wp_check_post_hierarchy_for_loops( int $post_parent, int $post_ID )

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 ( empty( $post_ID ) )
		return $post_parent;

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

	// Now look for larger loops.
	if ( !$loop = wp_find_hierarchy_loop( 'wp_get_post_parent_id', $post_ID, $post_parent ) )
		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;
}