bpo-33583: Add note in PyObject_GC_Resize() doc (GH-7021)
diff --git a/Doc/c-api/gcsupport.rst b/Doc/c-api/gcsupport.rst
index f5e0d7e..7f54b6a 100644
--- a/Doc/c-api/gcsupport.rst
+++ b/Doc/c-api/gcsupport.rst
@@ -49,7 +49,7 @@
.. c:function:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)
Resize an object allocated by :c:func:`PyObject_NewVar`. Returns the
- resized object or *NULL* on failure.
+ resized object or *NULL* on failure. *op* must not be tracked by the collector yet.
.. c:function:: void PyObject_GC_Track(PyObject *op)
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index 8ba1093..09c7356 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -1752,6 +1752,7 @@
{
const size_t basicsize = _PyObject_VAR_SIZE(Py_TYPE(op), nitems);
PyGC_Head *g = AS_GC(op);
+ assert(!IS_TRACKED(op));
if (basicsize > PY_SSIZE_T_MAX - sizeof(PyGC_Head))
return (PyVarObject *)PyErr_NoMemory();
g = (PyGC_Head *)PyObject_REALLOC(g, sizeof(PyGC_Head) + basicsize);