tests: Test for use after free in resource destruction signals

For years it's been common practice to free the object containing
the wl_listener inside resource destruction notifiers, but not
remove the listener from the list.

That is: It's been safe to assume (when only one listener is present)
that the wl_listener will never be touched again, since this is
a destruction callback.

Recently some patches were reviewed that made some positive changes
to our internal signal handling code, but would've violated this
assumption, and changed free()d memory in several existing compositors
(weston, mutter, enlightenment).

Since the breakage was extremely subtle, codify this assumption in
a test case (thus promoting it to an ABI promise).

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Markus Ongyerth <wl@ongy.net>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>

tests/resources-test.c

@@ -84,6 +84,17 @@ destroy_notify(struct wl_listener *l, void *data)
 {
 	assert(l && data);
 	notify_called = 1;
+
+	/* In real code it's common to free the structure holding the
+	 * listener at this point, but not to remove it from the list.
+	 *
+	 * That's fine since this is a destruction notification and
+	 * it's the last time this signal can fire.  We set these
+	 * to NULL so we can check them later to ensure no write after
+	 * "free" occurred.
+	 */
+	l->link.prev = NULL;
+	l->link.next = NULL;
 }
 
 TEST(destroy_res_tst)
@@ -119,6 +130,8 @@ TEST(destroy_res_tst)
 	assert(destroyed);
 	assert(notify_called); /* check if signal was emitted */
 	assert(wl_client_get_object(client, id) == NULL);
+	assert(destroy_listener.link.prev == NULL);
+	assert(destroy_listener.link.next == NULL);
 
 	res = wl_resource_create(client, &wl_seat_interface, 2, 0);
 	assert(res);
@@ -131,6 +144,8 @@ TEST(destroy_res_tst)
 	wl_client_destroy(client);
 	assert(destroyed);
 	assert(notify_called);
+	assert(destroy_listener.link.prev == NULL);
+	assert(destroy_listener.link.next == NULL);
 
 	wl_display_destroy(display);
 	close(s[1]);