Disable GC during hacked calls to PyType_Ready() that simulate a heap type for...

Disable GC during hacked calls to PyType_Ready() that simulate a heap type for an actual non-heap type, because it can lead to crashes if GC visits such a hacked type.
Closes https://github.com/cython/cython/issues/3603

Cython/Utility/ExtensionTypes.c

@@ -55,6 +55,31 @@ static int __Pyx_PyType_Ready(PyTypeObject *t) {
     }
 
 #if PY_VERSION_HEX >= 0x03050000
+    {
+        // Make sure GC does not pick up our non-heap type as heap type with this hack!
+        PyObject *ret, *py_status;
+        int gc_was_enabled;
+        PyObject *gc = PyImport_Import(PYUNICODE("gc"));
+        if (unlikely(!gc)) return -1;
+        py_status = PyObject_CallMethodObjArgs(gc, PYUNICODE("isenabled"), NULL);
+        if (unlikely(!py_status)) {
+            Py_DECREF(gc);
+            return -1;
+        }
+        gc_was_enabled = __Pyx_PyObject_IsTrue(py_status);
+        Py_DECREF(py_status);
+        if (gc_was_enabled > 0) {
+            ret = PyObject_CallMethodObjArgs(gc, PYUNICODE("disable"), NULL);
+            if (unlikely(!ret)) {
+                Py_DECREF(gc);
+                return -1;
+            }
+            Py_DECREF(ret);
+        } else if (unlikely(gc_was_enabled == -1)) {
+            Py_DECREF(gc);
+            return -1;
+        }
+
         // As of https://bugs.python.org/issue22079
         // PyType_Ready enforces that all bases of a non-heap type are
         // non-heap. We know that this is the case for the solid base but
@@ -69,6 +94,25 @@ static int __Pyx_PyType_Ready(PyTypeObject *t) {
 
 #if PY_VERSION_HEX >= 0x03050000
         t->tp_flags &= ~Py_TPFLAGS_HEAPTYPE;
+
+        if (gc_was_enabled) {
+            PyObject *t, *v, *tb;
+            PyErr_Fetch(&t, &v, &tb);
+            ret = PyObject_CallMethodObjArgs(gc, PYUNICODE("enable"), NULL);
+            if (likely(ret || r == -1)) {
+                Py_XDECREF(ret);
+                // do not overwrite exceptions raised by PyType_Ready() above
+                PyErr_Restore(t, v, tb);
+            } else {
+                // PyType_Ready() succeeded, but gc.enable() failed.
+                Py_XDECREF(t);
+                Py_XDECREF(v);
+                Py_XDECREF(tb);
+                r = -1;
+            }
+        }
+        Py_DECREF(gc);
+    }
 #endif
 
     return r;