simplify common type sharing code and fix ref-count inconsistency

Cython/Utility/CommonTypes.c

@@ -6,44 +6,25 @@ static PyTypeObject* __Pyx_FetchCommonType(PyTypeObject* type);
 
 static PyTypeObject* __Pyx_FetchCommonType(PyTypeObject* type) {
     static PyObject* fake_module = NULL;
-    PyObject* args = NULL;
     PyTypeObject* cached_type = NULL;
     const char* cython_module = "_cython_" CYTHON_ABI;
-    if (fake_module == NULL) {
-        PyObject* sys_modules = PyImport_GetModuleDict(); // borrowed
-        fake_module = PyDict_GetItemString(sys_modules, cython_module); // borrowed
-        if (fake_module != NULL) {
-            Py_INCREF(fake_module);
-        } else {
-            PyObject* py_cython_module;
-            args = PyTuple_New(1); if (args == NULL) goto bad;
-#if PY_MAJOR_VERSION >= 3
-            py_cython_module = PyUnicode_DecodeUTF8(cython_module, strlen(cython_module), NULL);
-#else
-            py_cython_module = PyBytes_FromString(cython_module);
-#endif
-            if (py_cython_module == NULL) goto bad;
-            PyTuple_SET_ITEM(args, 0, py_cython_module);
-            fake_module = PyObject_Call((PyObject*) &PyModule_Type, args, NULL);
-            if (PyDict_SetItemString(sys_modules, cython_module, fake_module) < 0)
-                goto bad;
-        }
+    if (!fake_module) {
+        fake_module = PyImport_AddModule(cython_module);
+        if (!fake_module) goto bad;
     }
 
-    if (PyObject_HasAttrString(fake_module, type->tp_name)) {
-        cached_type = (PyTypeObject*) PyObject_GetAttrString(fake_module, type->tp_name);
-    } else {
+    cached_type = (PyTypeObject*) PyObject_GetAttrString(fake_module, type->tp_name);
+    if (!cached_type) {
+        if (!PyErr_ExceptionMatches(PyExc_AttributeError)) goto bad;
+        PyErr_Clear();
         if (PyType_Ready(type) < 0) goto bad;
         if (PyObject_SetAttrString(fake_module, type->tp_name, (PyObject*) type) < 0)
             goto bad;
+        Py_INCREF(type);
         cached_type = type;
     }
 
-cleanup:
-    Py_XDECREF(args);
-    return cached_type;
-
 bad:
-    cached_type = NULL;
-    goto cleanup;
+    // NOTE: always returns owned reference, or NULL on error
+    return cached_type;
 }