race-condition.js

const express = require('express');
const fs = require('fs').promises;
const path = require('path');
const app = express();

const exportFilePath = path.join(__dirname, 'export.txt');

// Hardcoded data for user 1 and user 2
const userDataMap = {
    '1': 'User 1: John Doe, Balance: $1000\n',
    '2': 'User 2: Jane Smith, Balance: $1500\n'
};

app.get('/export', async (req, res) => {
    const userId = req.query.user;
    const userData = userDataMap[userId] || 'Unknown User\n';

    try {
        // Step 1: Write user data to the file
        await fs.writeFile(exportFilePath, userData);

        // Simulate processing delay
        await new Promise(resolve => setTimeout(resolve, 3000));

        // Step 2: Read the file content (which should be the user's data)
        const fileContent = await fs.readFile(exportFilePath, 'utf8');
        await fs.unlink(exportFilePath)

        res.send(`Exported data: ${fileContent}`);
    } catch (err) {
        res.status(500).send('Error exporting data');
    }
});

/*
http://localhost:3000/export?user=1
http://localhost:3000/export?user=2
If both requests are made nearly simultaneously, both endpoints gets user=2 information
*/

app.listen(3000, () => {
    console.log('Server running on port 3000');
});


/*FIX
Create random and unique name for each files
const exportFilePath = path.join(__dirname, `export_${userId}.txt`);

or 

const crypto = require('crypto');
const randomString = crypto.randomBytes(6).toString('hex');
const exportFilePath = path.join(__dirname, `export_${randomString}.txt`);
*/