import { ErrorRequestHandler, Request, Response } from "express";
import { HttpError } from "http-errors";
import { logError } from '../lib/logger';

/**
 * Final error handler that executes when an unhandled error occurs.
 * This prevents the server from crashing and ensures proper response handling.
 * 
 * @param err - HTTP error object
 * @param req - Express request object
 * @param res - Express response object
 * @param next - Express next function
 */
export const finalErrorRouter:ErrorRequestHandler = async (err:HttpError, req, res, next) => {

    const errorLogText:string = JSON.stringify({ message:err.message, name:err.name, stack:err.stack });

    logError("server error", `${err.status}; n${errorLogText}`);

    // `headersSent` will be TRUE if the router where the error occurred has already sent headers
    // If we try to set them again, it will throw an error and CRASH THE SERVER - we must prevent this
    if(!res.headersSent) {
        res.status(err.status);
        res.setHeader('Content-Type', "text/html");
        res.end(`unhandled server error`);
    } else {
        // If `end` hasn't been called - call it to finish processing the request
        // Otherwise the connection will remain open until timeout
        if(!res.writableEnded) {
            res.end();
        }
    }
};