Checks the given subset of the post hierarchy for hierarchy loops.

Prevents loops from forming and breaks those that it finds. Attached
to the {@see ‘wp_insert_post_parent’} filter.

function wp_check_post_hierarchy_for_loops( $post_parent, $post_ID ) {
	// Nothing fancy here - bail.
	if ( ! $post_parent ) {
		return 0;
	}

	// New post can't cause a loop.
	if ( ! $post_ID ) {
		return $post_parent;
	}

	// Can't be its own parent.
	if ( $post_parent == $post_ID ) {
		return 0;
	}

	// Now look for larger loops.
	$loop = wp_find_hierarchy_loop( 'wp_get_post_parent_id', $post_ID, $post_parent );
	if ( ! $loop ) {
		return $post_parent; // No loop.
	}

	// Setting $post_parent to the given value causes a loop.
	if ( isset( $loop[ $post_ID ] ) ) {
		return 0;
	}

	// There's a loop, but it doesn't contain $post_ID. Break the loop.
	foreach ( array_keys( $loop ) as $loop_member ) {
		wp_update_post(
			array(
				'ID'          => $loop_member,
				'post_parent' => 0,
			)
		);
	}

	return $post_parent;
}