyReferenced(self->data)
+            && PyBytes_CheckExact(data) && PyBytes_GET_SIZE(data) == 1) {
             /* XXX this code path unused in Python 3? */
             /* expat often generates single character data sections; handle
                the most common case by resizing the existing string... */

Modules/_functoolsmodule.c

@@ -291,7 +291,7 @@ partial_new(PyTypeObject *type, PyObject *args, PyObject *kw)
         if (kw == NULL) {
             pto->kw = PyDict_New();
         }
-        else if (Py_REFCNT(kw) == 1) {
+        else if (_PyObject_IsUniquelyReferenced(kw)) {
             pto->kw = Py_NewRef(kw);
         }
         else {
@@ -1076,7 +1076,7 @@ _functools_reduce_impl(PyObject *module, PyObject *func, PyObject *seq,
     for (;;) {
         PyObject *op2;
 
-        if (Py_REFCNT(args) > 1) {
+        if (!_PyObject_IsUniquelyReferenced(args)) {
             Py_DECREF(args);
             if ((args = PyTuple_New(2)) == NULL)
                 goto Fail;

Modules/itertoolsmodule.c

@@ -376,7 +376,7 @@ pairwise_next(PyObject *op)
     }
 
     result = po->result;
-    if (Py_REFCNT(result) == 1) {
+    if (_PyObject_IsUniquelyReferenced(result)) {
         Py_INCREF(result);
         PyObject *last_old = PyTuple_GET_ITEM(result, 0);
         PyObject *last_new = PyTuple_GET_ITEM(result, 1);
@@ -802,7 +802,7 @@ teedataobject_traverse(PyObject *op, visitproc visit, void * arg)
 static void
 teedataobject_safe_decref(PyObject *obj)
 {
-    while (obj && Py_REFCNT(obj) == 1) {
+    while (obj && _PyObject_IsUniquelyReferenced(obj)) {
         teedataobject *tmp = teedataobject_CAST(obj);
         PyObject *nextlink = tmp->nextlink;
         tmp->nextlink = NULL;
@@ -2129,7 +2129,7 @@ product_next_lock_held(PyObject *op)
         Py_ssize_t *indices = lz->indices;
 
         /* Copy the previous result tuple or re-use it if available */
-        if (Py_REFCNT(result) > 1) {
+        if (!_PyObject_IsUniquelyReferenced(result)) {
             PyObject *old_result = result;
             result = PyTuple_FromArray(_PyTuple_ITEMS(old_result), npools);
             if (result == NULL)
@@ -2364,7 +2364,7 @@ combinations_next_lock_held(PyObject *op)
         }
     } else {
         /* Copy the previous result tuple or re-use it if available */
-        if (Py_REFCNT(result) > 1) {
+        if (!_PyObject_IsUniquelyReferenced(result)) {
             PyObject *old_result = result;
             result = PyTuple_FromArray(_PyTuple_ITEMS(old_result), r);
             if (result == NULL)
@@ -2618,7 +2618,7 @@ cwr_next(PyObject *op)
         }
     } else {
         /* Copy the previous result tuple or re-use it if available */
-        if (Py_REFCNT(result) > 1) {
+        if (!_PyObject_IsUniquelyReferenced(result)) {
             PyObject *old_result = result;
             result = PyTuple_FromArray(_PyTuple_ITEMS(old_result), r);
             if (result == NULL)
@@ -2879,7 +2879,7 @@ permutations_next(PyObject *op)
             goto empty;
 
         /* Copy the previous result tuple or re-use it if available */
-        if (Py_REFCNT(result) > 1) {
+        if (!_PyObject_IsUniquelyReferenced(result)) {
             PyObject *old_result = result;
             result = PyTuple_FromArray(_PyTuple_ITEMS(old_result), r);
             if (result == NULL)
@@ -3847,7 +3847,7 @@ zip_longest_next(PyObject *op)
         return NULL;
     if (lz->numactive == 0)
         return NULL;
-    if (Py_REFCNT(result) == 1) {
+    if (_PyObject_IsUniquelyReferenced(result)) {
         Py_INCREF(result);
         for (i=0 ; i < tuplesize ; i++) {
             it = PyTuple_GET_ITEM(lz->ittuple, i);

Objects/bytesobject.c

@@ -3171,7 +3171,7 @@ PyBytes_Concat(PyObject **pv, PyObject *w)
         return;
     }
 
-    if (Py_REFCNT(*pv) == 1 && PyBytes_CheckExact(*pv)) {
+    if (_PyObject_IsUniquelyReferenced(*pv) && PyBytes_CheckExact(*pv)) {
         /* Only one reference, so we can resize in place */
         Py_ssize_t oldsize;
         Py_buffer wb;
@@ -3256,7 +3256,7 @@ _PyBytes_Resize(PyObject **pv, Py_ssize_t newsize)
         Py_DECREF(v);
         return 0;
     }
-    if (Py_REFCNT(v) != 1) {
+    if (!_PyObject_IsUniquelyReferenced(v)) {
         if (oldsize < newsize) {
             *pv = _PyBytes_FromSize(newsize, 0);
             if (*pv) {

Objects/dictobject.c

@@ -5667,22 +5667,10 @@ dictiter_iternext_threadsafe(PyDictObject *d, PyObject *self,
 
