Stop using None as the m_copy sentinel.

Author: ericsnowcurrently

Python/bltinmodule.c

@@ -3098,9 +3098,6 @@ _PyBuiltin_Init(PyInterpreterState *interp)
     }
     Py_DECREF(debug);
 
-    /* m_copy of Py_None means it is copied some other way. */
-    builtinsmodule.m_base.m_copy = Py_NewRef(Py_None);
-
     return mod;
 #undef ADD_TO_ALL
 #undef SETBUILTIN

Python/import.c

@@ -982,21 +982,27 @@ static PyObject *
 get_core_module_dict(PyInterpreterState *interp,
                      PyObject *name, PyObject *filename)
 {
-    if (filename != name && filename != NULL) {
-        /* It isn't a builtin module, which means it isn't core. */
-        return 0;
-    }
-    if (PyUnicode_CompareWithASCIIString(name, "sys") == 0) {
-        return interp->sysdict_copy;
-    }
-    assert(!PyErr_Occurred());
-    if (PyUnicode_CompareWithASCIIString(name, "builtins") == 0) {
-        return interp->builtins_copy;
+    /* Only builtin modules are core. */
+    if (filename == name) {
+        assert(!PyErr_Occurred());
+        if (PyUnicode_CompareWithASCIIString(name, "sys") == 0) {
+            return interp->sysdict_copy;
+        }
+        assert(!PyErr_Occurred());
+        if (PyUnicode_CompareWithASCIIString(name, "builtins") == 0) {
+            return interp->builtins_copy;
+        }
+        assert(!PyErr_Occurred());
     }
-    assert(!PyErr_Occurred());
     return 0;
 }
 
+static inline int
+is_core_module(PyInterpreterState *interp, PyObject *name, PyObject *filename)
+{
+    return get_core_module_dict(interp, name, filename) == NULL;
+}
+
 static int
 fix_up_extension(PyObject *mod, PyObject *name, PyObject *filename)
 {
@@ -1019,9 +1025,8 @@ fix_up_extension(PyObject *mod, PyObject *name, PyObject *filename)
     // bpo-44050: Extensions and def->m_base.m_copy can be updated
     // when the extension module doesn't support sub-interpreters.
     // XXX Why special-case the main interpreter?
-    if (_Py_IsMainInterpreter(tstate->interp) || def->m_size == -1) {
-        /* m_copy of Py_None means it is copied some other way. */
-        if (def->m_size == -1 && def->m_base.m_copy != Py_None) {
+    if (def->m_size == -1) {
+        if (!is_core_module(tstate->interp, name, filename)) {
             if (def->m_base.m_copy) {
                 /* Somebody already imported the module,
                    likely under a different name.
@@ -1037,7 +1042,9 @@ fix_up_extension(PyObject *mod, PyObject *name, PyObject *filename)
                 return -1;
             }
         }
+    }
 
+    if (_Py_IsMainInterpreter(tstate->interp) || def->m_size == -1) {
         if (_extensions_cache_set(filename, name, def) < 0) {
             return -1;
         }
@@ -1078,12 +1085,8 @@ import_find_extension(PyThreadState *tstate, PyObject *name,
         PyObject *m_copy = def->m_base.m_copy;
         /* Module does not support repeated initialization */
         if (m_copy == NULL) {
-            return NULL;
-        }
-        else if (m_copy == Py_None) {
             m_copy = get_core_module_dict(tstate->interp, name, filename);
             if (m_copy == NULL) {
-                _PyErr_SetString(tstate, PyExc_ImportError, "missing m_copy");
                 return NULL;
             }
         }

Python/sysmodule.c

@@ -3425,9 +3425,6 @@ _PySys_Create(PyThreadState *tstate, PyObject **sysmod_p)
         return _PyStatus_ERR("failed to create a module object");
     }
 
-    /* m_copy of Py_None means it is copied some other way. */
-    sysmodule.m_base.m_copy = Py_NewRef(Py_None);
-
     PyObject *sysdict = PyModule_GetDict(sysmod);
     if (sysdict == NULL) {
         goto error;