ability to have permanent callbacks that stay active when new blend files are loaded.

this works by tagging functions, eg:

  def my_func(scene):
      pass

  bpy.app.handlers.permanent_tag(my_func, True)  # <-- important bit
  bpy.app.handlers.frame_change_pre.append(my_func)

source/blender/python/intern/bpy_app_handlers.c

@@ -49,6 +49,11 @@ static PyStructSequence_Field app_cb_info_fields[]= {
 	{(char *)"save_post", NULL},
 	{(char *)"scene_update_pre", NULL},
 	{(char *)"scene_update_post", NULL},
+
+	/* sets the permanent tag */
+#   define APP_CB_OTHER_FIELDS 1
+	{(char *)"permanent_tag", NULL},
+
 	{NULL}
 };
 
@@ -65,6 +70,71 @@ static PyStructSequence_Desc app_cb_info_desc= {
 #endif
 */
 
+/* --------------------------------------------------------------------------*/
+/* permanent tagging code */
+#define PERMINENT_CB_ID "_bpy_permanent_tag"
+
+PyDoc_STRVAR(bpy_app_handlers_permanent_tag_doc,
+".. function:: permanent_tag(func, state=True)\n"
+"\n"
+"   Set the function as being permanent so its not cleared when new blend files are loaded.\n"
+"\n"
+"   :arg func: The function  to set as permanent.\n"
+"   :type func: function\n"
+"   :arg state: Set the permanent state to True or False.\n"
+"   :type state: bool\n"
+"   :return: the function argument\n"
+"   :rtype: function\n"
+);
+
+static PyObject *bpy_app_handlers_permanent_tag(PyObject *UNUSED(self), PyObject *args)
+{
+	PyObject *value;
+	int state= 1;
+
+	if(!PyArg_ParseTuple(args, "O|i:permanent_tag", &value, &state))
+		return NULL;
+
+	if (PyFunction_Check(value)) {
+		PyObject **dict_ptr= _PyObject_GetDictPtr(value);
+		if (dict_ptr == NULL) {
+			PyErr_SetString(PyExc_ValueError,
+			                "bpy.app.handlers.permanent_tag wasn't able to "
+			                "get the dictionary from the function passed");
+			return NULL;
+		}
+		else {
+			if (state) {
+				/* set id */
+				if (*dict_ptr == NULL) {
+					*dict_ptr= PyDict_New();
+				}
+
+				PyDict_SetItemString(*dict_ptr, PERMINENT_CB_ID, Py_None);
+			}
+			else {
+				/* clear id */
+				if (*dict_ptr) {
+					PyDict_DelItemString(*dict_ptr, PERMINENT_CB_ID);
+				}
+			}
+		}
+
+		Py_INCREF(value);
+		return value;
+	}
+	else {
+		PyErr_SetString(PyExc_ValueError,
+		                "bpy.app.handlers.permanent_tag expected a function");
+		return NULL;
+	}
+}
+
+static PyMethodDef meth_bpy_app_handlers_permanent_tag= {"permanent_tag", (PyCFunction)bpy_app_handlers_permanent_tag, METH_VARARGS, bpy_app_handlers_permanent_tag_doc};
+
+
+
+
 static PyObject *py_cb_array[BLI_CB_EVT_TOT]= {NULL};
 
 static PyObject *make_app_cb_info(void)
@@ -83,10 +153,13 @@ static PyObject *make_app_cb_info(void)
 		}
 		PyStructSequence_SET_ITEM(app_cb_info, pos, (py_cb_array[pos]= PyList_New(0)));
 	}
-	if (app_cb_info_fields[pos].name != NULL) {
+	if (app_cb_info_fields[pos + APP_CB_OTHER_FIELDS].name != NULL) {
 		Py_FatalError("invalid callback slots 2");
 	}
 
+	/* custom function */
+	PyStructSequence_SET_ITEM(app_cb_info, pos++, (PyObject *)PyCFunction_New(&meth_bpy_app_handlers_permanent_tag, NULL));
+
 	return app_cb_info;
 }
 
@@ -120,12 +193,46 @@ PyObject *BPY_app_handlers_struct(void)
 	return ret;
 }
 
-void BPY_app_handlers_reset(void)
+void BPY_app_handlers_reset(const short do_all)
 {
 	int pos= 0;
 
+	if (do_all) {
 	for (pos= 0; pos < BLI_CB_EVT_TOT; pos++) {
-		PyList_SetSlice(py_cb_array[pos], 0, PY_SSIZE_T_MAX, NULL);
+			/* clear list */
+			PyList_SetSlice(py_cb_array[pos], 0, PY_SSIZE_T_MAX, NULL);
+		}
+	}
+	else {
+		/* save string conversion thrashing */
+		PyObject *perm_id_str= PyUnicode_FromString(PERMINENT_CB_ID);
+
+		for (pos= 0; pos < BLI_CB_EVT_TOT; pos++) {
+			/* clear only items without PERMINENT_CB_ID */
+			PyObject *ls= py_cb_array[pos];
+			Py_ssize_t i;
+
+			PyObject *item;
+			PyObject **dict_ptr;
+
+			for(i= PyList_GET_SIZE(ls) - 1; i >= 0; i--) {
+
+				if (    (PyFunction_Check((item= PyList_GET_ITEM(ls, i)))) &&
+				        (dict_ptr= _PyObject_GetDictPtr(item)) &&
+				        (*dict_ptr) &&
+				        (PyDict_GetItem(*dict_ptr, perm_id_str) != NULL))
+				{
+					/* keep */
+				}
+				else {
+					/* remove */
+					/* PySequence_DelItem(ls, i); */ /* more obvious buw slower */
+					PyList_SetSlice(ls, i, i + 1, NULL);
+				}
+			}
+		}
+
+		Py_DECREF(perm_id_str);
 	}
 }