[BUG] Crash on `gio::ListStore::find_with_equal_func` when no instance of `item-type` exists

[pjungkamp]

Bug description

The find_with_equal_func function of gio::ListStore causes a segfault if no instance of item-type has been created yet.

Cause

GLib will only create a GType's class structure when a first instance of that type has been created. See https://docs.gtk.org/gobject/concepts.html#initialization-and-destruction

The gio::ListStore::find_with_equal_func function tries to work around the pre GLib 2.76 restriction that the item parameter cannot be NULL, by creating a "dummy" GObject instance. See https://gtk-rs.org/gtk-rs-core/stable/latest/docs/src/gio/list_store.rs.html#203-247

Note the call to glib::gobject_ffi::g_type_class_peek. This call should return a pointer to a GTypeClass for the item-type, but it won't if there has never been an instance of that type. This means that the g_class pointer of the "dummy" type instance will be NULL. The SEGFAULT is then caused by the dereference of this null-pointer as part of the g_type_is_a check at the beginning of the g_list_store_find_with_equal_func_full function. See https://gitlab.gnome.org/GNOME/glib/-/blob/main/gio/gliststore.c#L583-584

Solution

For gobject versions prior to 2.76: If self.n_items() == 0 return None without calling into C.

For current gobject versions: Skip the dummy item entirely and just pass a NULL item.