wp_maybe_generate_attachment_metadata( WP_Post $attachment )

function wp_maybe_generate_attachment_metadata( $attachment ) {
	if ( empty( $attachment ) || ( empty( $attachment->ID ) || ! $attachment_id = (int) $attachment->ID ) ) {
		return;
	}

	$file = get_attached_file( $attachment_id );
	$meta = wp_get_attachment_metadata( $attachment_id );
	if ( empty( $meta ) && file_exists( $file ) ) {
		$_meta = get_post_meta( $attachment_id );
		$regeneration_lock = 'wp_generating_att_' . $attachment_id;
		if ( ! array_key_exists( '_wp_attachment_metadata', $_meta ) && ! get_transient( $regeneration_lock ) ) {
			set_transient( $regeneration_lock, $file );
			wp_update_attachment_metadata( $attachment_id, wp_generate_attachment_metadata( $attachment_id, $file ) );
			delete_transient( $regeneration_lock );
		}
	}
}