 #endif
 
-static bool
-has_unique_reference(PyObject *op)
-{
-#ifdef Py_GIL_DISABLED
-    return (_Py_IsOwnedByCurrentThread(op) &&
-            op->ob_ref_local == 1 &&
-            _Py_atomic_load_ssize_relaxed(&op->ob_ref_shared) == 0);
-#else
-    return Py_REFCNT(op) == 1;
-#endif
-}
-
 static bool
 acquire_iter_result(PyObject *result)
 {
-    if (has_unique_reference(result)) {
+    if (_PyObject_IsUniquelyReferenced(result)) {
         Py_INCREF(result);
         return true;
     }
@@ -5831,7 +5819,7 @@ dictreviter_iter_lock_held(PyDictObject *d, PyObject *self)
     }
     else if (Py_IS_TYPE(di, &PyDictRevIterItem_Type)) {
         result = di->di_result;
-        if (Py_REFCNT(result) == 1) {
+        if (_PyObject_IsUniquelyReferenced(result)) {
             PyObject *oldkey = PyTuple_GET_ITEM(result, 0);
             PyObject *oldvalue = PyTuple_GET_ITEM(result, 1);
             PyTuple_SET_ITEM(result, 0, Py_NewRef(key));

Objects/longobject.c

@@ -352,7 +352,7 @@ _PyLong_Negate(PyLongObject **x_p)
     PyLongObject *x;
 
     x = (PyLongObject *)*x_p;
-    if (Py_REFCNT(x) == 1) {
+    if (_PyObject_IsUniquelyReferenced((PyObject *)x)) {
          _PyLong_FlipSign(x);
         return;
     }
@@ -5849,7 +5849,7 @@ _PyLong_GCD(PyObject *aarg, PyObject *barg)
             assert(size_a >= 0);
             _PyLong_SetSignAndDigitCount(c, 1, size_a);
         }
-        else if (Py_REFCNT(a) == 1) {
+        else if (_PyObject_IsUniquelyReferenced((PyObject *)a)) {
             c = (PyLongObject*)Py_NewRef(a);
         }
         else {
@@ -5863,7 +5863,8 @@ _PyLong_GCD(PyObject *aarg, PyObject *barg)
             assert(size_a >= 0);
             _PyLong_SetSignAndDigitCount(d, 1, size_a);
         }
-        else if (Py_REFCNT(b) == 1 && size_a <= alloc_b) {
+        else if (_PyObject_IsUniquelyReferenced((PyObject *)b)
+                 && size_a <= alloc_b) {
             d = (PyLongObject*)Py_NewRef(b);
             assert(size_a >= 0);
             _PyLong_SetSignAndDigitCount(d, 1, size_a);

Objects/setobject.c

@@ -2444,7 +2444,7 @@ set_init(PyObject *so, PyObject *args, PyObject *kwds)
     if (!PyArg_UnpackTuple(args, Py_TYPE(self)->tp_name, 0, 1, &iterable))
         return -1;
 
-    if (Py_REFCNT(self) == 1 && self->fill == 0) {
+    if (_PyObject_IsUniquelyReferenced((PyObject *)self) && self->fill == 0) {
         self->hash = -1;
         if (iterable == NULL) {
             return 0;
@@ -2774,7 +2774,7 @@ int
 PySet_Add(PyObject *anyset, PyObject *key)
 {
     if (!PySet_Check(anyset) &&
-        (!PyFrozenSet_Check(anyset) || Py_REFCNT(anyset) != 1)) {
+        (!PyFrozenSet_Check(anyset) || !_PyObject_IsUniquelyReferenced(anyset))) {
         PyErr_BadInternalCall();
         return -1;
     }

Objects/tupleobject.c

@@ -118,7 +118,7 @@ int
 PyTuple_SetItem(PyObject *op, Py_ssize_t i, PyObject *newitem)
 {
     PyObject **p;
-    if (!PyTuple_Check(op) || Py_REFCNT(op) != 1) {
+    if (!PyTuple_Check(op) || !_PyObject_IsUniquelyReferenced(op)) {
         Py_XDECREF(newitem);
         PyErr_BadInternalCall();
         return -1;
@@ -923,7 +923,7 @@ _PyTuple_Resize(PyObject **pv, Py_ssize_t newsize)
 
     v = (PyTupleObject *) *pv;
     if (v == NULL || !Py_IS_TYPE(v, &PyTuple_Type) ||
-        (Py_SIZE(v) != 0 && Py_REFCNT(v) != 1)) {
+        (Py_SIZE(v) != 0 && !_PyObject_IsUniquelyReferenced(*pv))) {
         *pv = 0;
         Py_XDECREF(v);
         PyErr_BadInternalCall();

Python/marshal.c

@@ -11,6 +11,7 @@
 #include "pycore_code.h"             // _PyCode_New()
 #include "pycore_hashtable.h"        // _Py_hashtable_t
 #include "pycore_long.h"             // _PyLong_IsZero()
+#include "pycore_object.h"           // _PyObject_IsUniquelyReferenced
 #include "pycore_pystate.h"          // _PyInterpreterState_GET()
 #include "pycore_setobject.h"        // _PySet_NextEntryRef()
 #include "pycore_unicodeobject.h"    // _PyUnicode_InternImmortal()
@@ -388,7 +389,7 @@ w_ref(PyObject *v, char *flag, WFILE *p)
      * But we use TYPE_REF always for interned string, to PYC file stable
      * as possible.
      */
-    if (Py_REFCNT(v) == 1 &&
+    if (_PyObject_IsUniquelyReferenced(v) &&
             !(PyUnicode_CheckExact(v) && PyUnicode_CHECK_INTERNED(v))) {
         return 0;
     }