Intercept personal data exporter page Ajax responses in order to assemble the personal data export file.

function wp_privacy_process_personal_data_export_page( $response, $exporter_index, $email_address, $page, $request_id, $send_as_email, $exporter_key ) {
	/* Do some simple checks on the shape of the response from the exporter.
	 * If the exporter response is malformed, don't attempt to consume it - let it
	 * pass through to generate a warning to the user by default Ajax processing.
	 */
	if ( ! is_array( $response ) ) {
		return $response;
	}

	if ( ! array_key_exists( 'done', $response ) ) {
		return $response;
	}

	if ( ! array_key_exists( 'data', $response ) ) {
		return $response;
	}

	if ( ! is_array( $response['data'] ) ) {
		return $response;
	}

	// Get the request.
	$request = wp_get_user_request( $request_id );

	if ( ! $request || 'export_personal_data' !== $request->action_name ) {
		wp_send_json_error( __( 'Invalid request ID when merging personal data to export.' ) );
	}

	$export_data = array();

	// First exporter, first page? Reset the report data accumulation array.
	if ( 1 === $exporter_index && 1 === $page ) {
		update_post_meta( $request_id, '_export_data_raw', $export_data );
	} else {
		$accumulated_data = get_post_meta( $request_id, '_export_data_raw', true );

		if ( $accumulated_data ) {
			$export_data = $accumulated_data;
		}
	}

	// Now, merge the data from the exporter response into the data we have accumulated already.
	$export_data = array_merge( $export_data, $response['data'] );
	update_post_meta( $request_id, '_export_data_raw', $export_data );

	// If we are not yet on the last page of the last exporter, return now.
	/** This filter is documented in wp-admin/includes/ajax-actions.php */
	$exporters        = apply_filters( 'wp_privacy_personal_data_exporters', array() );
	$is_last_exporter = count( $exporters ) === $exporter_index;
	$exporter_done    = $response['done'];
	if ( ! $is_last_exporter || ! $exporter_done ) {
		return $response;
	}

	// Last exporter, last page - let's prepare the export file.

	// First we need to re-organize the raw data hierarchically in groups and items.
	$groups = array();
	foreach ( (array) $export_data as $export_datum ) {
		$group_id    = $export_datum['group_id'];
		$group_label = $export_datum['group_label'];

		$group_description = '';
		if ( ! empty( $export_datum['group_description'] ) ) {
			$group_description = $export_datum['group_description'];
		}

		if ( ! array_key_exists( $group_id, $groups ) ) {
			$groups[ $group_id ] = array(
				'group_label'       => $group_label,
				'group_description' => $group_description,
				'items'             => array(),
			);
		}

		$item_id = $export_datum['item_id'];
		if ( ! array_key_exists( $item_id, $groups[ $group_id ]['items'] ) ) {
			$groups[ $group_id ]['items'][ $item_id ] = array();
		}

		$old_item_data                            = $groups[ $group_id ]['items'][ $item_id ];
		$merged_item_data                         = array_merge( $export_datum['data'], $old_item_data );
		$groups[ $group_id ]['items'][ $item_id ] = $merged_item_data;
	}

	// Then save the grouped data into the request.
	delete_post_meta( $request_id, '_export_data_raw' );
	update_post_meta( $request_id, '_export_data_grouped', $groups );

	/**
	 * Generate the export file from the collected, grouped personal data.
	 *
	 * @since 4.9.6
	 *
	 * @param int $request_id The export request ID.
	 */
	do_action( 'wp_privacy_personal_data_export_file', $request_id );

	// Clear the grouped data now that it is no longer needed.
	delete_post_meta( $request_id, '_export_data_grouped' );

	// If the destination is email, send it now.
	if ( $send_as_email ) {
		$mail_success = wp_privacy_send_personal_data_export_email( $request_id );
		if ( is_wp_error( $mail_success ) ) {
			wp_send_json_error( $mail_success->get_error_message() );
		}

		// Update the request to completed state when the export email is sent.
		_wp_privacy_completed_request( $request_id );
	} else {
		// Modify the response to include the URL of the export file so the browser can fetch it.
		$exports_url      = wp_privacy_exports_url();
		$export_file_name = get_post_meta( $request_id, '_export_file_name', true );
		$export_file_url  = $exports_url . $export_file_name;

		if ( ! empty( $export_file_url ) ) {
			$response['url'] = $export_file_url;
		}
	}

	return $response;
}