save_post not working for Pages/CPTs

Ran into a problem this weekend, where my custom hook onto save_post action was not firing for pages, or custom post types. I’ve done some debugging this morning, and it seems that save_post is not firing at all for pages (and I would assume CPTs). I’ve deactivated all plugins and activated Twenty Fifteen, tried different variations of the hook (save_page,save_post_page), and the action does not fire. I’m using simple code to test:

add_action('save_post','save_post_test');
function save_post_test($post_id) {
	exit('saving');
}

It does fire when saving a post, but not pages/CPTs. Is there something I’ve missed in 4.0.1?

Edit: it gets weirder: i just edited wp-includes/post.php to exit() with output just before each (two) do_action('save_post'), but still nothing! how is that possible?? script does exit() when placed at beginning of wp_insert_post(), so I’m going to work my way down through the function and see where it returns. ??