Source

File: wp-includes/media.php

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

	$attachment_id = (int) $attachment->ID;
	$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 );
		$_lock = 'wp_generating_att_' . $attachment_id;

		if ( ! array_key_exists( '_wp_attachment_metadata', $_meta ) && ! get_transient( $_lock ) ) {
			set_transient( $_lock, $file );
			wp_update_attachment_metadata( $attachment_id, wp_generate_attachment_metadata( $attachment_id, $file ) );
			delete_transient( $_lock );
		}
	}
}