gh-133073: avoid NULL + 0 arithmetic in list_extend_* functions (#133074)

Author: picnixz

Objects/listobject.c

@@ -1315,9 +1315,15 @@ list_extend_set(PyListObject *self, PySetObject *other)
 {
     Py_ssize_t m = Py_SIZE(self);
     Py_ssize_t n = PySet_GET_SIZE(other);
-    if (list_resize(self, m + n) < 0) {
+    Py_ssize_t r = m + n;
+    if (r == 0) {
+        return 0;
+    }
+    if (list_resize(self, r) < 0) {
         return -1;
     }
+
+    assert(self->ob_item != NULL);
     /* populate the end of self with iterable's items */
     Py_ssize_t setpos = 0;
     Py_hash_t hash;
@@ -1327,7 +1333,7 @@ list_extend_set(PyListObject *self, PySetObject *other)
         FT_ATOMIC_STORE_PTR_RELEASE(*dest, key);
         dest++;
     }
-    Py_SET_SIZE(self, m + n);
+    Py_SET_SIZE(self, r);
     return 0;
 }
 
@@ -1337,10 +1343,15 @@ list_extend_dict(PyListObject *self, PyDictObject *dict, int which_item)
     // which_item: 0 for keys and 1 for values
     Py_ssize_t m = Py_SIZE(self);
     Py_ssize_t n = PyDict_GET_SIZE(dict);
-    if (list_resize(self, m + n) < 0) {
+    Py_ssize_t r = m + n;
+    if (r == 0) {
+        return 0;
+    }
+    if (list_resize(self, r) < 0) {
         return -1;
     }
 
+    assert(self->ob_item != NULL);
     PyObject **dest = self->ob_item + m;
     Py_ssize_t pos = 0;
     PyObject *keyvalue[2];
@@ -1351,7 +1362,7 @@ list_extend_dict(PyListObject *self, PyDictObject *dict, int which_item)
         dest++;
     }
 
-    Py_SET_SIZE(self, m + n);
+    Py_SET_SIZE(self, r);
     return 0;
 }
 
@@ -1360,10 +1371,15 @@ list_extend_dictitems(PyListObject *self, PyDictObject *dict)
 {
     Py_ssize_t m = Py_SIZE(self);
     Py_ssize_t n = PyDict_GET_SIZE(dict);
-    if (list_resize(self, m + n) < 0) {
+    Py_ssize_t r = m + n;
+    if (r == 0) {
+        return 0;
+    }
+    if (list_resize(self, r) < 0) {
         return -1;
     }
 
+    assert(self->ob_item != NULL);
     PyObject **dest = self->ob_item + m;
     Py_ssize_t pos = 0;
     Py_ssize_t i = 0;
@@ -1379,7 +1395,7 @@ list_extend_dictitems(PyListObject *self, PyDictObject *dict)
         i++;
     }
 
-    Py_SET_SIZE(self, m + n);
+    Py_SET_SIZE(self, r);
     return 0;
 }