Created attachment 366722 [details] [review]
Suggested fix

The following code snippet fails to change the sort order from the default GTK_SORT_ASCENDING to GTK_SORT_DESCENDING.

=== Begin ===
GtkTreeModel *sorted = gtk_tree_model_sort_new_with_model(my_model);

gtk_tree_sortable_set_default_sort_func(
    GTK_TREE_SORTABLE(GTK_TREE_MODEL_SORT(sorted)),
    my_sort_func, NULL, NULL);

gtk_tree_sortable_set_sort_column_id(
    GTK_TREE_SORTABLE(GTK_TREE_MODEL_SORT(sorted)),
    GTK_TREE_SORTABLE_DEFAULT_SORT_COLUMN_ID,
    GTK_SORT_DESCENDING);
=== End ===

This is due to a bug in gtk_tree_model_sort_set_sort_column_id(). The function returns without updating sort order when:
1) there is no change to column ID, and
2) column ID is GTK_TREE_SORTABLE_DEFAULT_SORT_COLUMN_ID.

A work-around is to call gtk_tree_sortable_set_sort_column_id() twice. First time to set sort order by using a dummy column ID (e.g. the value zero), and then a second time with the values you want applied.

Patch with suggested fix is attached.

Reviewing the master branch showed the bug is present there too.