PEP 489: Multi-phase extension module initialization

Known limitations of the current implementation:

- documentation changes are incomplete
- there's a reference leak I haven't tracked down yet

The leak is most visible by running:

  ./python -m test -R3:3 test_importlib

However, you can also see it by running:

  ./python -X showrefcount

Importing the array or _testmultiphase modules, and
then deleting them from both sys.modules and the local
namespace shows significant increases in the total
number of active references each cycle. By contrast,
with _testcapi (which continues to use single-phase
initialisation) the global refcounts stabilise after
a couple of cycles.

Modules/xxmodule.c

@@ -334,26 +334,10 @@ static PyMethodDef xx_methods[] = {
 PyDoc_STRVAR(module_doc,
 "This is a template module just for instruction.");
 
-/* Initialization function for the module (*must* be called PyInit_xx) */
 
-
-static struct PyModuleDef xxmodule = {
-    PyModuleDef_HEAD_INIT,
-    "xx",
-    module_doc,
-    -1,
-    xx_methods,
-    NULL,
-    NULL,
-    NULL,
-    NULL
-};
-
-PyMODINIT_FUNC
-PyInit_xx(void)
+static int
+xx_exec(PyObject *m)
 {
-    PyObject *m = NULL;
-
     /* Due to cross platform compiler issues the slots must be filled
      * here. It's required for portability to Windows without requiring
      * C++. */
@@ -366,11 +350,6 @@ PyInit_xx(void)
     if (PyType_Ready(&Xxo_Type) < 0)
         goto fail;
 
-    /* Create the module and add the functions */
-    m = PyModule_Create(&xxmodule);
-    if (m == NULL)
-        goto fail;
-
     /* Add some symbolic constants to the module */
     if (ErrorObject == NULL) {
         ErrorObject = PyErr_NewException("xx.error", NULL, NULL);
@@ -389,8 +368,33 @@ PyInit_xx(void)
     if (PyType_Ready(&Null_Type) < 0)
         goto fail;
     PyModule_AddObject(m, "Null", (PyObject *)&Null_Type);
-    return m;
+    return 0;
  fail:
     Py_XDECREF(m);
-    return NULL;
+    return -1;
+}
+
+static struct PyModuleDef_Slot xx_slots[] = {
+    {Py_mod_exec, xx_exec},
+    {0, NULL},
+};
+
+static struct PyModuleDef xxmodule = {
+    PyModuleDef_HEAD_INIT,
+    "xx",
+    module_doc,
+    0,
+    xx_methods,
+    xx_slots,
+    NULL,
+    NULL,
+    NULL
+};
+
+/* Export function for the module (*must* be called PyInit_xx) */
+
+PyMODINIT_FUNC
+PyInit_xx(void)
+{
+    return PyModuleDef_Init(&xxmodule);
 }